Network Solution for Windows – Windows system, Linux system, Ubuntu server configuration tip

Another problem that I have seen on occasions involves IP address conflicts among dynamically configured addresses. When you create a DHCP scope, it is the DHCP server’s responsibility to make sure that addresses within the scope are only leased to one client at a time. If that’s the case, then how is it possible to have an IP address conflict for dynamically assigned addresses?

There are two situations that I’ve run into that can cause this problem. The first time that I ever ran into this problem, I was able to determine which PCs had been assigned at the duplicate addresses. When I checked the TCP/IP configuration on those machines, I found that one of the machine’s IP addresses had been manually configured. It’s kind of a long story, but that machine’s user was running an unauthorized application that required a static IP address. The user got tired of having to reconfigure the application every time they used it, so they took the address that had been dynamically assigned to them, and entered it as a static address.

The likelihood of this happening today is fairly slim. When a particular situation occurred, Windows 98 was the current operating system. Windows 98 lacks many of the security features that we take for granted today. A properly secured workstation running Windows XP or Windows Vista should be resistant to end user reconfigurations. Even so, I wanted to at least mention this issue because it gives you something to look for if you have trouble solving the problem.

A much more common cause of this problem is that multiple DHCP servers are in use, and those DHCP servers have overlapping scopes. If you only have a single DHCP server on your network, do not make the mistake of immediately dismissing this idea as a possible cause of your problem. In all likelihood, there is probably a rogue DHCP server that is conflicting with your primary DHCP server.

Windows 2000 Server and Windows Server 2003 are both designed in such a way to prevent rogue DHCP servers from causing problems. A DHCP server can only issue IP addresses after it has been authorized by the Active Directory. The problem is that this only applies to Windows-based DHCP servers. DHCP servers running other operating systems are free to lease IP addresses to clients without having to be authorized by the Active Directory.

So has a user really gone through the trouble of installing a rogue, Linux based DHCP server? Probably not. A much more likely explanation is that a wireless access point, or a router intended for cable or DSL Internet connections is causing your problem. Such devices almost always have DHCP server’s built in. These devices typically use a scope range of 192.168.0.x or 192.168.1.x. If this happens to be the same IP address scope that your primary DHCP server uses, then you may run into a situation in which both DHCP servers are issuing addresses from the same address pool.