Internet Protocols - CCIE Routing and Switching Exam Certification Guide - page 278

Four Class C networks can be summarized at the /22 bit level. For example, networks 200.1.100.0,
200.1.101.0, 200.1.102.0, and 200.1.103.0 share common bits, as shown in Table 6-14. The
resulting network is 200.1.100.0/22, which you can use for a 1000 node LAN.

Common Bits Within Class C Networks Table 6-14

Binary Address IP Address

11001000 00000001 011001 00 00000000 200.1.100.0

11001000 00000001 011001 01 00000000 200.1.101.0

11001000 00000001 011001 10 00000000 200.1.102.0

11001000 00000001 011001 11 00000000 200.1.103.0

Network addresses can be summarized by aggregating the networks at a bit boundary. A
neighboring router can receive one summarized route, rather than receiving 8, 16, 32, or more
routes, depending on the level of summarization. Figure 6-4 shows an example of the route
summarization. Router A sends a single route to its upstream router. In Table 6-15, all the Class
C networks share a bit boundary with 21 common bits.

Route Summarization

Figure 6-4

199.100.16.0/21

Router A

199.100.16.0/23
199.100.18.0/23

199.100.20.0/22

199.100.16.0/24
199.100.17.0/24

199.100.18.0/24
199.100.19.0/24

199.100.20.0/24
199.100.21.0/24
199.100.22.0/24
199.100.23.0/24

Summarization of Networks

Table 6-15

Binary Address IP Network

11000111 01100100 00010 000 00000000 199.100.16.0

11000111 01100100 00010 001 00000000 199.100.17.0

11000111 01100100 00010 010 00000000 199.100.18.0

11000111 01100100 00010 011 00000000 199.100.19.0