I have installed FC4 in my computer,and I have an ethernet NIC in my computer.I can connect to network via the school local area network under Windows XP.In linux I configed the IP Gateway and DNS,even I have changed my NIC's MAC because the original MAC is illegal.After I have done all these,when I ping a computer in the LAN,it respond that"The network is not reachable".I don't know what can I do.

su root and type:
mii-tool

report back the results here.

How is that possible? By definition, a MAC address can't be illegal, as it is wrote physically in your network card "chipset". It is very unlikely to be filtered by some kind of firewall too, as MAC address is way way harder to spoof than ip (and since it is a "physical" address, you can't just filter your clients MAC).

Anyways, once you "spoofed" your MAC address, your gateway/router will no longer recongnize you, as his ARP table will be wrong, you will have, at least, to call back the DHCP...

...

Still, this is... suspect?

I can't connect the network using the original MAC ,and after I changed the MAC only with nothing else being changed,the computer can connect to the network.

mii-tool
It shows
SIOCGMIIPHY on 'eth' failedperation not supported
no MII Interface found

Just a question... assuming you have to change your MAC address to connect to your network, how do you do that in windoze XP? I believe this is somehow harder to do on XP and since the MAC is always the same...

In the Network's attribute under Windows XP,you can set the Net address

My network card is NVIDIA nForce MCP Networking Controller.And what can I do now?Thankyou

errr...
what's your MAC address right now? What's the "illegal" MAC address?

The MAC address right now is
00-0D-61-C2-D2-35
the original MAC
4D-00-47-DE-39-1C