ss that references a network. The prefix length is the number of bits set to 1 in the subnet mask. Networks are described in the format network-address/prefix-length. For example, the network 10.0.0.0 with a subnet mask of 255.0.0.0 would be described as 10.0.0.0/8. When shown in this format, the route is called simply a prefix. The network 10.0.0.0/24 is said to be a longer prefix than the network 10.0.0.0/8. The more bits that are used to identify the network portion of the address, the longer the prefix is said to be. The RIB may include multiple routes to the same network. For example, in Fig- ure 9-1, R2 learns the network 10.0.0.0 from two sources: R1 advertises the route 10.0.0.0/8, and R3 advertises the route 10.0.0.0/24. Because the prefix lengths are dif- ferent, these are considered to be different routes. As a result, they will both end up in the routing table. 10.0.0.0/8 10.0.0.0/24 192.168.1.0/24 192.168.2.0/24 10.0.0.0/8 10.0.0.0/24 Figure 9-1. Same network with different prefix lengths Here are the routes as seen in R2: 10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks 10.0.0.0/8 [90/30720] via 192.168.1.1, 00:12:01, FastEtherneto/o 10.0.0.0/24 [90/30720] via 192.168.2.3, 00:12:01, FastEtherneto/1 When a packet is received in R2, the destination IP address is matched against the routing table. If R2 receives a packet destined for 10.0.0.1, which route will it choose? There are two routes in the table that seem to match: 10.0.0.0/8 and 10.0.0.0/24. The route with the longest prefix length (also called the most specific route) is the more desirable route. Thus, when a packet destined for 10.0.0.1 arrives on R2, it will be forwarded to R3. The important thing to realize about this example is that there may Chapter Routing and Routers be legitimate addresses within the 10.0.0.0/24 range behind R1 that R2 will never be able to access. Technically, 10.0.0.0/8 is a network and 10.0.0.0/24 is a subnet. Read on for further clarification. Route Types The routing table can contain six types of routes: Hos