RIP is intended to allow hosts and gateways to exchange information for computing routes through an IP-based network. RIP is a distance vector protocol. Thus, it has the general features described in section 2. RIP may be implemented by both hosts and gateways. As in most IP documentation, the term "host" will be used here to cover either. RIP is used to convey information about routes to "destinations", which may be individual hosts, networks, or a special destination used to convey a default route.
Any host that uses RIP is assumed to have interfaces to one or more networks. These are referred to as its "directly-connected networks". The protocol relies on access to certain information about each of these networks. The most important is its metric or "cost". The metric of a network is an integer between 1 and 15 inclusive. It is set in some manner not specified in this protocol. Most existing implementations always use a metric of 1. New implementations should allow the system administrator to set the cost of each network. In addition to the cost, each network will have an IP network number and a subnet mask associated with it. These are to be set by the system administrator in a manner not specified in this protocol.
Note that the rules specified in section 3.2 assume that there is a single subnet mask applying to each IP network, and that only the subnet masks for directly-connected networks are known. There may be systems that use different subnet masks for different subnets within a single network. There may also be instances where it is desirable for a system to know the subnets masks of distant networks. However, such situations will require modifications of the rules which govern the spread of subnet information. Such modifications raise issues of interoperability, and thus must be viewed as modifying the protocol.
Each host that implements RIP is assumed to have a routing table. This table has one entry for every destination that is reachable through the system described by RIP. Each entry contains at least the following information:
The entries for the directly-connected networks are set up by the host, using information gathered by means not specified in this protocol. The metric for a directly-connected network is set to the cost of that network. In existing RIP implementations, 1 is always used for the cost. In that case, the RIP metric reduces to a simple hop-count. More complex metrics may be used when it is desirable to show preference for some networks over others, for example because of differences in bandwidth or reliability.
Implementors may also choose to allow the system administrator to enter additional routes. These would most likely be routes to hosts or networks outside the scope of the routing system.
Entries for destinations other these initial ones are added and updated by the algorithms described in the following sections.
In order for the protocol to provide complete information on routing, every gateway in the system must participate in it. Hosts that are not gateways need not participate, but many implementations make provisions for them to listen to routing information in order to allow them to maintain their routing tables.