Routing Tables

Internet hosts use routing tables to compute the next hop for a packet.

Routing tables can take many forms, but here is a simple model that can explain most Internet routing. Each entry in a routing table has at least two fields - IP Address Prefix and Next Hop. The Next Hop is the IP address of another host or router that is directly reachable via an Ethernet, serial link, or some other physical connection. The IP Address Prefix specifies a set of destinations for which the routing entry is valid for. In order to be in this set, the beginning of the destination IP address must match the IP Address Prefix, which can have from 0 to 32 significant bits. For example, a IP Address Prefix of 128.8.0.0/16 would match any IP Destination Address of the form 128.8.X.X.

Bridged networks are regarded as single connections.

If no routing table entries match a packet's Destination Address, the packet is discarded as undeliverable (possibly with an ICMP notification to the sender). If multiple routing tables entries match, the longest match is preferred. The longest match is the entry with the most 1 bits in its Routing Mask.

To avoid needing routing entries for every possible Internet destination, most hosts and routers use a default route (some routing tables contain nothing but a single default route). A default route has a Routing Address/Mask pair of 0.0.0.0/0.0.0.0. In other words, it matches every IP address, but since there are no 1 bits in its Routing Mask, any other match would be selected by the longest match rule. The default route will only be used if there are no others matches in the routing table, thus its name. Default routes are quite common, and are put to best use on networks with only a single link connecting to the global Internet. On such a network, routing tables will have entries for local nets and subnets, as well as a single default route leading to the outbound link. However, remember that all Next Hops must be directly reachable, so the default routes won't necessarily point to the same IP address. Also, some networks (large Internet service providers, mostly) use defaultless routing tables that must be able to match every IP address in the global net.