Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 273

from the left portion of the mask; the bits set to 0 are also contiguous to the right portion of the
mask. Table 6-5 shows the default masks for Class A, B, and C addresses.

Default Address Masks Table 6-5

Class Binary Mask Dotted Decimal Mask

A 11111111 00000000 00000000 00000000 255.0.0.0

B 11111111 11111111 00000000 00000000 255.255.0.0

C 11111111 11111111 11111111 00000000 255.255.255.0

Consider, for example, a company with 200 hosts is assigned the Class C network of 200.1.1.0/
24. The 200 hosts are in six different LANs. The Class C network can be subnetted by using
a mask of 255.255.255.224. Looking at the mask in binary (11111111 11111111 11111111
11100000), the first three bytes are the network part, the first three bits of the fourth byte
determine the subnets, and the five remaining 0 bits are for host addressing.

Table 6-6 shows the subnetworks created with a mask of 255.255.255.224. Using this mask,
2 n subnets are created, where n is the number of bits taken from the host part for the subnet
mask. In this example, you use 3 bits, so that 2 3 = 8 subnets. With Cisco routers, the all 1s subnet
(LAN 7) can be used for a subnet broadcast. You can use the all 0s subnet with the ip subnet-
zero command.

Subnets for Network 200.1.1.0 Table 6-6

LAN Fourth Byte Subnet Number First Host Broadcast

LAN 0 000 00000 200.1.1.0 200.1.1.1 200.1.1.31

LAN 1 001 00000 200.1.1.32 200.1.1.33 200.1.1.63

LAN 2 010 00000 200.1.1.64 200.1.1.65 200.1.1.95

LAN 3 011 00000 200.1.1.96 200.1.1.97 200.1.1.127

LAN 4 100 00000 200.1.1.128 200.1.1.129 200.1.1.159

LAN 5 101 00000 200.1.1.160 200.1.1.161 200.1.1.191

LAN 6 110 00000 200.1.1.192 200.1.1.193 200.1.1.223

LAN 7 111 00000 200.1.1.224 200.1.1.225 200.1.1.255

In this example, you use a fixed-length subnet mask. The entire network has the same subnet
mask, 255.255.255.224. Routing protocols such as Routing Information Protocol version 1
(RIPv1) and IGRP use fixed-length subnet masks. They do not support variable-length subnet
masks (VLSM). VLSMs are covered in this chapter.