Must I Manually Assign Xbox's IP Address (DMZ)

[Rodeojones]

I have:

D-Link DI-604 (wired) router, F/W 3.51
Comcast (ISP) Cable Modem
Windows XP

I have had problems speaking and hearing two friends (specifically) in COD 2. We cannot invite each to games, etc.

I forwarded the proper ports, added the Xbox's IP address to the DMZ and have a NAT setting of open.

I DID NOT manually assign (change) my Xbox's IP address though, I left it in the automatic setting and applied that address to the DMZ.

Could this be my problem and is it necessary to manually (change) assign the address to the Xbox when entering into the DMZ of the router?

Thanks in advance

[R1CH4RD 21]

Wouldnt have thought that you would have to manually enter the IP address changing that shouldnt make any difference to your problem, although i wouldnt quote me on that. Is this a permenant issue or just occasional as sometimes mine plays up but thats only once in a blue moon.