I have a workgroup network contains 12 computers including the Server, i have Sygate personal Firewall installed on the server, so sometimes no computer can reach the other i mean can't access any other computer on my network and my firewall tells me that there is (MAC SPOOFING) on my network, is it a viruse can do that and if so then what kinda anti virus should i use to remove it? gentelmen

MAC spoofing is real and not very hard accomplish. Example: My ISP assigns my IP via DHCP. The same IP continues to be renewed to my router because it's based on the router's MAC address. If the MAC changes, so will my IP. The router I own has a setting for changing the MAC address to whatever I want it to be (ie: spoof). So theoretically, I could sit for endelss hours changing the MAC address within my router to have the ISP generate me a new (different) IP each time. MAC spoofing is easily accomplished on any Linux/UNIX based computer and can be done with only slightly more hassle on a Windows box. Having said that.....it makes no sense that Sygate is reporting spoofing internally within your network unless someone actually is MAC spoofing. I would highly recommend you hit Sygate's web site and check their support page for FAQ's on known and/or common issues. You may find information pertaining to the problem you're having there.

Just to throw it out there, does the server or any other computers have dual NICs with both connected to the network....? Assume that I already did an Internet search.

There is also the rare issue of mismanufactured nics not containing unique mac addresses. Might want to do a ipconfig /all at each pc and note the physical address [mac address] to see if any match. Imagine the power knowing how to web search gives you.

...I could see that, but in a different way. I've actually run across D'Link network adapters that would sometimes come up with a MAC of 00-00-00-00-00-00. This was fixed by installing and updated driver for the NIC. (We had this happen to a college student who's laptop could not connect to the campus WLAN because of a duplicate IP address, which we figured to be another D'Link with the same problem connected to their network.) Assume that I already did an Internet search.