Module Overview
10.1 Routed
Protocol
10.1.1 Routable
and routed protocols
10.1.2 IP as a
routed protocol
10.1.3 Packet
propagation and switching within a router
10.1.4
Connectionless and connection-oriented delivery
10.1.5 Anatomy of
an IP packet
10.2 IP Routing
Protocols
10.2.1 Routing
overview
10.2.2 Routing
versus switching
10.2.3 Routed
versus routing
10.2.4 Path
determination
10.2.5 Routing
tables
10.2.6 Routing
algorithms and metrics
10.2.7 IGP and
EGP
10.2.8 Link state
and distance vector
10.2.9 Routing
protocols
10.3 The
Mechanics of Subnetting
10.3.1 Classes of
network IP addresses
10.3.2
Introduction to and reason for subnetting
10.3.3 Establishing
the subnet mask address
10.3.4 Applying
the subnet mask
10.3.5 Subnetting
Class A and B networks
10.3.6
Calculating the resident subnetwork through ANDing
Module: Summary
Overview
Internet Protocol
(IP) is the main routed protocol of the Internet. IP addresses are used to
route packets from a source to a destination through the best available path.
The propagation of packets, encapsulation changes, and connection-oriented and
connectionless protocols are also critical to ensure that data is properly
transmitted to its destination. This module will provide an overview for each.
The difference
between routing and routed protocols is a common source of confusion. The two
words sound similar but are quite different. Routers use routing protocols to
build tables that are used to determine the best path to a host on the
Internet.
Not all
organizations can fit into the three class system of A, B, and C addresses.
Flexibility exists within the class system through subnets. Subnets allow network
administrators to determine the size of the network they will work with. After
they decide how to segment their networks, they can use subnet masks to
determine the location of each device on a network.
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:
- Describe routed protocols
- List the steps of data
encapsulation in an internetwork as data is routed to Layer 3 devices
- Describe connectionless and
connection-oriented delivery
- Name the IP packet fields
- Describe how data is routed
- Compare and contrast different
types of routing protocols
- List and describe several
metrics used by routing protocols
- List several uses for
subnetting
- Determine the subnet mask for a
given situation
- Use a subnet mask to determine
the subnet ID
10.1
Routed Protocol
10.1.1 Routable and routed protocols
This page will
define routed and routable protocols.
A protocol is a
set of rules that determines how computers communicate with each other across
networks. Computers exchange data messages to communicate with each other. To
accept and act on these messages, computers must have sets of rules that
determine how a message is interpreted. Examples include messages used to
establish a connection to a remote machine, e-mail messages, and files
transferred over a network.
A protocol
describes the following:
- The required format of a
message
- The way that computers must
exchange messages for specific activities
A routed protocol
allows the router to forward data between nodes on different networks. A routable protocol must provide the ability
to assign a network number and a host number to each device. Some protocols,
such as IPX, require only a network number. These protocols use the MAC address
of the host for the host number. Other protocols, such as IP, require an
address with a network portion and a host portion. These protocols also require
a network mask to differentiate the two numbers. The network address is
obtained by ANDing the address with the network mask.
The reason that a
network mask is used is to allow groups of sequential IP addresses to be
treated as a single unit. If this
grouping were not allowed, each host would have to be mapped individually for
routing. This would be impossible, because according to the Internet Software
Consortium there are approximately 250,000,000 hosts on the Internet.
The next page
will discuss IP.
10.1 Routed
Protocol
10.1.2 IP as a
routed protocol
This page
describes the features and functions of IP.
IP is the most
widely used implementation of a hierarchical network-addressing scheme. IP is a
connectionless, unreliable, best-effort delivery protocol. The term connectionless
means that no dedicated circuit connection is established prior to
transmission. IP determines the most efficient route for data based on the
routing protocol. The terms unreliable and best-effort do not imply that the
system is unreliable and does not work well. They indicate that IP does not
verify that data sent on the network reaches its destination. If required,
verification is handled by upper layer protocols.
As information
flows down the layers of the OSI model, the data is processed at each
layer. At the network layer, the data is
encapsulated into packets. These packets are also known as datagrams. IP determines the contents of the IP packet
header, which includes address information. However, it is not concerned with
the actual data. IP accepts whatever data is passed down to it from the upper
layers.
The next page
examines how a packet travels through a network.
10.1
Routed Protocol
10.1.3 Packet propagation and switching within a
router
This page will
explain the process that occurs as a packet moves through a network.
As a packet
travels through an internetwork to its final destination, the Layer 2 frame
headers and trailers are removed and replaced at every Layer 3 device. This is because Layer 2 data units, or
frames, are for local addressing. Layer 3 data units, or packets, are for
end-to-end addressing.
Layer 2 Ethernet
frames are designed to operate within a broadcast domain with the MAC address
that is burned into the physical device. Other Layer 2 frame types include PPP
serial links and Frame Relay connections, which use different Layer 2
addressing schemes. Regardless of the type of Layer 2 addressing used, frames
are designed to operate within a Layer 2 broadcast domain. When the data is
sent to a Layer 3 device the Layer 2 information changes.
As a frame is received at a router interface, the
destination MAC address is extracted. The address is checked to see if the
frame is directly addressed to the router interface, or if it is a broadcast.
In either situation, the frame is accepted. Otherwise, the frame is discarded
since it is destined for another device on the collision domain.
The CRC
information is extracted from the frame trailer of an accepted frame. The CRC
is calculated to verify that the frame data is without error.
If the check
fails, the frame is discarded. If the check is valid, the frame header and
trailer are removed and the packet is passed up to Layer 3. The packet is then
checked to see if it is actually destined for the router, or if it is to be
routed to another device in the internetwork. If the destination IP address
matches one of the router ports, the Layer 3 header is removed and the data is
passed up to the Layer 4. If the packet is to be routed, the destination IP address
will be compared to the routing table. If a match is found or there is a
default route, the packet will be sent to the interface specified in the
matched routing table statement. When the packet is switched to the outgoing
interface, a new CRC value is added as a frame trailer, and the proper frame
header is added to the packet. The frame is then transmitted to the next
broadcast domain on its trip to the final destination.
The next page
will describe two types of delivery services.
10.1 Routed Protocol
10.1.4 Connectionless and connection-oriented
delivery
This page will
introduce two types of delivery systems, which are connectionless and
connection-oriented.
These two
services provide the actual end-to-end delivery of data in an internetwork.
Most network
services use a connectionless delivery system.
Different packets may take different paths to get through the network.
The packets are reassembled after they arrive at the destination. In a
connectionless system, the destination is not contacted before a packet is
sent. A good comparison for a connectionless system is a postal system. The
recipient is not contacted to see if they will accept the letter before it is
sent. Also, the sender does not know if the letter arrived at the destination.
In
connection-oriented systems, a connection is established between the sender and
the recipient before any data is transferred.
An example of a connection-oriented network is the telephone system. The
caller places the call, a connection is established, and then communication
occurs.
Connectionless
network processes are often referred to as packet-switched processes. As the
packets pass from source to destination, packets can switch to different paths,
and possibly arrive out of order. Each packet contains the instructions, such
as destination address and order in a message, that coordinate its arrival with
other associated packets. Packets are reassembled into the proper sequence at
the destination. Devices make the path determination for each packet based on a
variety of criteria. Some of the criteria, such as available bandwidth, may
differ from packet to packet.
Connection-oriented
network processes are often referred to as circuit-switched processes. A
dedicated connection between the originator and the recipient is first
established, and then data transfer begins. All packets travel sequentially
across the same physical or virtual circuit in one continuous stream.
The Internet is a
gigantic, connectionless network in which the majority of packet deliveries are
handled by IP. TCP adds Layer 4 connection-oriented reliability services to
connectionless IP communications.
The next page
will discuss the IP header.
10.1 Routed Protocol
10.1.5 Anatomy of an IP packet
IP packets
consist of the data from upper layers plus an IP header. This page will discuss
the information contained in the IP header:
- Version
– Specifies the format of the IP packet header. The 4-bit version field
contains the number 4 if it is an IPv4 packet and 6 if it is an IPv6
packet. However, this field is not used to distinguish between IPv4 and
IPv6 packets. The protocol type field present in the Layer 2 envelope is
used for that.
- IP header length (HLEN)
– Indicates the datagram header length in 32-bit words. This is the total
length of all header information and includes the two variable-length
header fields.
- Type of service (ToS)
– 8 bits that specify the level of importance that has been assigned by a
particular upper-layer protocol.
- Total length
– 16 bits that specify the length of the entire packet in bytes. This
includes the data and header. To get the length of the data payload
subtract the HLEN from the total length.
- Identification –
16 bits that identify the current datagram. This is the sequence number.
- Flags
– A 3-bit field in which the two low-order bits control fragmentation. One
bit specifies if the packet can be fragmented and the other indicates if
the packet is the last fragment in a series of fragmented packets.
- Fragment offset
– 13 bits that are used to help piece together datagram fragments. This
field allows the previous field to end on a 16-bit boundary.
- Time to Live (TTL)
– A field that specifies the number of hops a packet may travel. This
number is decreased by one as the packet travels through a router. When
the counter reaches zero the packet is discarded. This prevents packets
from looping endlessly.
- Protocol –
8 bits that indicate which upper-layer protocol such as TCP or UDP
receives incoming packets after the IP processes have been completed.
- Header checksum
– 16 bits that help ensure IP header integrity.
- Source address
– 32 bits that specify the IP address of the node from which the packet
was sent.
- Destination address
– 32 bits that specify the IP address of the node to which the data is
sent.
- Options
– Allows IP to support various options such as security. The length of
this field varies.
- Padding
– Extra zeros are added to this field to ensure that the IP header is
always a multiple of 32 bits.
- Data
– Contains upper-layer information and has a variable length of up to 64
bits.
While the IP
source and destination addresses are important, the other header fields have
made IP very flexible. The header fields list the source and destination
address information of the packet and often indicate the length of the message
data. The information for routing the message is also contained in IP headers,
which can get long and complex
This page
concludes this lesson. The next lesson will focus on IP routing protocols. The
first page provides a routing overview.
10.2 IP Routing Protocols
10.2.1 Routing overview
This page will
discuss routing and the two main functions of a router.
Routing is an OSI
Layer 3 function. Routing is a
hierarchical organizational scheme that allows individual addresses to be
grouped together. These individual addresses are treated as a single unit until
the destination address is needed for final delivery of the data. Routing finds the most efficient path from
one device to another. The primary device that performs the routing process is
the router.
The following are
the two key functions of a router:
- Routers must maintain routing
tables and make sure other routers know of changes in the network
topology. They use routing protocols to communicate network information
with other routers.
- When packets arrive at an
interface, the router must use the routing table to determine where to
send them. The router switches the packets to the appropriate interface,
adds the frame information for the interface, and then transmits the
frame.
A router is a
network layer device that uses one or more routing metrics to determine the
optimal path along which network traffic should be forwarded. Routing metrics
are values that are used to determine the advantage of one route over
another. Routing protocols use various
combinations of metrics to determine the best path for data.
Routers
interconnect network segments or entire networks. Routers pass data frames
between networks based on Layer 3 information. Routers make logical decisions
about the best path for the delivery of data. Routers then direct packets to
the appropriate output port to be encapsulated for transmission. Stages of the encapsulation and
de-encapsulation process occur each time a packet transfers through a router.
The router must de-encapsulate the Layer 2 data frame to access and examine the
Layer 3 address. As shown in Figure , the complete process of sending data from
one device to another involves encapsulation and de-encapsulation on all seven
OSI layers. The encapsulation process breaks up the data stream into segments,
adds the appropriate headers and trailers, and then transmits the data. The
de-encapsulation process removes the headers and trailers and then recombines the
data into a seamless stream.
This course
focuses on the most common routable protocol, which is IP. Other examples of
routable protocols include IPX/SPX and AppleTalk. These protocols provide Layer
3 support. Non-routable protocols do not provide Layer 3 support. The most
common non-routable protocol is NetBEUI. NetBEUI is a small, fast, and
efficient protocol that is limited to frame delivery within one segment.
The next page will compare routing and switching.
10.2 IP Routing Protocols
10.2.2 Routing versus switching
This page will
compare and contrast routing and switching.
Routers and switches may seem to perform the same function. The primary
difference is that switches operate at Layer 2 of the OSI model and routers
operate at Layer 3. This distinction indicates that routers and switches use
different information to send data from a source to a destination.
The relationship
between switching and routing can be compared to local and long-distance
telephone calls. When a telephone call is made to a number within the same area
code, a local switch handles the call. The local switch can only keep track of
its local numbers. The local switch cannot handle all the telephone numbers in
the world. When the switch receives a request for a call outside of its area
code, it switches the call to a higher-level switch that recognizes area codes.
The higher-level switch then switches the call so that it eventually gets to
the local switch for the area code dialed.
The router
performs a function similar to that of the higher-level switch in the telephone
example. Figure shows the ARP tables for
Layer 2 MAC addresses and routing tables for Layer 3 IP addresses. Each
computer and router interface maintains an ARP table for Layer 2 communication.
The ARP table is only effective for the broadcast domain to which it is
connected. The router also maintains a routing table that allows it to route
data outside of the broadcast domain. Each ARP table entry contains an IP-MAC
address pair.
The Layer 2
switch builds its forwarding table using MAC addresses. When a host has data
for a non-local IP address, it sends the frame to the closest router. This
router is also known as its default gateway. The host uses the MAC address of
the router as the destination MAC address.
A switch
interconnects segments that belong to the same logical network or
subnetwork. For non-local hosts, the
switch forwards the frame to the router based on the destination MAC address.
The router examines the Layer 3 destination address of the packet to make the
forwarding decision. Host X knows the IP address of the router because the IP
configuration of the host contains the IP address of the default gateway.
Just as a switch
keeps a table of known MAC addresses, the router keeps a table of IP addresses
known as a routing table. MAC addresses
are not logically organized. IP addresses are organized in a hierarchy. A
switch can handle a limited number of unorganized MAC addresses since it only
has to search its table for addresses within its segment. Routers require an
organized address system that can group similar addresses together and treat
them as a single network unit until the data reaches the destination segment.
If IP addresses
were not organized, the Internet would not work. This could be compared to a
library that contained millions of individual pages of printed material in a
large pile. This material is useless because it is impossible to locate an
individual document. If the pages are identified and organized into books and
each book is listed in a book index, it will be a lot easier to locate and use
the data.
Another
difference between switched and routed networks is switched networks do not
block broadcasts. As a result, switches
can be overwhelmed by broadcast storms. Routers block LAN broadcasts, so a
broadcast storm only affects the broadcast domain from which it originated.
Since routers block broadcasts, they also provide a higher level of security
and bandwidth control than switches.
The Interactive
Media Activity will help students learn the differences between routing and
switching.
The next page
will compare routing and routed protocols.
10.2 IP Routing Protocols
10.2.3 Routed versus routing
This page
explains the differences between routing protocols and routed protocols.
Routed or
routable protocols are used at the network layer to transfer data from one host
to another across a router. Routed protocols transport data across a network.
Routing protocols allow routers to choose the best path for data from a source
to a destination.
Some functions of
a routed protocol are as follows:
- Includes any network protocol
suite that provides enough information in its network layer address to
allow a router to forward it to the next device and ultimately to its
destination
- Defines the format and use of
the fields within a packet
The Internet Protocol (IP) and Novell Internetwork Packet
Exchange (IPX) are examples of routed protocols. Other examples include DECnet,
AppleTalk, Banyan VINES, and Xerox Network Systems (XNS).
Routers use
routing protocols to exchange routing tables and share routing information. In
other words, routing protocols enable routers to route routed protocols.
Some functions of
a routing protocol are as follows:
- Provides processes used to
share route information
- Allows routers to communicate
with other routers to update and maintain the routing tables
Examples of routing protocols that support the IP routed
protocol include RIP, IGRP, OSPF, BGP, and EIGRP.
The Interactive
Media Activity will help students learn the differences between routed and
routing protocols.
The next page
will discuss path determination.
10.2 IP Routing Protocols
10.2.4 Path determination
This page will
explain how path determination occurs.
Path
determination occurs at the network layer.
A router uses path determination to compare a destination address to the
available routes in its routing table and select the best path. The routers
learn of these available routes through static routing or dynamic routing.
Routes configured manually by the network administrator are static routes.
Routes learned by others routers using a routing protocol are dynamic routes.
The router uses
path determination to decide which port to send a packet out of to reach its
destination. This process is also
referred to as routing the packet. Each router that the packet encounters along
the way is called a hop. The hop count is the distanced traveled. Path
determination can be compared to a person who drives from one location in a
city to another. The driver has a map that shows which streets lead to the
destination, just as a router has a routing table. The driver travels from one
intersection to another just as a packet travels from one router to another in
each hop. At any intersection, the driver can choose to turn left, turn right,
or go straight ahead. This is similar to how a router chooses the outbound port
through which a packet is sent.
The decisions of
a driver are influenced by factors such as traffic, the speed limit, the number
of lanes, tolls, and whether or not a road is frequently closed. Sometimes it
is faster to take a longer route on a smaller, less crowded back street instead
of a highway with a lot of traffic. Similarly, routers can make decisions based
on the load, bandwidth, delay, cost, and reliability of a network link.
The following
process is used to determine the path for every packet that is routed:
- The router compares the IP
address of the packet that it received to the IP tables that it has.
- The destination address is
obtained from the packet.
- The mask of the first entry in
the routing table is applied to the destination address.
- The masked destination and the
routing table entry are compared.
- If there is a match, the packet
is forwarded to the port that is associated with that table entry.
- If there is not a match, the
next entry in the table is checked.
- If the packet does not match
any entries in the table, the router checks to see if a default route has been
set.
- If a default route has been
set, the packet is forwarded to the associated port. A default route is a
route that is configured by the network administrator as the route to use
if there are no matches in the routing table.
- If there is no default route,
the packet is discarded. A message is often sent back to the device that
sent the data to indicate that the destination was unreachable.
The Interactive Media Activity will help students
understand path determination.
The next page
will explain how routing protocols build and maintain routing tables.
10.2 IP
Routing Protocols
10.2.5 Routing tables
This page will describe the functions of a routing table.
Routers use
routing protocols to build and maintain routing tables that contain route
information. This aids in the process of path determination. Routing protocols
fill routing tables with a variety of route information. This information
varies based on the routing protocol used. Routing tables contain the
information necessary to forward data packets across connected networks. Layer
3 devices interconnect broadcast domains or LANs. A hierarchical address scheme
is required for data transfers.
Routers keep
track of the following information in their routing tables:
- Protocol type
– Identifies the type of routing protocol that created each entry.
- Next-hop associations
– Tell a router that a destination is either directly connected to the
router or that it can be reached through another router called the
next-hop on the way to the destination. When a router receives a packet,
it checks the destination address and attempts to match this address with
a routing table entry.
- Routing metric
– Different routing protocols use different routing metrics. Routing
metrics are used to determine the desirability of a route. For example,
RIP uses hop count as its only routing metric. IGRP uses bandwidth, load,
delay, and reliability metrics to create a composite metric value.
- Outbound interfaces
– The interface that the data must be sent out of to reach the final
destination.
Routers
communicate with one another to maintain their routing tables through the
transmission of routing update messages. Some routing protocols transmit update
messages periodically. Other protocols send them only when there are changes in
the network topology. Some protocols transmit the entire routing table in each
update message and some transmit only routes that have changed. Routers analyze
the routing updates from directly-connected routers to build and maintain their
routing tables.
The next page
will explain routing algorithms and metrics.
10.2 IP Routing Protocols
10.2.6 Routing algorithms and metrics
This page will
define algorithms and metrics as they relate to routers.
An algorithm is a
detailed solution to a problem. Different routing protocols use different
algorithms to choose the port to which a packet should be sent. Routing
algorithms depend on metrics to make these decisions.
Routing protocols
often have one or more of the following design goals:
- Optimization
– This is the capability of a routing algorithm to select the best route.
The route will depend on the metrics and metric weights used in the
calculation. For example, one algorithm may use both hop count and delay
metrics, but may consider delay metrics as more important in the
calculation.
- Simplicity and low overhead
– The simpler the algorithm, the more efficiently it will be processed by
the CPU and memory in the router. This is important so that the network
can scale to large proportions, such as the Internet.
- Robustness and stability
– A routing algorithm should perform correctly when confronted by unusual
or unforeseen circumstances, such as hardware failures, high load
conditions, and implementation errors.
- Flexibility –
A routing algorithm should quickly adapt to a variety of network changes.
These changes include router availability, router memory, changes in
bandwidth, and network delay.
- Rapid convergence
– Convergence is the process of agreement by all routers on available
routes. When a network event causes changes in router availability,
updates are needed to reestablish network connectivity. Routing algorithms
that converge slowly can cause data to be undeliverable.
Routing
algorithms use different metrics to determine the best route. Each routing algorithm interprets what is
best in its own way. A routing algorithm generates a number called a metric
value for each path through a network. Sophisticated routing algorithms base
route selection on multiple metrics that are combined in a composite metric
value. Typically, smaller metric values indicate preferred paths.
Metrics can be
based on a single characteristic of a path, or can be calculated based on
several characteristics. The following metrics are most commonly used by
routing protocols:
- Bandwidth
– Bandwidth is the data capacity of a
link. Normally, a 10-Mbps Ethernet link is preferable to a 64-kbps leased
line.
- Delay
– Delay is the length of time required
to move a packet along each link from a source to a destination. Delay
depends on the bandwidth of intermediate links, the amount of data that
can be temporarily stored at each router, network congestion, and physical
distance.
- Load
– Load is the amount of activity on a
network resource such as a router or a link.
- Reliability
– Reliability is usually a reference to the error rate of each network
link.
- Hop
count – Hop count is the number of routers
that a packet must travel through before reaching its destination. Each
router is equal to one hop. A hop count of four indicates that data would
have to pass through four routers to reach its destination. If multiple
paths are available to a destination, the path with the least number of
hops is preferred.
- Ticks
– The delay on a data link using IBM PC clock ticks. One tick is
approximately 1/18 second.
- Cost
– Cost is an arbitrary value, usually
based on bandwidth, monetary expense, or other measurement, that is
assigned by a network administrator.
The next page
will discuss two types of routing protocols.
10.2 IP
Routing Protocols
10.2.7
IGP and EGP
This page will
introduce two types of routing protocols.
An autonomous
system is a network or set of networks under common administrative control,
such as the cisco.com domain. An autonomous system consists of routers that
present a consistent view of routing to the external world.
Two families of
routing protocols are Interior Gateway Protocols (IGPs) and Exterior Gateway
Protocols (EGPs).
IGPs route data
within an autonomous system:
- RIP and RIPv2
- IGRP
- EIGRP
- OSPF
- Intermediate
System-to-Intermediate System (IS-IS) protocol
EGPs route data
between autonomous systems. An example of an EGP is BGP.
The next page
will define link-state and distance vector protocols.
10.2 IP Routing Protocols
10.2.8 Link state and distance vector
Routing protocols
can be classified as either IGPs or EGPs. Which type is used depends on whether
a group of routers is under a single administration or not. IGPs can be further
categorized as either distance-vector or link-state protocols. This page describes distance-vector and
link-state routing and explains when each type of routing protocol is used.
The
distance-vector routing approach determines the distance and direction, vector,
to any link in the internetwork. The distance may be the hop count to the link.
Routers using distance-vector algorithms send all or part of their routing
table entries to adjacent routers on a periodic basis. This happens even if
there are no changes in the network. By receiving a routing update, a router
can verify all the known routes and make changes to its routing table. This
process is also known as “routing by rumor”. The understanding that a router
has of the network is based upon the perspective of the adjacent router of the
network topology.
Examples of
distance-vector protocols include the following:
- Routing Information Protocol
(RIP) – The most common IGP in the
Internet, RIP uses hop count as its only routing metric.
- Interior Gateway Routing
Protocol (IGRP) – This IGP was developed by
Cisco to address issues associated with routing in large, heterogeneous
networks.
- Enhanced IGRP (EIGRP)
– This Cisco-proprietary IGP includes many of the features of a link-state
routing protocol. Because of this, it has been called a balanced-hybrid
protocol, but it is really an advanced distance-vector routing protocol.
Link-state
routing protocols were designed to overcome limitations of distance vector
routing protocols. Link-state routing protocols respond quickly to network
changes sending trigger updates only when a network change has occurred.
Link-state routing protocols send periodic updates, known as link-state
refreshes, at longer time intervals, such as every 30 minutes.
When a route or
link changes, the device that detected the change creates a link-state
advertisement (LSA) concerning that link. The LSA is then transmitted to all
neighboring devices. Each routing device takes a copy of the LSA, updates its
link-state database, and forwards the LSA to all neighboring devices. This
flooding of LSAs is required to ensure that all routing devices create
databases that accurately reflect the network topology before updating their
routing tables.
Link-state
algorithms typically use their databases to create routing table entries that
prefer the shortest path. Examples of link-state protocols include Open
Shortest Path First (OSPF) and Intermediate System-to-Intermediate System
(IS-IS).
The Interactive
Media Activity will identify the differences between link-state and distance
vector routing protocols.
The next page
will discuss routing protocols.
10.2 IP
Routing Protocols
10.2.9 Routing protocols
This page will
describe different types of router protocols.
RIP is a distance
vector routing protocol that uses hop count as its metric to determine the
direction and distance to any link in the internetwork. If there are multiple
paths to a destination, RIP selects the path with the least number of hops.
However, because hop count is the only routing metric used by RIP, it does not
always select the fastest path to a destination. Also, RIP cannot route a
packet beyond 15 hops. RIP Version 1 (RIPv1) requires that all devices in the
network use the same subnet mask, because it does not include subnet mask
information in routing updates. This is also known as classful routing.
RIP Version 2
(RIPv2) provides prefix routing, and does send subnet mask information in
routing updates. This is also known as classless routing. With classless routing
protocols, different subnets within the same network can have different subnet
masks. The use of different subnet masks within the same network is referred to
as variable-length subnet masking (VLSM).
IGRP is a
distance-vector routing protocol developed by Cisco. IGRP was developed
specifically to address problems associated with routing in large networks that
were beyond the range of protocols such as RIP. IGRP can select the fastest
available path based on delay, bandwidth, load, and reliability. IGRP also has
a much higher maximum hop count limit than RIP. IGRP uses only classful
routing.
OSPF is a
link-state routing protocol developed by the Internet Engineering Task Force
(IETF) in 1988. OSPF was written to address the needs of large, scalable internetworks
that RIP could not.
Intermediate
System-to-Intermediate System (IS-IS) is a link-state routing protocol used for
routed protocols other than IP. Integrated IS-IS is an expanded implementation
of IS-IS that supports multiple routed protocols including IP.
Like IGRP, EIGRP
is a proprietary Cisco protocol. EIGRP is an advanced version of IGRP.
Specifically, EIGRP provides superior operating efficiency such as fast
convergence and low overhead bandwidth. EIGRP is an advanced distance-vector
protocol that also uses some link-state protocol functions. Therefore, EIGRP is
sometimes categorized as a hybrid routing protocol.
Border Gateway
Protocol (BGP) is an example of an External Gateway Protocol (EGP). BGP
exchanges routing information between autonomous systems while guaranteeing
loop-free path selection. BGP is the principal route advertising protocol used
by major companies and ISPs on the Internet. BGP4 is the first version of BGP
that supports classless interdomain routing (CIDR) and route aggregation.
Unlike common Internal Gateway Protocols (IGPs), such as RIP, OSPF, and EIGRP,
BGP does not use metrics like hop count, bandwidth, or delay. Instead, BGP
makes routing decisions based on network policies, or rules using various BGP
path attributes.
The Lab Activity
will help students understand the price of a small router.
This page
concludes this lesson. The next lesson will focus on the mechanics of
subnetting. The first page covers the different classes of IP addresses.
10.3 The Mechanics of Subnetting
10.3.1 Classes of network IP addresses
This page will
review the classes of IP addresses. The combined classes of IP addresses offer
a range from 256 to 16.8 million hosts.
To efficiently
manage a limited supply of IP addresses, all classes can be subdivided into
smaller subnetworks. Figure provides an
overview of the division between networks and hosts.
The next page
will explain why subnetting is important.
10.3 The
Mechanics of Subnetting
10.3.2
Introduction to and reason for subnetting
This page will
describe how subnetting works and why it is important.
To create the
subnetwork structure, host bits must be reassigned as network bits. This is
often referred to as ‘borrowing’ bits. However, a more accurate term would be
‘lending’ bits. The starting point for this process is always the leftmost host
bit, the one closest to the last network octet.
Subnet addresses
include the Class A, Class B, and Class C network portion, plus a subnet field
and a host field. The subnet field and the host field are created from the
original host portion of the major IP address. This is done by re-assigning
bits from the host portion to the original network portion of the address. - The
ability to divide the original host portion of the address into the new subnet
and host fields provides addressing flexibility for the network administrator.
In addition to
the need for manageability, subnetting enables the network administrator to
provide broadcast containment and low-level security on the LAN. Subnetting
provides some security since access to other subnets is only available through
the services of a router. Further, access security may be provided through the
use of access lists. These lists can permit or deny access to a subnet, based
on a variety of criteria, thereby providing more security. Access lists will be
studied later in the curriculum. Some owners of Class A and B networks have
also discovered that subnetting creates a revenue source for the organization
through the leasing or sale of previously unused IP addresses.
Subnetting is an
internal function of a network. From the outside, a LAN is seen as a single
network with no details of the internal network structure. This view of the
network keeps the routing tables small and efficient. Given a local node
address of 147.10.43.14 on subnet 147.10.43.0, the world outside the LAN sees
only the advertised major network number of 147.10.0.0. The reason for this is
that the local subnet address of 147.10.43.0 is only valid within the LAN where
subnetting is applied.
The next page
will discuss subnet masks.
10.3 The
Mechanics of Subnetting
10.3.3
Establishing the subnet mask address
This page
provides detailed information about subnet masks and how they are established
on a network.
Selecting the
number of bits to use in the subnet process will depend on the maximum number
of hosts required per subnet. An understanding of basic binary math and the
position value of the bits in each octet is necessary when calculating the
number of subnetworks and hosts created when bits were borrowed.
The last two bits
in the last octet, regardless of the IP address class, may never be assigned to
the subnetwork. These bits are referred to as the last two significant bits.
Use of all the available bits to create subnets, except these last two, will
result in subnets with only two usable hosts. This is a practical address
conservation method for addressing serial router links. However, for a working
LAN this would result in prohibitive equipment costs.
The subnet mask
gives the router the information required to determine in which network and
subnet a particular host resides. The
subnet mask is created by using binary ones in the network bit positions. The
subnet bits are determined by adding the position value of the bits that were
borrowed. If three bits were borrowed, the mask for a Class C address would be
255.255.255.224. This mask may also be
represented, in the slash format, as /27. The number following the slash is the
total number of bits that were used for the network and subnetwork portion.
To determine the
number of bits to be used, the network designer needs to calculate how many
hosts the largest subnetwork requires and the number of subnetworks needed. As
an example, the network requires six subnetworks of 25 hosts each. A shortcut
to determine how many bits to reassign is by using the subnetting chart. By consulting the row titled ”Usable
Subnets”, the chart indicates that for six usable subnets three additional bits
are required in the subnet mask. The chart also shows that this creates 30
usable hosts per subnet, which will satisfy the requirements of this scheme.
The difference between usable hosts and total hosts is a result of using the
first available address as the ID and the last available address as the
broadcast for each subnetwork. Borrowing the appropriate number of bits to
accommodate required subnetworks and hosts per subnetwork can be a balancing
act and may result in unused host addresses in multiple subnetworks. The
ability to use these addresses is not provided with classful routing. However,
classless routing, which will be covered later in the course can recover many
of these lost addresses.
The method that
was used to create the subnet chart can be used to solve all subnetting
problems. This method uses the following
formula:
Number of usable
subnets = two to the power of the assigned subnet bits or borrowed bits, minus
two. The minus two is for the reserved addresses of network ID and network
broadcast.
2 power of
borrowed bits - 2 = usable subnets
2 3 - 2 = 6
Number of usable
hosts = two to the power of the bits remaining, minus two (reserved addresses
for subnet id and subnet broadcast).
2 power of
remaining host bits - 2 = usable hosts
2 5 - 2 = 30
The next page
will explain how a subnet mask is applied.
10.3 The Mechanics of Subnetting
10.3.4 Applying the subnet mask
This page will teach students how to apply a subnet mask.
Once the subnet
mask has been established it then can be used to create the subnet scheme. The
chart in Figure is an example of the
subnets and addresses created by assigning three bits to the subnet field. This
will create eight subnets with 32 hosts per subnet. Start with zero (0) when
numbering subnets. The first subnet is always referenced as the zero subnet.
When filling in
the subnet chart three of the fields are automatic, others require some
calculation. The subnetwork ID of subnet zero is the same as the major network
number, in this case 192.168.10.0. The broadcast ID for the whole network is
the largest number possible, in this case 192.168.10.255. The third number that
is given is the subnetwork ID for subnet number seven. This number is the three
network octets with the subnet mask number inserted in the fourth octet
position. Three bits were assigned to the subnet field with a cumulative value
of 224. The ID for subnet seven is
192.168.10.224. By inserting these numbers, checkpoints have been established
that will verify the accuracy when the chart is completed.
When consulting
the subnetting chart or using the formula, the three bits assigned to the
subnet field will result in 32 total hosts assigned to each subnet. This information provides the step count for
each subnetwork ID. Adding 32 to each preceding number, starting with subnet
zero, the ID for each subnet is established.
Notice that the subnet ID has all binary 0s in the host portion.
The broadcast
field is the last number in each subnetwork, and has all binary ones in the
host portion. This address has the ability to broadcast only to the members of
a single subnet. Since the subnetwork ID
for subnet zero is 192.168.10.0 and there are 32 total hosts the broadcast ID
would be 192.168.10.31. Starting at zero the 32nd sequential number is 31. It
is important to remember that zero (0) is a real number in the world of
networking.
The balance of
the broadcast ID column can be filled in using the same process that was used
in the subnetwork ID column. Simply add 32 to the preceding broadcast ID of the
subnet. Another option is to start at the bottom of this column and work up to
the top by subtracting one from the preceding subnetwork ID.
The next page
will discuss subnetting for Class A, B, and C networks.
10.3 The Mechanics of Subnetting
10.3.5 Subnetting Class A and B networks
This page will
describe the process used to subnet Class A, B, and C networks.
The Class A and B
subnetting procedure is identical to the process for Class C, except there may
be significantly more bits involved. The available bits for assignment to the
subnet field in a Class A address is 22 bits while a Class B address has 14
bits.
Assigning 12 bits
of a Class B address to the subnet field creates a subnet mask of
255.255.255.240 or /28. All eight bits were assigned in the third octet
resulting in 255, the total value of all eight bits. Four bits were assigned in
the fourth octet resulting in 240. Recall that the slash mask is the sum total
of all bits assigned to the subnet field plus the fixed network bits.
Assigning 20 bits
of a Class A address to the subnet field creates a subnet mask of
255.255.255.240 or /28. All eight bits of the second and third octets were
assigned to the subnet field and four bits from the fourth octet.
In this
situation, it is apparent that the subnet mask for the Class A and Class B
addresses appear identical. Unless the mask is related to a network address it
is not possible to decipher how many bits were assigned to the subnet field.
Whichever class
of address needs to be subnetted, the following rules are the same:
Total subnets = 2
to the power of the bits borrowed
Total hosts = 2
to the power of the bits remaining
Usable subnets =
2 to the power of the bits borrowed minus 2
Usable hosts = 2
to the power of the bits remaining minus 2
The next page
will discuss logical ANDing.
10.3 The
Mechanics of Subnetting
10.3.6
Calculating the resident subnetwork through ANDing
This page will
explain the concept of ANDing.
Routers use
subnet masks to determine the home subnetwork for individual nodes. This
process is referred to as logical ANDing. ANDing is a binary process by which
the router calculates the subnetwork ID for an incoming packet. ANDing is similar to multiplication.
This process is
handled at the binary level. Therefore, it is necessary to view the IP address
and mask in binary. The IP address and
the subnetwork address are ANDed with the result being the subnetwork ID. The
router then uses that information to forward the packet across the correct
interface.
Subnetting is a
learned skill. It will take many hours performing practice exercises to gain a
development of flexible and workable schemes. A variety of subnet calculators
are available on the web. However, a network administrator must know how to
manually calculate subnets in order to effectively design the network scheme
and assure the validity of the results from a subnet calculator. The subnet
calculator will not provide the initial scheme, only the final addressing.
Also, no calculators, of any kind, are permitted during the certification exam.
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.
IP is referred to
as a connectionless protocol because no dedicated circuit connection is
established between source and destination prior to transmission, IP is
referred to as unreliable because does not verify that the data reached its
destination. If verification of delivery is required then a combination of IP
and a connection-oriented transport protocol such as TCP is required. If
verification of error-free delivery is not required IP can be used in
combination with a connectionless transport protocol such as UDP.
Connectionless network processes are often referred to as packet switched
processes. Connection-oriented network processes are often referred to as
circuit switched processes.
Protocols at each
layer of the OSI model add control information to the data as it moves through
the network. Because this information is added at the beginning and end of the
data, this process is referred to as encapsulating the data. Layer 3 adds
network, or logical, address information to the data and Layer 2 adds local, or
physical, address information.
Layer 3 routing
and Layer 2 switching are used to direct and deliver data throughout the
network. Initially, the router receives a Layer 2 frame with a Layer 3 packet
encapsulated within it. The router must strip off the Layer 2 frame and examine
the Layer 3 packet. If the packet is destined for local delivery the router
must encapsulate it in a new frame with the correct local MAC address as the
destination. If the data must be forwarded to another broadcast domain, the
router must encapsulate the Layer 3 packet in a new Layer 2 frame that contains
the MAC address of the next internetworking device. In this way a frame is
transmitted through networks from broadcast domain to broadcast domain and
eventually delivered to the correct host.
Routed protocols,
such as IP, transport data across a network. Routing protocols allow routers to
choose the best path for data from source to destination. These routes can be
either static routes, which are entered manually, or dynamic routes, which are
learned through routing protocols. When dynamic routing protocols are used,
routers use routing update messages to communicate with one another and
maintain their routing tables. Routing algorithms use metrics to process
routing updates and populate the routing table with the best routes.
Convergence describes the speed at which all routers agree on a change in the
network.
Interior gateway
protocols (IGP) are routing protocols that route data within autonomous
systems, while exterior gateway protocols (EGP) route data between autonomous
systems. IGPs can be further categorized as either distance-vector or
link-state protocols. Routers using distance-vector routing protocols
periodically send routing updates consisting of all or part of their routing
tables. Routers using link-state routing protocols use link-state
advertisements (LSAs) to send updates only when topological changes occur in
the network, and send complete routing tables much less frequently.
As a packet
travels through the network devices need a method of determining what portion
of the IP address identifies the network and what portion identifies the host.
A 32-bit address mask, called a subnet mask, is used to indicate the bits of an
IP address that are being used for the network address. The default subnet mask
for a Class A address is 255.0.0.0. For a Class B address, the subnet mask
always starts out as 255.255.0.0, and a Class C subnet mask begins as
255.255.255.0. The subnet mask can be used to split up an existing network into
subnetworks, or subnets.
Subnetting
reduces the size of broadcast domains, allows LAN segments in different
geographical locations to communicate through routers and provides improved
security by separating one LAN segment from another.
Custom subnet
masks use more bits than the default subnet masks by borrowing these bits from
the host portion of the IP address. This creates a three-part address:
- The original network address
- The subnet address made up of
the bits borrowed
- The host address made up of the
bits left after borrowing some for subnets
Routers use
subnet masks to determine the subnetwork portion of an address for an incoming
packet. This process is referred to as logical ANDing.
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