Module
Overview
9.1 Introduction
to TCP/IP
9.1.1 History and
future of TCP/IP
9.1.2 Application
layer
9.1.3 Transport
layer
9.1.4 Internet
layer
9.1.5 Network
access layer
9.1.6 The OSI
model and the TCP/IP model
9.1.7 Internet
architecture
9.2 Internet
Addresses
9.2.1 IP
addressing
9.2.2 Decimal and
binary conversion
9.2.3 IPv4
addressing
9.2.4 Class A, B,
C, D, and E IP addresses
9.2.5 Reserved IP
addresses
9.2.6 Public and
private IP addresses
9.2.7
Introduction to subnetting
9.2.8 IPv4 versus
IPv6
9.3 Obtaining an
IP address
9.3.1 Obtaining
an Internet address
9.3.2 Static
assignment of an IP address
9.3.3 RARP IP
address assignment
9.3.4 BOOTP IP
address assignment
9.3.5 DHCP IP
address management
9.3.6 Problems in
address resolution
9.3.7 Address
Resolution Protocol (ARP)
Module: Summary
Overview
The Internet was
developed to provide a communication network that could function in wartime.
Although the Internet has evolved from the original plan, it is still based on
the TCP/IP protocol suite. The design of TCP/IP is ideal for the decentralized
and robust Internet. Many common protocols were designed based on the
four-layer TCP/IP model.
It is useful to
know both the TCP/IP and OSI network models. Each model uses its own structure
to explain how a network works. However, there is much overlap between the two
models. A system administrator should be familiar with both models to
understand how a network functions.
Any device on the
Internet that wants to communicate with other Internet devices must have a
unique identifier. The identifier is known as the IP address because routers
use a Layer 3 protocol called the IP protocol to find the best route to that
device. The current version of IP is IPv4. This was designed before there was a
large demand for addresses. Explosive growth of the Internet has threatened to
deplete the supply of IP addresses. Subnets, Network Address Translation (NAT),
and private addresses are used to extend the supply of IP addresses. IPv6
improves on IPv4 and provides a much larger address space. Administrators can
use IPv6 to integrate or eliminate the methods used to work with IPv4.
In addition to
the physical MAC address, each computer needs a unique IP address to be part of
the Internet. This is also called the logical address. There are several ways
to assign an IP address to a device. Some devices always have a static address.
Others have a temporary address assigned to them each time they connect to the
network. When a dynamically assigned IP address is needed, a device can obtain
it several ways.
For efficient
routing to occur between devices, issues such as duplicate IP addresses must be
resolved.
This module
covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND
640-811 exams.
Students who
complete this module should be able to perform the following tasks:
- Explain why the Internet was
developed and how TCP/IP fits the design of the Internet
- List the four layers of the
TCP/IP model
- Describe the functions of each
layer of the TCP/IP model
- Compare the OSI model and the
TCP/IP model
- Describe the function and
structure of IP addresses
- Understand why subnetting is
necessary
- Explain the difference between
public and private addressing
- Understand the function of
reserved IP addresses
- Explain the use of static and
dynamic addressing for a device
- Understand how dynamic
addresses can be assigned with RARP, BootP, and DHCP
- Use ARP to obtain the MAC
address to send a packet to another device
- Understand the issues related
to addressing between networks
9.1 Introduction to TCP/IP
9.1.1 History and future of TCP/IP
This page
discusses the history and the future of TCP/IP.
The U.S.
Department of Defense (DoD) created the TCP/IP reference model because it
wanted a network that could survive any conditions. To illustrate further,
imagine a world, crossed by multiple cable runs, wires, microwaves, optical
fibers, and satellite links. Then imagine a need for data to be transmitted
without regard for the condition of any particular node or network. The U.S.
DoD required reliable data transmission to any destination on the network under
any circumstances. The creation of the TCP/IP model helped to solve this
difficult design problem. The TCP/IP model has since become the standard on
which the Internet is based.
Think about the
layers of the TCP/IP model layers in relation to the original intent of the
Internet. This will help reduce confusion. The four layers of the TCP/IP model
are the application layer, transport layer, Internet layer, and network access
layer. Some of the layers in the TCP/IP
model have the same name as layers in the OSI model. It is critical not to
confuse the layer functions of the two models because the layers include
different functions in each model. The present version of TCP/IP was
standardized in September of 1981.
The next page
will discuss the application layer of TCP/IP.
9.1
Introduction to TCP/IP
9.1.2 Application layer
This page
describes the functions of the TCP/IP application layer.
The application
layer handles high-level protocols, representation, encoding, and dialog
control. The TCP/IP protocol suite combines all application related issues into
one layer. It ensures that the data is properly packaged before it is passed on
to the next layer. TCP/IP includes Internet and transport layer specifications
such as IP and TCP as well as specifications for common applications. TCP/IP
has protocols to support file transfer, e-mail, and remote login, in addition
to the following:
- File Transfer Protocol (FTP)
– FTP is a reliable, connection-oriented service that uses TCP to transfer
files between systems that support FTP. It supports bi-directional binary
file and ASCII file transfers.
- Trivial File Transfer Protocol
(TFTP) – TFTP is a connectionless service
that uses the User Datagram Protocol (UDP). TFTP is used on the router to
transfer configuration files and Cisco IOS images, and to transfer files
between systems that support TFTP. It is useful in some LANs because it
operates faster than FTP in a stable environment.
- Network File System (NFS)
– NFS is a distributed file system protocol suite developed by Sun
Microsystems that allows file access to a remote storage device such as a
hard disk across a network.
- Simple Mail Transfer Protocol
(SMTP) – SMTP administers the transmission
of e-mail over computer networks. It does not provide support for
transmission of data other than plain text.
- Telnet –
Telnet provides the capability to remotely access another computer. It
enables a user to log into an Internet host and execute commands. A Telnet
client is referred to as a local host. A Telnet server is referred to as a
remote host.
- Simple Network Management
Protocol (SNMP) – SNMP is a protocol that
provides a way to monitor and control network devices. SNMP is also used
to manage configurations, statistics, performance, and security.
- Domain Name System (DNS)
– DNS is a system used on the Internet to translate domain names and
publicly advertised network nodes into IP addresses.
The Interactive Media Activity will help students become
familiar with the application layer protocols.
The next page
will discuss the transport layer.
9.1 Introduction
to TCP/IP
9.1.3 Transport
layer
This page will
explain how the transport layer provides transport services from the source
host to the destination host.
The transport
layer provides a logical connection between a source host and a destination
host. Transport protocols segment and
reassemble data sent by upper-layer applications into the same data stream, or
logical connection, between end points.
The Internet is
often represented by a cloud. The transport layer sends data packets from a
source to a destination through the cloud.
The primary duty of the transport layer is to provide end-to-end control
and reliability as data travels through this cloud. This is accomplished
through the use of sliding windows, sequence numbers, and acknowledgments. The
transport layer also defines end-to-end connectivity between host applications.
Transport layer protocols include TCP and UDP.
The functions of
TCP and UDP are as follows:
- Segment upper-layer application
data
- Send segments from one end
device to another
The functions of
TCP are as follows:
- Establish end-to-end operations
- Provide flow control through
the use of sliding windows
- Ensure reliability through the
use of sequence numbers and acknowledgments
The Interactive Media Activity will help students become
familiar with the transport layer protocols.
The next page
will describe the Internet layer.
9.1 Introduction to TCP/IP
9.1.4 Internet layer
This page
explains the functions of the TCP/IP Internet layer.
The purpose of
the Internet layer is to select the best path through the network for packets
to travel. The main protocol that functions at this layer is IP. Best path
determination and packet switching occur at this layer.
The following
protocols operate at the TCP/IP Internet layer:
- IP provides connectionless,
best-effort delivery routing of packets. IP is not concerned with the
content of the packets but looks for a path to the destination.
- Internet Control Message
Protocol (ICMP) provides control and messaging capabilities.
- Address Resolution Protocol
(ARP) determines the data link layer address, or MAC address, for known IP
addresses.
- Reverse Address Resolution
Protocol (RARP) determines the IP address for a known MAC address.
IP performs the following
operations:
- Defines a packet and an
addressing scheme
- Transfers data between the
Internet layer and network access layer
- Routes packets to remote hosts
IP is sometimes referred to as an unreliable protocol.
This does not mean that IP will not accurately deliver data across a network.
IP is unreliable because it does not perform error checking and correction.
That function is handled by upper layer protocols from the transport or
application layers.
The Interactive
Media Activity will help students become familiar with the protocols used in
the Internet layer.
The next page
will discuss the network access layer.
9.1
Introduction to TCP/IP
9.1.5
Network access layer
This page will
discuss the TCP/IP network access layer, which is also called the
host-to-network layer.
The network
access layer allows an IP packet to make a physical link to the network media.
It includes the LAN and WAN technology details and all the details contained in
the OSI physical and data link layers.
Drivers for
software applications, modem cards, and other devices operate at the network
access layer. The network access layer defines the procedures used to interface
with the network hardware and access the transmission medium. Modem protocol
standards such as Serial Line Internet Protocol (SLIP) and Point-to-Point
Protocol (PPP) provide network access through a modem connection. Many
protocols are required to determine the hardware, software, and
transmission-medium specifications at this layer. This can lead to confusion
for users. Most of the recognizable protocols operate at the transport and
Internet layers of the TCP/IP model.
Network access
layer protocols also map IP addresses to physical hardware addresses and
encapsulate IP packets into frames. The network access layer defines the
physical media connection based on the hardware type and network interface.
Here is an
example of a network access layer configuration that involves a Windows system
set up with a third party NIC. The NIC would automatically be detected by some
versions of Windows and then the proper drivers would be installed. In an older
version of Windows, the user would have to specify the network card driver. The
card manufacturer supplies these drivers on disks or CD-ROMs.
The Interactive
Media Activity will help students become familiar with the network access layer
protocols.
The next page
explains the similarities and differences between the TCP/IP model and the OSI
reference model.
9.1
Introduction to TCP/IP
9.1.6
The OSI model and the TCP/IP model
This page
provides a comparison of the OSI model and the TCP/IP model.
The OSI and
TCP/IP models have many similarities:
- Both have layers.
- Both have application layers,
though they include different services.
- Both have comparable transport
and network layers.
- Both use packet-switched
instead of circuit-switched technology.
- Networking professionals need
to know both models.
Here are some differences of the OSI and TCP/IP models:
- TCP/IP combines the OSI application,
presentation, and session layers into its application layer.
- TCP/IP combines the OSI data
link and physical layers into its network access layer.
- TCP/IP appears simpler because
it has fewer layers.
- When the TCP/IP transport layer
uses UDP it does not provide reliable delivery of packets. The transport
layer in the OSI model always does.
The Internet was
developed based on the standards of the TCP/IP protocols. The TCP/IP model
gains credibility because of its protocols. The OSI model is not generally used
to build networks. The OSI model is used as a guide to help students understand
the communication process.
The Interactive
Media Activity will help students understand the differences between the TCP/IP
and OSI reference models.
The next page
examines the basic architecture of the Internet.
9.1 Introduction to TCP/IP
9.1.7 Internet architecture
This page will
examine the basic architecture of the Internet.
The Internet
enables nearly instantaneous worldwide data communications between anyone,
anywhere, at any time.
LANs are networks
within limited geographic areas. However, LANs are limited in scale. Although
there have been technological advances to improve the speed of communications,
such as Metro Optical, Gigabit, and 10-Gigabit Ethernet, distance is still a
problem.
Students can
focus on the communications between source and destination computers or
intermediate computers at the application layer to get an overview of the
Internet architecture. Identical instances of an application could be placed on
all the computers in a network to ease the delivery of messages. However, this
does not scale well. New software would require new applications to be
installed on every computer in the network. For new hardware to function properly,
the software would need to be modified. Any failure of an intermediate computer
or computer application would cause a break in the chain of the messages that
are passed.
The Internet uses
the principle of network layer interconnection. The goal is to build the
functionality of the network in independent modules. This allows a diversity of
LAN technologies at Layers 1 and 2 of the OSI model and a diversity of
applications at Layers 5, 6, and 7. The OSI model provides a mechanism where
the details of the lower and the upper layers are separated. This allows
intermediate networking devices to relay traffic without details about the LAN.
This leads to the
concept of internetworks, or networks that consist of many networks. A network
of networks is called an internetwork, which is indicated with the lowercase i.
The network on which the World Wide Web (www) runs is the Internet, which is
indicated with a capital I. Internetworks must be scalable with regard to the
number of networks and computers attached. They must also be able to handle the
transport of data across vast distances. An internetwork must be flexible to
account for constant technological innovations. It must be able to adjust to
dynamic conditions on the network. And internetworks must be cost-effective.
Internetworks must be designed to permit data communications to anyone,
anywhere, at any time.
Figure summarizes the connection of one physical
network to another through a special purpose computer called a router. These
networks are described as directly connected to the router. The router is
needed to handle any path decisions required for the two networks to
communicate. Many routers are needed to handle large volumes of network
traffic.
Figure extends the idea to three physical networks
connected by two routers. Routers make complex decisions to allow users on all
the networks to communicate with each other. Not all networks are directly
connected to one another. The router must have some method to handle this
situation.
One option is for
a router to keep a list of all computers and all the paths to them. The router
would then decide how to forward data packets based on this reference table.
Packets would be forwarded based on the IP address of the destination computer.
This option would become difficult as more users were added to the network.
Scalability is introduced when the router keeps a list of all networks, but
leaves the local delivery details to the local physical networks. In this
situation, the routers pass messages to other routers. Each router shares
information about its connected network.
Figure shows the transparency that users require.
However, the physical and logical structures inside the Internet cloud can be
extremely complex as shown in Figure . The Internet has grown rapidly to allow
more and more users. The fact that the Internet has grown so large, with more
than 90,000 core routes and 300,000,000 end users, proves the effectiveness of
the Internet architecture.
Two computers
located anywhere in the world that follow certain hardware, software, and
protocol specifications can communicate reliably. The standardization of ways
to move data across networks has made the Internet possible.
This page
concludes this lesson. The next lesson will discuss Internet addressing. The
first page covers IP addressing.
9.2 Internet Addresses
9.2.1 IP addressing
This page will describe IP addressing.
For any two
systems to communicate, they must be able to identify and locate each other.
The addresses in Figure are not actual
network addresses. They represent and show the concept of address grouping.
A computer may be
connected to more than one network. In
this situation, the system must be given more than one address. Each address
will identify the connection of the computer to a different network. Each
connection point, or interface, on a device has an address to a network. This
will allow other computers to locate the device on that particular network. The
combination of the network address and the host address creates a unique
address for each device on a network. Each computer in a TCP/IP network must be
given a unique identifier, or IP address. This address, which operates at Layer
3, allows one computer to locate another computer on a network. All computers
also have a unique physical address, which is known as a MAC address. These are
assigned by the manufacturer of the NIC. MAC addresses operate at Layer 2 of
the OSI model.
An IP address is
a 32-bit sequence of ones and zeros. Figure
shows a sample 32-bit number. To make the IP address easier to work
with, it is usually written as four decimal numbers separated by periods. For
example, an IP address of one computer is 192.168.1.2. Another computer might
have the address 128.10.2.1. This is called the dotted decimal format. Each
part of the address is called an octet because it is made up of eight binary
digits. For example, the IP address 192.168.1.8 would be
11000000.10101000.00000001.00001000 in binary notation. The dotted decimal
notation is an easier method to understand than the binary ones and zeros
method. This dotted decimal notation also prevents a large number of
transposition errors that would result if only the binary numbers were used.
Both the binary
and decimal numbers in Figure represent
the same values. However, the address is easier to understand in dotted decimal
notation. This is one of the common problems associated with binary numbers.
The long strings of repeated ones and zeros make errors more likely.
It is easy to see
the relationship between the numbers 192.168.1.8 and 192.168.1.9. The binary
values 11000000.10101000.00000001.00001000 and
11000000.10101000.00000001.00001001 are not as easy to recognize. It is more
difficult to determine that the binary values are consecutive numbers.
The next page
will discuss the conversion of binary and decimal numbers.
9.2 Internet Addresses
9.2.2 Decimal and binary conversion
There are several
ways to convert decimal numbers to binary numbers. This page will describe one
method.
The student may
find other methods easier. It is a matter of personal preference.
When converting a
decimal number to binary, the biggest power of two that will fit into the
decimal number must be determined. If
this process is designed to be working with computers, the most logical place
to start is with the largest values that will fit into a byte or two bytes. As
mentioned earlier, the most common grouping of bits is eight, which make up one
byte. However, sometimes the largest value that can be held in one byte is not
large enough for the values needed. To accommodate this, bytes are combined.
Instead of having two eight-bit numbers, one 16-bit number is created. Instead
of three eight-bit numbers, one 24-bit number is created. The same rules apply
as they did for eight-bit numbers. Multiply the previous position value by two
to get the present column value.
Since working
with computers often is referenced by bytes it is easiest to start with byte
boundaries and calculate from there.
Start by calculating a couple of examples, the first being 6,783. Since
this number is greater than 255, the largest value possible in a single byte,
two bytes will be used. Start calculating from 215. The binary equivalent of
6,783 is 00011010 01111111.
The second
example is 104. Since this number is less than 255, it can be represented by
one byte. The binary equivalent of 104 is 01101000.
This method works
for any decimal number. Consider the decimal number one million. Since one
million is greater than the largest value that can be held in two bytes, 65535,
at least three bytes will be needed. By multiplying by two until 24 bits, three
bytes, is reached, the value will be 16,777,215. This means that the largest
value that 24 bits can hold is 16,777,215. So starting at the 24-bit, follow
the process until zero is reached. Continuing with the procedure described, it
is determined that the decimal number one million is equal to the binary number
00001111 01000010 01000000.
Figure includes some decimal to binary conversion
exercises.
Binary to decimal
conversion is just the opposite. Simply place the binary in the table and if
there is a one in a column position add that value into the total. Convert 00000100 00011101 to decimal. The
answer is 1053.
Figure includes some binary to decimal conversion
exercises.
The next page
will discuss IPv4 addressing.
9.2 Internet Addresses
9.2.3 IPv4 addressing
This page will
discuss IPv4 addressing.
A router uses IP
to forward packets from the source network to the destination network. The
packets must include an identifier for both the source and destination
networks. A router uses the IP address
of the destination network to deliver a packet to the correct network. When the
packet arrives at a router connected to the destination network, the router
uses the IP address to locate the specific computer on the network. This system
works in much the same way as the national postal system. When the mail is
routed, the zip code is used to deliver it to the post office at the
destination city. That post office must use the street address to locate the
final destination in the city.
Every IP address
also has two parts. The first part
identifies the network where the system is connected and the second part
identifies the system. As is shown Figure , each octet ranges from 0 to 255.
Each one of the octets breaks down into 256 subgroups and they break down into
another 256 subgroups with 256 addresses in each. By referring to the group
address directly above a group in the hierarchy, all of the groups that branch
from that address can be referenced as a single unit.
This kind of
address is called a hierarchical address, because it contains different levels.
An IP address combines these two identifiers into one number. This number must
be a unique number, because duplicate addresses would make routing impossible.
The first part identifies the system's network address. The second part, called
the host part, identifies which particular machine it is on the network.
IP addresses are
divided into classes to define the large, medium, and small networks. Class A
addresses are assigned to larger networks. Class B addresses are used for
medium-sized networks, and Class C for small networks. The first step in determining which part of
the address identifies the network and which part identifies the host is
identifying the class of an IP address.
The Interactive
Media Activity will require students to identify the different classes of
addresses.
The next page
will provide more information about Class A, B, C, D, and E
IP addresses.
9.2
Internet Addresses
9.2.4 Class A, B, C, D, and E
IP addresses
This page will
describe the five IP address classes.
To accommodate
different size networks and aid in classifying these networks, IP addresses are
divided into groups called classes. This
is known as classful addressing. Each complete 32-bit IP address is broken down
into a network part and a host part. A
bit or bit sequence at the start of each address determines the class of the
address. There are five IP address classes as shown in Figure .
The Class A
address was designed to support extremely large networks, with more than 16
million host addresses available. Class
A IP addresses use only the first octet to indicate the network address. The
remaining three octets provide for host addresses.
The first bit of
a Class A address is always 0. With that first bit a 0, the lowest number that
can be represented is 00000000, decimal 0. The highest number that can be
represented is 01111111, decimal 127. The numbers 0 and 127 are reserved and
cannot be used as network addresses. Any address that starts with a value
between 1 and 126 in the first octet is a Class A address.
The 127.0.0.0
network is reserved for loopback testing. Routers or local machines can use
this address to send packets back to themselves. Therefore, this number cannot
be assigned to a network.
The Class B
address was designed to support the needs of moderate to large-sized networks. A Class B IP address uses the first two of
the four octets to indicate the network address. The other two octets specify
host addresses.
The first two
bits of the first octet of a Class B address are always 10. The remaining six
bits may be populated with either 1s or 0s. Therefore, the lowest number that
can be represented with a Class B address is 10000000, decimal 128. The highest
number that can be represented is 10111111, decimal 191. Any address that
starts with a value in the range of 128 to 191 in the first octet is a Class B
address.
The Class C
address space is the most commonly used of the original address classes. This address space was intended to support
small networks with a maximum of 254 hosts.
A Class C address
begins with binary 110. Therefore, the lowest number that can be represented is
11000000, decimal 192. The highest number that can be represented is 11011111,
decimal 223. If an address contains a number in the range of 192 to 223 in the
first octet, it is a Class C address.
The Class D
address class was created to enable multicasting in an IP address. A multicast address is a unique network
address that directs packets with that destination address to predefined groups
of IP addresses. Therefore, a single station can simultaneously transmit a
single stream of data to multiple recipients.
The Class D
address space, much like the other address spaces, is mathematically
constrained. The first four bits of a Class D address must be 1110. Therefore,
the first octet range for Class D addresses is 11100000 to 11101111, or 224 to
239. An IP address that starts with a value in the range of 224 to 239 in the
first octet is a Class D address.
A Class E address
has been defined. However, the Internet
Engineering Task Force (IETF) reserves these addresses for its own research.
Therefore, no Class E addresses have been released for use in the Internet. The
first four bits of a Class E address are always set to 1s. Therefore, the first
octet range for Class E addresses is 11110000 to 11111111, or 240 to 255.
Figure shows the IP address range of the first octet
both in decimal and binary for each IP address class.
The next page
will discuss reserved IP addresses.
9.2 Internet Addresses
9.2.5 Reserved IP addresses
This page will
describe the types of reserved IP addresses.
Certain host
addresses are reserved and cannot be assigned to devices on a network. These
reserved host addresses include the following:
- Network
address – Used to identify the network itself
In Figure , the
section that is identified by the upper box represents the 198.150.11.0
network. Data that is sent to any host on that network (198.150.11.1-
198.150.11.254) will be seen outside of the local area network as 198.159.11.0.
The only time that the host numbers matter is when the data is on the local
area network. The LAN that is contained in the lower box is treated the same as
the upper LAN, except that its network number is 198.150.12.0.
- Broadcast address
– Used for broadcasting packets to all the devices on a network
In Figure , the
section that is identified by the upper box represents the 198.150.11.255
broadcast address. Data that is sent to the broadcast address will be read by
all hosts on that network (198.150.11.1- 198.150.11.254). The LAN that is
contained in the lower box is treated the same as the upper LAN, except that
its broadcast address is 198.150.12.255.
An IP address
that has binary 0s in all host bit positions is reserved for the network
address. In a Class A network example, 113.0.0.0 is the IP address of the
network, known as the network ID, containing the host 113.1.2.3. A router uses
the network IP address when it forwards data on the Internet. In a Class B
network example, the address 176.10.0.0 is a network address, as shown in
Figure .
In a Class B
network address, the first two octets are designated as the network portion.
The last two octets contain 0s because those 16 bits are for host numbers and
are used to identify devices that are attached to the network. The IP address,
176.10.0.0, is an example of a network address. This address is never assigned
as a host address. A host address for a device on the 176.10.0.0 network might
be 176.10.16.1. In this example, “176.10” is the network portion and “16.1” is
the host portion.
To send data to
all the devices on a network, a broadcast address is needed. A broadcast occurs when a source sends data
to all devices on a network. To ensure that all the other devices on the
network process the broadcast, the sender must use a destination IP address
that they can recognize and process. Broadcast IP addresses end with binary 1s
in the entire host part of the address.
In the network
example, 176.10.0.0, the last 16 bits make up the host field or host part of
the address. The broadcast that would be
sent out to all devices on that network would include a destination address of
176.10.255.255. This is because 255 is the decimal value of an octet containing
11111111.
The next page
will discuss public and private IP addresses.
9.2 Internet Addresses
9.2.6 Public and private IP addresses
This page
describes public and private IP addresses.
The stability of
the Internet depends directly on the uniqueness of publicly used network
addresses. In Figure , there is an issue with the network addressing scheme. In
looking at the networks, both have a network address of 198.150.11.0. The
router in this illustration will not be able to forward the data packets
correctly. Duplicate network IP addresses prevent the router from performing
its job of best path selection. Unique addresses are required for each device
on a network.
A procedure was
needed to make sure that addresses were in fact unique. Originally, an
organization known as the Internet
Network Information
Center
Public IP
addresses are unique. No two machines that connect to a public network can have
the same IP address because public IP addresses are global and standardized.
All machines connected to the Internet agree to conform to the system. Public
IP addresses must be obtained from an Internet service provider (ISP) or a
registry at some expense.
With the rapid
growth of the Internet, public IP addresses were beginning to run out. New
addressing schemes, such as classless interdomain routing (CIDR) and IPv6 were
developed to help solve the problem. CIDR and IPv6 are discussed later in the
course.
Private IP
addresses are another solution to the problem of the impending exhaustion of
public IP addresses. As mentioned, public networks require hosts to have unique
IP addresses. However, private networks that are not connected to the Internet
may use any host addresses, as long as each host within the private network is
unique. Many private networks exist alongside public networks. However, a
private network using just any address is strongly discouraged because that
network might eventually be connected to the Internet. RFC 1918 sets aside
three blocks of IP addresses for private, internal use. These three blocks consist of one Class A, a
range of Class B addresses, and a range of Class C addresses. Addresses that
fall within these ranges are not routed on the Internet backbone. Internet
routers immediately discard private addresses. If addressing a nonpublic
intranet, a test lab, or a home network, these private addresses can be used
instead of globally unique addresses.
Private IP addresses can be intermixed, as shown in the graphic, with
public IP addresses. This will conserve the number of addresses used for
internal connections.
Connecting a
network using private addresses to the Internet requires translation of the
private addresses to public addresses. This translation process is referred to
as Network Address Translation (NAT). A router usually is the device that
performs NAT. NAT, along with CIDR and IPv6 are covered in more depth later in
the curriculum.
The next page
will introduce subnetting.
9.2
Internet Addresses
9.2.7
Introduction to subnetting
This page will
explain how subnetting is used to manage IP addresses.
Subnetting is one
method used to manage IP addresses, as shown in example , the 131.108.0.0
network is subnetted into the 131.108.1.0, 131.108.2.0 and 131.108.3.0 subnets.
This method of dividing full network address classes into smaller pieces has
prevented complete IP address exhaustion. It is impossible to cover TCP/IP
without mentioning subnetting. As a system administrator it is important to
understand subnetting as a means of dividing and identifying separate networks
throughout the LAN. It is not always necessary to subnet a small network.
However, for large or extremely large networks, subnetting is required. Subnetting a network means to use the subnet
mask to divide the network and break a large network up into smaller, more
efficient and manageable segments, or subnets. An example would be the U.S.
The system
administrator must resolve these issues when adding and expanding the network.
It is important to know how many subnets or networks are needed and how many
hosts will be needed on each network. With subnetting, the network is not
limited to the default Class A, B, or C network masks and there is more
flexibility in the network design.
Subnet addresses
include the network portion, plus a subnet field and a host field. The subnet
field and the host field are created from the original host portion for the
entire network. The ability to decide how to divide the original host portion
into the new subnet and host fields provides addressing flexibility for the
network administrator.
To create a
subnet address, a network administrator borrows bits from the host field and
designates them as the subnet field. The
minimum number of bits that can be borrowed is two. When creating a subnet,
where only one bit was borrowed the network number would be the .0 network. The
broadcast number would then be the .255 network. The maximum number of bits
that can be borrowed can be any number that leaves at least two bits remaining,
for the host number.
The Lab Activity
will help students become familiar with the different classes of IP addresses.
The next page
will introduce IP Version 6 (IPv6).
9.2 Internet Addresses
9.2.8 IPv4 versus IPv6
This page will
compare IPv4 and IPv6.
When TCP/IP was
adopted in the 1980s, it relied on a two-level addressing scheme. At the time
this offered adequate scalability. Unfortunately, the designers of TCP/IP could
not have predicted that their protocol would eventually sustain a global
network of information, commerce, and entertainment. Over twenty years ago, IP
Version 4 (IPv4) offered an addressing strategy that, although scalable for a
time, resulted in an inefficient allocation of addresses.
The Class A and B
addresses make up 75 percent of the IPv4 address space, however fewer than
17,000 organizations can be assigned a Class A or B network number. Class C network addresses are far more
numerous than Class A and Class B addresses, although they account for only
12.5 percent of the possible four billion IP addresses.
Unfortunately,
Class C addresses are limited to 254 usable hosts. This does not meet the needs
of larger organizations that cannot acquire a Class A or B address. Even if
there were more Class A, B, and C addresses, too many network addresses would
cause Internet routers to come to a stop under the burden of the enormous size
of routing tables required to store the routes to reach each of the networks.
As early as 1992,
the Internet Engineering Task Force (IETF) identified the following two
specific concerns:
- Exhaustion of the remaining,
unassigned IPv4 network addresses. At the time, the Class B space was on
the verge of depletion.
- The rapid and large increase in
the size of Internet routing tables occurred as more Class C networks came
online. The resulting flood of new network information threatened the
ability of Internet routers to cope effectively.
Over the past two
decades, numerous extensions to IPv4 have been developed. These extensions are
specifically designed to improve the efficiency with which the 32-bit address
space can be used. Two of the more important of these are subnet masks and
classless interdomain routing (CIDR), which are discussed in more detail in
later lessons.
Meanwhile, an
even more extendible and scalable version of IP, IP Version 6 (IPv6), has been
defined and developed. IPv6 uses 128
bits rather than the 32 bits currently used in IPv4. IPv6 uses hexadecimal
numbers to represent the 128 bits. IPv6 provides 640 sextrillion addresses.
This version of IP should provide enough addresses for future communication
needs.
Figure shows an IPv4 address and an IPv6 address. IPv4
addresses are 32 bits long, written in decimal form, and separated by periods.
IPv6 addresses are 128-bits long and are identifiers for individual interfaces
and sets of interfaces. IPv6 addresses are assigned to interfaces, not nodes.
Since each interface belongs to a single node, any of the unicast addresses
assigned to the interfaces of the node may be used as an identifier for the
node. IPv6 addresses are written in hexadecimal, and separated by colons. IPv6
fields are 16 bits long. To make the addresses easier to read, leading zeros
can be omitted from each field. The field :0003: is written :3:. IPv6 shorthand
representation of the 128 bits uses eight 16-bit numbers, shown as four
hexadecimal digits.
After years of
planning and development, IPv6 is slowly being implemented in select networks.
Eventually, IPv6 may replace IPv4 as the dominant Internet protocol.
This page
concludes this lesson. The next lesson will explain how IP addresses are
obtained. The first page will discuss Internet addresses.
9.3 Obtaining an IP address
9.3.1 Obtaining an Internet address
This page will explain how an Internet address is
obtained.
A network host
needs to obtain a globally unique address in order to function on the Internet.
The physical or MAC address that a host has is only locally significant,
identifying the host within the local area network. Since this is a Layer 2
address, the router does not use it to forward outside the LAN.
IP addresses are
the most commonly used addresses for Internet communications. This protocol is
a hierarchical addressing scheme that allows individual addresses to be
associated together and treated as groups. These groups of addresses allow
efficient transfer of data across the Internet.
Network administrators
use two methods to assign IP addresses. These methods are static and dynamic.
Later in this lesson, static addressing and three variations of dynamic
addressing will be covered. Regardless of which addressing scheme is chosen, no
two interfaces can have the same IP address. Two hosts that have the same IP
address could create a conflict that might cause both of the hosts involved not
to operate properly. As shown in Figure , the hosts have a physical address by
having a network interface card that allows connection to the physical medium.
The next page
will focus on static IP address assignments.
9.3 Obtaining an IP address
9.3.2 Static assignment of an IP address
This page will discuss static assignments.
Static assignment
works best on small, infrequently changing networks. The system administrator
manually assigns and tracks IP addresses for each computer, printer, or server
on the intranet. Good recordkeeping is
critical to prevent problems which occur with duplicate IP addresses. This is
possible only when there are a small number of devices to track.
Servers should be
assigned a static IP address so workstations and other devices will always know
how to access needed services. Consider
how difficult it would be to phone a business that changed its phone number
every day.
Other devices
that should be assigned static IP addresses are network printers, application
servers, and routers.
The next page
will introduce Reverse Address Resolution Protocol (RARP).
9.3 Obtaining an IP address
9.3.3 RARP IP address assignment
This page will discuss RARP address assignment.
Reverse Address
Resolution Protocol (RARP) associates a known MAC addresses with an IP
addresses. This association allows network devices to encapsulate data before
sending the data out on the network. A network device, such as a diskless
workstation, might know its MAC address but not its IP address. RARP allows the
device to make a request to learn its IP address. Devices using RARP require
that a RARP server be present on the network to answer RARP requests.
Consider an
example where a source device wants to send data to another device. In this
example, the source device knows its own MAC address but is unable to locate
its own IP address in the ARP table. The source device must include both its
MAC address and IP address in order for the destination device to retrieve
data, pass it to higher layers of the OSI model, and respond to the originating
device. Therefore, the source initiates a process called a RARP request. This
request helps the source device detect its own IP address. RARP requests are
broadcast onto the LAN and are responded to by the RARP server which is usually
a router.
RARP uses the
same packet format as ARP. However, in a RARP request, the MAC headers and
operation code are different from an ARP request. The RARP packet format contains places for
MAC addresses of both the destination and source devices. The source IP address
field is empty. The broadcast goes to all devices on the network. Figures , ,
and depict the destination MAC address
as FF:FF:FF:FF:FF:FF. Workstations running RARP have codes in ROM that direct
them to start the RARP process. A step-by-step layout of the RARP process is
illustrated in Figures through .
The next page
will discuss the Bootstrap Protocol (BOOTP).
9.3 Obtaining an IP address
9.3.4 BOOTP IP address assignment
This page will
introduce BOOTP.
The bootstrap
protocol (BOOTP) operates in a client-server environment and only requires a
single packet exchange to obtain IP information. However, unlike RARP, BOOTP packets can
include the IP address, as well as the address of a router, the address of a
server, and vendor-specific information.
One problem with
BOOTP, however, is that it was not designed to provide dynamic address
assignment. With BOOTP, a network administrator creates a configuration file
that specifies the parameters for each device. The administrator must add hosts
and maintain the BOOTP database. Even though the addresses are dynamically
assigned, there is still a one to one relationship between the number of IP
addresses and the number of hosts. This means that for every host on the
network there must be a BOOTP profile with an IP address assignment in it. No
two profiles can have the same IP address. Those profiles might be used at the
same time and that would mean that two hosts have the same IP address.
A device uses
BOOTP to obtain an IP address when starting up. BOOTP uses UDP to carry
messages. The UDP message is encapsulated in an IP packet. A computer uses
BOOTP to send a broadcast IP packet using a destination IP address of all 1s,
255.255.255.255 in dotted decimal notation. A BOOTP server receives the
broadcast and then sends back a broadcast. The client receives a frame and
checks the MAC address. If the client finds its own MAC address in the
destination address field and a broadcast in the IP destination field, it takes
and stores the IP address and other information supplied in the BOOTP reply
message. A step-by-step description of the process is shown in Figures through .
The next page
will discuss Dynamic Host Configuration Protocol (DHCP).
9.3 Obtaining an IP address
9.3.5 DHCP IP address management
This page will
explain the features and benefits of DHCP.
Dynamic host
configuration protocol (DHCP) is the successor to BOOTP. Unlike BOOTP, DHCP
allows a host to obtain an IP address dynamically without the network
administrator having to set up an individual profile for each device. All that
is required when using DHCP is a defined range of IP addresses on a DHCP
server. As hosts come online, they contact the DHCP server and request an
address. The DHCP server chooses an address and leases it to that host. With
DHCP, the entire network configuration of a computer can be obtained in one
message. This includes all of the data
supplied by the BOOTP message, plus a leased IP address and a subnet mask.
The major
advantage that DHCP has over BOOTP is that it allows users to be mobile. This mobility
allows the users to freely change network connections from location to
location. It is no longer required to keep a fixed profile for every device
attached to the network as was required with the BOOTP system. The importance
to this DHCP advancement is its ability to lease an IP address to a device and
then reclaim that IP address for another user after the first user releases it.
This means that DHCP offers a one to many ratio of IP addresses and that an
address is available to anyone who connects to the network. A step-by-step
description of the process is shown in Figures
through .
The Lab Activity
will help students set up a network computer as a DHCP client.
The next page
describes common problems in address resolution.
9.3 Obtaining an IP address
9.3.6 Problems in address resolution
This page will discuss address resolution problems.
One of the major
problems in networking is how to communicate with other network devices. In TCP/IP communications, a datagram on a
local-area network must contain both a destination MAC address and a
destination IP address. These addresses must be correct and match the
destination MAC and IP addresses of the host device. If it does not match, the
datagram will be discarded by the destination host. Communications within a LAN
segment require two addresses. There needs to be a way to automatically map IP
to MAC addresses. It would be too time consuming for the user to create the
maps manually. The TCP/IP suite has a protocol, called Address Resolution
Protocol (ARP), which can automatically obtain MAC addresses for local
transmission. Different issues are raised when data is sent outside of the
local area network.
Communications
between two LAN segments have an additional task. Both the IP and MAC addresses
are needed for both the destination host and the intermediate routing device.
TCP/IP has a variation on ARP called Proxy ARP that will provide the MAC
address of an intermediate device for transmission outside the LAN to another
network segment.
The next page
will describe Address Resolution Protocol (ARP).
9.3 Obtaining an IP address
9.3.7 Address Resolution Protocol (ARP)
This page
provides an explanation of how ARP works.
With TCP/IP
networking, a data packet must contain both a destination MAC address and a
destination IP address. If the packet is missing either one, the data will not
pass from Layer 3 to the upper layers. In this way, MAC addresses and IP
addresses act as checks and balances for each other. After devices determine
the IP addresses of the destination devices, they can add the destination MAC
addresses to the data packets.
Some devices will
keep tables that contain MAC addresses and IP addresses of other devices that
are connected to the same LAN. These are
called Address Resolution Protocol (ARP) tables. ARP tables are stored in RAM
memory, where the cached information is maintained automatically on each of the
devices. It is very unusual for a user to have to make an ARP table entry
manually. Each device on a network maintains its own ARP table. When a network
device wants to send data across the network, it uses information provided by
the ARP table.
When a source
determines the IP address for a destination, it then consults the ARP table in
order to locate the MAC address for the destination. If the source locates an
entry in its table, destination IP address to destination MAC address, it will
associate the IP address to the MAC address and then uses it to encapsulate the
data. The data packet is then sent out over the networking media to be picked
up by the destination device.
There are two
ways that devices can gather MAC addresses that they need to add to the
encapsulated data. One way is to monitor the traffic that occurs on the local
network segment. All stations on an
Ethernet network will analyze all traffic to determine if the data is for them.
Part of this process is to record the source IP and MAC address of the datagram
to an ARP table. So as data is transmitted on the network, the address pairs
populate the ARP table. Another way to get an address pair for data
transmission is to broadcast an ARP request.
The computer that
requires an IP and MAC address pair broadcasts an ARP request. All the other
devices on the local area network analyze this request. If one of the local
devices matches the IP address of the request, it sends back an ARP reply that
contains its IP-MAC pair. If the IP address is for the local area network and
the computer does not exist or is turned off, there is no response to the ARP
request. In this situation, the source device reports an error. If the request
is for a different IP network, there is another process that can be used.
Routers do not
forward broadcast packets. If the feature is turned on, a router performs a
proxy ARP. Proxy ARP is a variation of
the ARP protocol. In this variation, a router sends an ARP response with the
MAC address of the interface on which the request was received, to the
requesting host. The router responds with the MAC addresses for those requests
in which the IP address is not in the range of addresses of the local subnet.
Another method to
send data to the address of a device that is on another network segment is to
set up a default gateway. The default
gateway is a host option where the IP address of the router interface is stored
in the network configuration of the host. The source host compares the
destination IP address and its own IP address to determine if the two IP
addresses are located on the same segment. If the receiving host is not on the
same segment, the source host sends the data using the actual IP address of the
destination and the MAC address of the router. The MAC address for the router
was learned from the ARP table by using the IP address of that router.
If the default
gateway on the host or the proxy ARP feature on the router is not configured,
no traffic can leave the local area network. One or the other is required to
have a connection outside of the local area network.
The Lab Activity
will introduce the arp -a command.
The Interactive
Media Activity will help students understand the ARP process.
This page
concludes this lesson. The next page will summarize the main points from the
module.
Summary
This page
summarizes the topics discussed in this module.
The U.S.
Department of Defense (DoD) TCP/IP reference model has four layers: the
application layer, transport layer, Internet layer, and the network access
layer. The application layer handles high-level protocols, issues of
representation, encoding, and dialog control. The transport layer provides
transport services from the source host to the destination host. The purpose of
the Internet layer is to select the best path through the network for packet
transmissions. The network access layer is concerned with the physical link to
the network media.
Although some
layers of the TCP/IP reference model correspond to the seven layers of the OSI
model, there are differences. The TCP/IP model combines the presentation and
session layer into its application layer. The TCP/IP model combines the OSI
data link and physical layers into its network access layer.
Routers use the
IP address to move data packets between networks. IP addresses are thirty-two
bits long according to the current version IPv4 and are divided into four
octets of eight bits each. They operate at the network layer, Layer 3, of the
OSI model, which is the Internet layer of the TCP/IP model.
The IP address of
a host is a logical address and can be changed. The Media Access Control (MAC)
address of the workstation is a 48-bit physical address. This address is
usually burned into the network interface card (NIC) and cannot change unless
the NIC is replaced. TCP/IP communications within a LAN segment require both a
destination IP address and a destination MAC address for delivery. While IP
address are unique and routable throughout the Internet, when a packet arrives
at the destination network there needs to be a way to automatically map the IP
address to a MAC address. The TCP/IP suite has a protocol, called Address
Resolution Protocol (ARP), which can automatically obtain MAC addresses for
local transmission. A variation on ARP called Proxy ARP will provide the MAC
address of an intermediate device for transmission to another network segment.
There are five
classes of IP addresses, A through E. Only the first three classes are used
commercially. Depending on the class, the network and host part of the address
will use a different number of bits. The Class D address is used for multicast
groups. Class E addresses are reserved for research use only.
An IP address
that has binary zeros in all host bit positions is used to identify the network
itself. An address in which all of the host bits are set to one is the
broadcast address and is used for broadcasting packets to all the devices on a
network.
Public IP
addresses are unique. No two machines that connect to a public network can have
the same IP address because public IP addresses are global and standardized.
Private networks that are not connected to the Internet may use any host
addresses, as long as each host within the private network is unique. Three
blocks of IP addresses are reserved for private, internal use. These three
blocks consist of one Class A, a range of Class B addresses, and a range of
Class C addresses. Addresses that fall within these ranges are discarded by
routers and not routed on the Internet backbone.
Subnetting is
another means of dividing and identifying separate networks throughout the LAN.
Subnetting a network means to use the subnet mask to divide the network and
break a large network up into smaller, more efficient and manageable segments,
or subnets. Subnet addresses include the network portion, plus a subnet field
and a host field. The subnet field and the host field are created from the
original host portion for the entire network.
A more extendible
and scalable version of IP, IP Version 6 (IPv6), has been defined and
developed. IPv6 uses 128 bits rather than the 32 bits currently used in IPv4.
IPv6 uses hexadecimal numbers to represent the 128 bits. IPv6 is being
implemented in select networks and may eventually replace IPv4 as the dominant
Internet protocol.
IP addresses are
assigned to hosts in the following ways:
- Statically
– manually, by a network administrator
- Dynamically
– automatically, using reverse address resolution protocol, bootstrap
protocol (BOOTP), or Dynamic Host Configuration Protocol (DHCP)
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