Is a default gateway required if you want to communicate on the LAN by IP addresses only?

Answer 1

A default gateway entry is only needed if the two computers are on different subnets. This is true whether you use IP addresses or have some sort of name resolution. The default gateway points to a router that routes packets between subnets.

For instance:

Computer 1 - 201.13.16.99, subnet mask 255.255.0.0

Computer 2 - 199.27.12.43, subnet mask 255.255.0.0

These two computers are on different subnets, and would require a router AND a default gateway entry pointing to that router to communicate.

Computer 1 - 192.168.2.100, subnet mask 255.255.255.0

Computer 2 - 192.168.2.67, subnet mask 255.255.255.0

These two computers are on the same subnet, and will be able to communicate as long as they have a physical ethernet connection between the two. No default gateway entry or router is needed.

The default gateway IP is never used by the TCP/IP stack unless a connection outside the local subnet is needed.

Note that this has nothing to do with whether you use IP's or netbios names resolved by a WINS server or DNS names resolved by a DNS server. All these schemes eventually resolve down to an IP (and then to a MAC address for the actual communication) so the above still applies.