What is Subnetting ?

IP Default Subnet Masks For Address Classes A, B and C

Subnetting is the process of dividing a Class A, B or C network into subnets, as we've seen in the preceding topics. In order to better understand how this “division of the whole” is accomplished, it's worth starting with a look at how the “whole” class A, B and C networks are represented in a subnetted environment. This is also of value because there are situations where you may need to define an unsubnetted network using subnetting notation.

This might seem like a strange concept—if you aren't going to bother creating subnets, why do you need to consider how the old-fashioned classes are used under subnetting? The answer is that after subnetting became popular, most operating systems and networking hardware and software were designed under the assumption that subnetting would be used. Even if you decide not to subnet, you may need to express your unsubnetted network using a subnet mask.

In essence, a non-subnetted class A, B or C network can be considered the “default case” of the more general, custom-subnetted network. Specifically, it is the case where we choose to divide the host ID so that zero bits are used for the subnet ID and all the bits are used for the host ID. I realize that this seems like a bit of a semantic game. However, this default case is the basis for the more practical subnetting we will examine in the next topic.

Just as is always the case, the subnet mask for a default, unsubnetted class A, B or C network has ones for each bit that is used for network ID or subnet ID, and zeroes for the host ID bits. Of course, we just said we aren't subnetting, so there are no subnet ID bits! Thus, the subnet mask for this default case has 1s for the network ID portion and 0s for the host ID portion. This is called the default subnet mask for each of the IP address classes.

Since classes A, B and C divide the network ID from the host ID on octet boundaries, the subnet mask will always have all ones or all zeroes in an octet. Therefore, the default subnet masks will always have 255s or 0s when expressed in decimal notation. Table 1 summarizes the default subnet masks for each of the classes; they are also shown graphically in Figure 1.

Table 1: Default Subnet Masks for Class A, Class B and Class C Networks
IP Address Class	Total # Of Bits For Network ID / Host ID	Default Subnet Mask
		First Octet	Second Octet	Third Octet	Fourth Octet
Class A	8 / 24	11111111 (255)	00000000 (0)	00000000 (0)	00000000 (0)
Class B	16 / 16	11111111 (255)	11111111 (255)	00000000 (0)	00000000 (0)
Class C	24 / 8	11111111 (255)	11111111 (255)	11111111 (255)	00000000 (0)

Figure 68: Default Subnet Masks for Class A, Class B and Class C Networks

So, the three default subnet masks are 255.0.0.0 for Class A, 255.255.0.0 for class B, and 255.255.255.0 for Class C. Note that while all default subnet masks use only “255” and “0”, not all subnet masks with “255” and “0” are defaults. There are a small number of custom subnets that divide on octet boundaries as well. These are:

• 255.255.0.0:,This is the default mask for Class B, but can also be the custom subnet mask for dividing a Class A network using 8 bits for the subnet ID (leaving 16 bits for the host ID).
• 255.255.255.0: This is the default subnet mask for Class C, but can be a custom Class A with 16 bits for the subnet ID or a Class B with 8 bits for the subnet ID.

Subnet masks

Network IDs and host IDs within an IP address are distinguished by using a subnet mask. Each subnet mask is a 32-bit number that uses consecutive bit groups of all ones (1) to identify the network ID and all zeroes (0) to identify the host ID portions of an IP address.

For example, the subnet mask normally used with the IP address 131.107.16.200 is the following 32-bit binary number:

11111111 11111111 00000000 00000000

This subnet mask number is 16 one-bits followed by 16 zero-bits, indicating that the network ID and host ID sections of this IP address are both 16 bits in length. Normally, this subnet mask is displayed in dotted decimal notation as 255.255.0.0.

The following table displays subnet masks for the Internet address classes.

Address class	Bits for subnet mask	Subnet mask
Class A	11111111 00000000 00000000 00000000	255.0.0.0
Class B	11111111 11111111 00000000 00000000	255.255.0.0
Class C	11111111 11111111 11111111 00000000	255.255.255.0

Typically, default subnet mask values (as shown in the previous table) are acceptable for most networks with no special requirements and where each IP network segment corresponds to a single physical network.

In some cases, you can use customized subnet masks to implement IP subnetting. With IP subnetting, you can subdivide the default host ID portion of an IP address to specify subnets, which are subdivisions of the original class-based network ID.

By customizing the subnet mask length, you can reduce the number of bits that are used for the actual host ID. For more information about using a custom subnet mask for subnetting your network, see the Windows 2000 Resource Kit.

Important To prevent addressing and routing problems, you should make sure all TCP/IP computers on any network segment use the same subnet mask.