Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 276

Subnets with a /20 Mask (Continued) Table 6-10

Third Octet Subnetwork

1010 0000 172.16.160.0/20

1011 0000 172.16.176.0/20

1100 0000 172.16.192.0/20

1101 0000 172.16.208.0/20

1110 0000 172.16.224.0/20

1111 0000 172.16.240.0/20

With fixed-length subnet masks, the network only supports 16 networks. Any LAN or WAN
link must use a /20 subnet. This is an inefficient use of address space. With VLSMs, the /20
subnets can be further subnetted.

For example, take 172.16.64.0/20 and subdivide it to support LANs with approximately 500
hosts. Use a /23 mask. This produces the subnetworks shown in Table 6-11.

Subnetworks for 172.16.64.0/20 Table 6-11

Third Octet Subnetwork

0100 000 0 172.16.64.0/23

0100 001 0 172.16.66.0/23

0100 010 0 172.16.68.0/23

0100 011 0 172.16.70.0/23

0100 100 0 172.16.72.0/23

0100 101 0 172.16.74.0/23

0100 110 0 172.16.76.0/23

0100 111 0 172.16.78.0/23

With VLSMs, these subnetworks can be even further subdivided. Take subnetwork 172.16.76.0/
23 and use it for two LANs that have fewer than 250 hosts. This produces subnetworks
172.16.76.0/24 and 172.16.77.0/24. Also, subdivide 172.16.78.0/23 for serial links. Because
each point-to-point serial link needs only two IP addresses, you can use a /30 mask. The
subnetworks that are produced are shown in Table 6-12.