RTP - LAN/network question

Greetings.

I have a little LAN with 3 PCs going through a Netwinder OfficeServer
(Linux).
I’ve set the Gateway to the Netwinder’s address and I can get on the
internet.

My question is: how do I access the other computers?

I noticed that if I try to manually input the Netmask, it allows me to
type in
the 255.255.255.0, but when I hit apply, it resets it to 0.0.0.0. Do I
need to
put something in the Destination field, also? If so, what goes there?

TIA

thomas