An IP address is divided into two parts. The first part designates the network address while the second part designates the host address. When designing network, an IP addressing scheme is used to uniquely identify hosts and devices within the network. The IP addressing scheme should be hierarchical and should build on the traditional logical hierarchical model. This allows the addressing scheme to provide designated points in the network where effective route summarization can be performed.
Summarization reduces the amount of information that routers must process, which allows for faster convergence within the network. Summarization also restricts the size of the area that is affected by network changes by hiding detailed topology information from certain areas within the network. This concept is illustrated in Figure 1-1:
Fig. 1-1. Route Summarization
Referencing Figure 1-1, the branch offices (Access layer) are dual-homed to the regional office routers (Distribution layer). Using a hierarchical addressing scheme allows the Distribution layer routers to advertise a summary route for the branch office subnets to the Core layer. This protects the core from the effects of any route flapping between the Distribution and Access layer routers. This is because a summary route will not flap until every last one of the more specific prefixes from which it is derived in removed from the routing table. This increases stability within the area. In addition to this the routing table size at the Core is further reduced.