3.3.1.1 Local/Remote Decision

To decide if the destination is on a connected network, the following algorithm MUST be used [see IP:3]:

1. The address mask (particular to a local IP address for a multihomed host) is a 32-bit mask that selects the network number and subnet number fields of the corresponding IP address.

2. If the IP destination address bits extracted by the address mask match the IP source address bits extracted by the same mask, then the destination is on the corresponding connected network, and the datagram is to be transmitted directly to the destination host.

3. If not, then the destination is accessible only through a gateway. Selection of a gateway is described below (3.3.1.2).

A special-case destination address is handled as follows:

• For a limited broadcast or a multicast address, simply pass the datagram to the link layer for the appropriate interface.

• For a (network or subnet) directed broadcast, the datagram can use the standard routing algorithms.

The host IP layer MUST operate correctly in a minimal network environment, and in particular, when there are no gateways. For example, if the IP layer of a host insists on finding at least one gateway to initialize, the host will be unable to operate on a single isolated broadcast net.