stion. If a device recognizes the address, it will reply to the request where it will update its ARP cache with that information. The ARP cache contains the mapping tables maintained by the ARP module. There is also a reverse address resolution protocol (RARP) (Ref. 8). It works in a fashion similar to that of the ARP, but in reverse order. RARP provides an IP address to a device when the device returns its own hardware address. This is particularly useful when certain devices are booted and only know their own hardware address. Routing with IP involves a term called hop. A hop is defined as a link connecting adjacent nodes (gateways) in a connectivity involving IP. A hop count indicates how many gateways (nodes) must be traversed between source and destination. One part of an IP routing algorithm can be source routing. Here an upper-layer protocol (ULP) determines how an IP datagram is to be routed. One option is that the ULP passes a listing of Internet addresses to the IP layer. In this case information is provided on the intermediate nodes required for transit of a datagram in question to its final destination. Each gateway makes its routing decision based on a resident routing list or routing table. If a destination resides in another network, a routing decision is required by the IP gateway to implement a route to that other network. In many cases, multiple hops are involved and each gateway must carry out routing decisions based on its own routing table. A routing table can be static or dynamic. The table contains IP addressing information for each reachable network and closest gateway for the network, and it is based on the concept of shortest routing, thus routing through the closest gateway. Involved in IP shortest routing is the distance metric, which is a value expressing minimum number of hops between a gateway and a datagram's destination. An IP gateway tries to match the destination network address contained in the header of a datagram with ENTERPRISE NETWORKS II: WIDE AREA NETWORKS a network addre