Networking Tips: Understanding and Resolving IP Address Conflicts Conflict Resolution: Shared IP Addresses Joseph Moran
Picture the following: You're sitting at the computer minding your own business. Then, all of a sudden, Windows presents you with a warning: "There is an IP address conflict with another system on the network." You then quickly realize that you no longer have Internet access, or your connection becomes so intermittent it becomes unusable.
This probably happens to many of us every so often, and as is typical, the cryptic Windows error message doesn't give you much to go in terms of helping to identify and fix the problem. Fortunately, IP address conflicts are usually temporary and often eventually resolve themselves.
But you can't count that to always happen. Even when it does, the resolution is not always immediate, causing many to resort to time-consuming reboots that may or may not remedy the problem. This week we'll examine likely causes of IP address conflicts and some ways to address them (forgive the pun).
Conflict Resolution
Let's start by briefly defining exactly what an IP address conflict is. In order for a system to communicate on a network it must have an IP address, and by definition that IP addresses must be unique — unique, at least, to your own network.
When two devices somehow wind up with the same IP address, this is detected and the network interface on both on them gets disabled, causing each system to lose its connectivity until the conflict has been resolved.
So what are some possible causes of IP address conflicts? A classic one is when two systems are given an identical static (manually assigned) address. This scenario isn't as common as it once was, however, because on most home or small business networks DHCP is now used to automatically manage and assign IP addresses.
DHCP servers — which are built into most routers — use a defined pool of IP addresses (called a scope) from which they dole out addresses in response to system requests. But since the default DHCP scope for most routers includes every available network address, even a single device with a static address assignment can cause a conflict if that address has already been given out by DHCP.
You can minimize the chances of this happening by reducing the number of addresses in the DHCP pool to slightly more than you need (most routers let you do this), which allows you to have some free and available when you need static addresses.
But even avoiding the use of static addresses and relying entirely on DHCP doesn't eliminate the potential for address conflicts. For example, in some cases you may (often unknowingly) have more than one similarly configured DHCP server handing out overlapping addresses, especially if you have multiple wireless access points or other network devices with embedded DHCP servers (which are usually turned on by default). In this case, too many cooks spoil the broth — you should only have one DHCP server running and make sure all others are turned off.
Another common cause of address conflicts is when a system comes back online after an extended period in stand-by or hibernate mode, especially if the system was previously part of another network (say, a work notebook brought home). A conflict will promptly occur if the IP address the system was previously configured with is the same as one that's already in use on the network.
Absent any of the above causes, conflicts can also be caused by internal DHCP server malfunctions, which cause the server to lose track of which systems are using which addresses. This can be surprisingly common with some routers.
