[robespierre]

esp doesn't exist in IRIX 5.3.
"unknown host" means that the resolver could not resolve the host name into a network address, so this is not a test of the network. Use numeric addresses to test if your network is routed correctly, not names.
Once you have packets passing over the correct routes you can work on the DNS.

To your other question, if two computers are on the same network segment, they do not require a router to communicate. This means they must be assigned addresses in the same subnet.