n- ning tree is only concerned with loops and with preventing them from a physical and layer-2 perspective. In other words, spanning tree is concerned more with determining that all possible paths within its domain are loop-free than with determining the paths along which data should be sent. Routing Tables When a packet arrives at a router, the router determines whether the packet needs to be forwarded to another network. If it does, the RIB is checked to see whether it con- tains a route to the destination network. If there is a match, the packet is adjusted and forwarded on to where it belongs. (See Chapter 15 for more information on this process.) If no match is found in the RIB, the packet is forwarded to the default gateway, if one exists. If no default gateway exists, the packet is dropped. Originally, the destination network was described by a network address and a sub- net mask. Today, destination networks are often described by a network address and a prefix length. The network address is an IP address 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 Figure 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 