Summarization has been examined in several chapters so far. Summarization conserves internetwork resources by reducing the size of routing tables and route advertisements. The smaller, simpler routing tables can also make management and troubleshooting easier. A summary address is an address that represents several, sometimes many, more-specific addresses. For example, the following four subnets:
can be summarized with the single address 192.168.200.128/25. When examined in binary, the addresses reveal that the summary address is less specific because it consists of fewer network and subnet bits than the addresses being summarized. So put crudely, it might be said that as more zeros are added to the host space and as fewer network bits are used, more addresses are summarized. Taking this concept to its limit, what if so many zeros are added to the host space that no network bits remain ? In other words, what if the summary address consists of 32 zeros (0.0.0.0)? This address summarizes every possible IP address. Note The IP default address is 0.0.0.0 0.0.0.0 is the IP default address, and a route to 0.0.0.0 is a default route. [1] Every other IP address is more specific than the default address, so when a default route exists in a routing table, that route will be matched only if a more specific match cannot be made.
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