Your question is pretty commom, and has probably been answered here before. Since you haven't included specifics (make/model of NIC, error messages you've encountered, any troubleshooting steps you've already tried), here's the general drill:
Try using the
Forum Search to review past threads dealing with the issue. Also review the
LNO archives. As search criteria, try using your NIC's model name, the module name (tulip), and/or the text of any error messages you receive.
Also, I've posted a (very) generic outline of things to try when your NIC/network setup is giving you trouble in a couple of threads, one of which is
here. Give the suggestions a try; if you still have problems, repost, and include any pertinent messages/errors/info.