How to Resolve IP Address Conflict Mac

There is an IP address conflict

For some time now, both of my computers have been getting an IP error display in the Windows info field (bottom right on the desktop). The message is on

Windows XP computer: "There is an IP address conflict with another system in the network." Windows 7 computer: "An IP address conflict has been detected. Another computer has the same IP address as this computer. ... "

Specifically, it appears that both computers want to access the Internet with one and the same IP (a typical Telekom address 178.191.247.XXX). Which of course has to fail.

I have now searched the Pirelli PRGAV4202N modem via the management console to see whether there are any setting options for it. Unfortunately not. You could only supply the LAN addresses with static IPs, but of course not the addresses in the WAN. In this respect, I cannot help myself here, because the IPs in the WAN are assigned by the DHCP router at the telephone exchange in my opinion.

Am I correct in my opinion here and can only Telekom solve the problem or can I help myself?