2.2. Changes to Host Software to Support Subnets

In most implementations of IP, there is code in the module that handles outgoing datagrams to decide if a datagram can be sent directly to the destination on the local network or if it must be sent to a gateway.

Generally the code is something like this:

         IF ip_net_number(dg.ip_dest) = ip_net_number(my_ip_addr)
             THEN
                 send_dg_locally(dg, dg.ip_dest)
             ELSE
                 send_dg_locally(dg,
                                  gateway_to(ip_net_number(dg.ip_dest)))

To support subnets, it is necessary to store one more 32-bit quantity, called my_ip_mask. This is a bit-mask with bits set in the fields corresponding to the IP network number, and additional bits set corresponding to the subnet number field.

The code then becomes:

         IF bitwise_and(dg.ip_dest, my_ip_mask)
                                   = bitwise_and(my_ip_addr, my_ip_mask)
             THEN
                 send_dg_locally(dg, dg.ip_dest)
             ELSE
                 send_dg_locally(dg,
                        gateway_to(bitwise_and(dg.ip_dest, my_ip_mask)))

It may or may not be necessary to modify the "gateway_to" function, so that it too takes the subnet field bits into account when performing comparisons.

To support multiply-connected hosts, the code can be changed to keep the "my_ip_addr" and "my_ip_mask" quantities on a per-interface basis; the expression in the conditional must then be evaluated for each interface.