What can I check if I can't ping other machines?

First, are you sure that the address, netmask and broadcast are correct? Your broadcast should be the inverse of of your netmask. For example, if your ip address is 192.168.2.3 and your netmask is 255.255.255.0, your broadcast should be 192.168.2.255, but if the netmask were 255.255.0.0, the broadcast would be 192.168.255.255.

If the netmask does not match the netmask of the OTHER machines on the network, you can have strange results like being able to ping machine X but machine X can't ping you.

Ping by ip address rather than name. If that works, your problem is name resolution, not anything else.

Are you sure your ip address is unique? Try pinging it from some other machine with this one turned off- obviously you should get nothing.

Do you have a physical connection? You should be seeing network traffic on the lights of your nic if you are on a hub (not if you are connected to a switch unless its broadcast traffic or specifically going to your ip address).

If you unplug the patch cord, the lights at the hub or switch should go out- if they don't, obviously you aren't plugged in where you think you are.

Is your cable good? Swap it with a working system to check.

Move the machine to another place on the network- maybe the wiring is bad just here.

Routes all look good? netstat -rn shows you. If you are seeing unexpected routes, a rip enabled router may be feeding you bad info. Turn off rip by killing the routed process and make sure it doesn't restart by editing it out of /etc/tcp. Find the line that reads "/etc/routed &" and put a "#" in front of it.