Expert: John Teodoro - 10/11/2005

Question
I have an NT network using DHCP with about 50 computers attached. Most computers get their ip address via dhcp; a few have static addresses. One with a static address is the NT server with an address of 192.168.1.100. For the past couple of months, a message pops up on the NT server every couple of days saying that 192.168.1.100 is having an ip address conflict with a device that has the mac address 00:12:F0:3A:8F:E2. I have not been able to find a way to identify the device with this mac address. Any ideas?

Answer
Hi Mark,

First of all DHCP for your client computers is really the way to go, however make sure that the scope of your DHCP is not within the same range of your fixed IP's for the servers. You can specify the range to be say 192.168.1.100-175. Make all your server 192.168.1.10, 11, 12, etc... and then keep your DHCP for VPN in the 200-250 range. Makes this much easier to administrate and no conflicts can occur. A server by the way should always have a fixed IP...which is what you've done anyways...so your setup sounds good, except for maybe the range of addresses.

Now as for the conflict, you can trace a mac address to an IP address with the arp command. There is also allot of software out there to help you "sniff out" an IP address on your network. as well.

In a command shell if you type arp/? it will give you all the switches. Now I know this command is available in 2000 and 2003 server, I do not remember if it is available in Windows NT. If not, you can download it from Microsoft.

There is also a program called ARP assistant available at:

http://techrepublic.com.com/5138-1035_11-729260.html

I have never used this program but it claims to have a GUI that makes it easier if you are not comfortable with a command shell. If you are an admin in a company, I would suggest getting used to using the command shell more often.

Once you find that address, i suggest you change it at the workstation level, not the server, since this will affect the server services....but you probably know this already.

By the way might I ask why your company is still using NT? It is no longer supported by Microsoft and with 50 users behind you it does not seem like a small business...you should really look into migrating to Server 2003, especially when Longhorn is already comming out next year...anyways just my 2 cents.

I hope this helps and thank you for writing.

Have a nice evening,
John