As indicated in section 2, distance vector routing can be used to describe routes to individual hosts or to networks. The RIP protocol allows either of these possibilities. The destinations appearing in request and response messages can be networks, hosts, or a special code used to indicate a default address. In general, the kinds of routes actually used will depend upon the routing strategy used for the particular network. Many networks are set up so that routing information for individual hosts is not needed. If every host on a given network or subnet is accessible through the same gateways, then there is no reason to mention individual hosts in the routing tables. However, networks that include point to point lines sometimes require gateways to keep track of routes to certain hosts. Whether this feature is required depends upon the addressing and routing approach used in the system. Thus, some implementations may choose not to support host routes. If host routes are not supported, they are to be dropped when they are received in response messages. (See section 3.4.2.)
The RIP packet formats do not distinguish among various types of address. Fields that are labeled "address" can contain any of the following:
host address
subnet number
network number
0, indicating a default route
Entities that use RIP are assumed to use the most specific information available when routing a datagram. That is, when routing a datagram, its destination address must first be checked against the list of host addresses. Then it must be checked to see whether it matches any known subnet or network number. Finally, if none of these match, the default route is used.
When a host evaluates information that it receives via RIP, its interpretation of an address depends upon whether it knows the subnet mask that applies to the net. If so, then it is possible to determine the meaning of the address. For example, consider net 128.6. It has a subnet mask of 255.255.255.0. Thus 128.6.0.0 is a network number, 128.6.4.0 is a subnet number, and 128.6.4.1 is a host address. However, if the host does not know the subnet mask, evaluation of an address may be ambiguous. If there is a non-zero host part, there is no clear way to determine whether the address represents a subnet number or a host address. As a subnet number would be useless without the subnet mask, addresses are assumed to represent hosts in this situation. In order to avoid this sort of ambiguity, hosts must not send subnet routes to hosts that cannot be expected to know the appropriate subnet mask. Normally hosts only know the subnet masks for directly-connected networks. Therefore, unless special provisions have been made, routes to a subnet must not be sent outside the network of which the subnet is a part.
This filtering is carried out by the gateways at the "border" of the subnetted network. These are gateways that connect that network with some other network. Within the subnetted network, each subnet is treated as an individual network. Routing entries for each subnet are circulated by RIP. However, border gateways send only a single entry for the network as a whole to hosts in other networks. This means that a border gateway will send different information to different neighbors. For neighbors connected to the subnetted network, it generates a list of all subnets to which it is directly connected, using the subnet number. For neighbors connected to other networks, it makes a single entry for the network as a whole, showing the metric associated with that network. (This metric would normally be the smallest metric for the subnets to which the gateway is attached.)
Similarly, border gateways must not mention host routes for hosts within one of the directly-connected networks in messages to other networks. Those routes will be subsumed by the single entry for the network as a whole. We do not specify what to do with host routes for "distant" hosts (i.e., hosts not part of one of the directly- connected networks). Generally, these routes indicate some host that is reachable via a route that does not support other hosts on the network of which the host is a part.
The special address 0.0.0.0 is used to describe a default route. A default route is used when it is not convenient to list every possible network in the RIP updates, and when one or more closely- connected gateways in the system are prepared to handle traffic to the networks that are not listed explicitly. These gateways should create RIP entries for the address 0.0.0.0, just as if it were a network to which they are connected. The decision as to how gateways create entries for 0.0.0.0 is left to the implementor. Most commonly, the system administrator will be provided with a way to specify which gateways should create entries for 0.0.0.0. However, other mechanisms are possible. For example, an implementor might decide that any gateway that speaks EGP should be declared to be a default gateway. It may be useful to allow the network administrator to choose the metric to be used in these entries. If there is more than one default gateway, this will make it possible to express a preference for one over the other. The entries for 0.0.0.0 are handled by RIP in exactly the same manner as if there were an actual network with this address. However, the entry is used to route any datagram whose destination address does not match any other network in the table. Implementations are not required to support this convention. However, it is strongly recommended. Implementations that do not support 0.0.0.0 must ignore entries with this address. In such cases, they must not pass the entry on in their own RIP updates. System administrators should take care to make sure that routes to 0.0.0.0 do not propagate further than is intended. Generally, each autonomous system has its own preferred default gateway. Thus, routes involving 0.0.0.0 should generally not leave the boundary of an autonomous system. The mechanisms for enforcing this are not specified in this document.