Cisco CCNA

Planning for Address Summarization

In a classless world, address summarization (also called aggregation) allows a router to consolidate multiple network prefixes into a single, less specific prefix. Example 1-2 in this chapter uses a single prefix 192.168.4.0/22 to summarize the address space of four prefixes that resemble class C addresses (192.168.4.0/24, 192.168.5.0/24, 192.168.6.0/24, and 192.168.7.0/24). A router can view the address block as the four /24 prefixes or as the single HI prefix—it's the same address space, but using the single ill prefix is more efficient. To extend the idea further, you could summarize all prefixes that start with 192.168 with a single less specific prefix, 192.168.0.0/16. Again, a prefix is equivalent in meaning to a subnet.

Figure 1-9 illustrates an address summarization scenario. Figure 1-9 A Router Using Address Summarization

In Figure 1-9, Router B advertises a summary route 202.100.0.0/16 to tell Router A that all prefixes starting with 202.100 are reachable through it. Advertising a single generalized route is more efficient than babbling 255 specific routes with a /24 mask. Router A needs to receive and process only one route—not 255 separate ones.

Summarization reduces the number of network prefixes managed and communicated between routers. With large networks, especially the Internet» managing too many specific prefixes wastes router memory and network bandwidth; therefore, if at all possible, plan for summarization by deploying addresses as contiguous groups. Then, you can use routing protocols, such as OSPF. EIGRP, or BGP. to summarize address blocks and exchange fewer and less specific routes between routers.

Conserving Subnets with IP Unnumbered 29