But I'll chime in anyway.
Either the interface between the motherboard and the NIC isn't working (maybe a software issue), or the interface between the NIC and the network isn't working. That is, the computer can talk to itself, and the network can talk to itself, but packets to/from the CPU aren't getting out on the wire. Since the computer can find the NIC, I'd guess its on the other side - the network side.
Could the CAT-5 (or whatever) socket on the NIC be bad?
Is there anyway to network the Sun box via a different port (e.g. PCs can be networked via parallel ports, etc.)? If so, that might tell you something about the network software if it also doesn't work.
Did it ever work before? If so, what changed? Can you put things back the way they were?
HTH a bit. Good luck.
Cheers,
Scott.