Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 275

As an example, take the IP address 200.1.1.70 with a subnet mask of 255.255.255.224. The
three bolded bits in the subnet mask extend the default Class C mask (/24) to a mask of /27.
Perform an AND operation of the IP address with the subnet mask to obtain the subnetwork, as
shown in Table 6-9. The broadcast number is obtained by making all the host bits 1.

Subnetwork of IP Address 200.1.1.70 Table 6-9

IP Address 11001000 00000001 00000001 010 00110 200.1.1.70

Subnet Mask 11111111 11111111 11111111 111 00000 255.255.255.224

Subnetwork 11001000 00000001 00000001 010 00000 200.1.1.64

Network Subnet Host

Broadcast Address 11001000 00000001 00000001 010 11111 200.1.1.95

VLSM

VLSMs further subdivide a network to prevent the wasting of IP addresses. If a Class C network
uses 255.255.255.240 as a subnet mask, there are 16 subnets, each with 14 IP addresses. If a point-
to-point link needs only 2 IP addresses, 12 IP addresses are wasted. This problem scales further
with Class B and Class A address space. With VLSMs, small LANs can use /28 subnets with 14
hosts, whereas larger LANs can use /23 or /22 masks with 510 and 1022 hosts, respectively.

Consider Class B network 172.16.0.0/16 as an example. Using a /20 mask produces 16 subnet-
works. Table 6-10 shows the subnetworks. With the /20 subnet masks, the first four bits of the
third octet determine the subnets.

Subnets with a /20 Mask

Table 6-10

Third Octet Subnetwork

0000 0000 172.16.0.0/20

0001 0000 172.16.16.0/20

0010 0000 172.16.32.0/20

0011 0000 172.16.48.0/20

0100 0000 172.16.64.0/20

0101 0000 172.16.80.0/20

0110 0000 172.16.96.0/20

0111 0000 172.16.112.0/20

1000 0000 172.16.128.0/20

1001 0000 172.16.144.0/20

continues