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Prevent IP address conflicts on your wireless network with DHCP scopes

Many wireless APs are set by default to act as DHCP servers, which can create IP address conflicts in your wireless network. Learn how to use DHCP scopes to manage your wireless environment.

One of the problems with adding a wireless access point, or AP, into a wired network is many are configured by...

default to act as DHCP servers. Unless you plan ahead, this can cause some problems if your network already makes use of Dynamic Host Configuration Protocol.

There are two main problems that wireless access points can cause when they act as DHCP servers.

What is an IP conflict?

First is the potential for IP address conflicts. An IP address conflict is a situation in which two devices on the same subnet are assigned duplicate IP addresses. The consequences of an IP address conflict tend to vary, because different devices and operating systems are designed to handle IP conflicts in different ways. At least one of the devices with a duplicate address will lose the ability to communicate on the network, however, and there's a good chance both of the conflicting devices will fail.

The majority of wireless access points on the market are configured by default to assign clients IP addresses in the 192.168.0.x range. This can be a problem because it is common for an organization to include multiple wireless access points in an effort to extend coverage to a larger area. If two or more wireless access points are configured to assign the same block of IP addresses, conflicts are bound to occur. Keep in mind these address assignments may not be limited to wireless clients. Some wireless access points will assign IP addresses to clients on the wired network, as well, so you could potentially have clients on your wired network conflicting with wireless clients.

Although IP address conflicts can be caused by overlapping DHCP scopes -- the consecutive range of IP addresses the DHCP server can lease to clients on a subnet -- that is far from being the only possible cause. For example, an IP address conflict can also occur if someone manually assigns an IP address to a device and that address is already in use, or exists within a pool of addresses that can be assigned by a DHCP server.

The second problem with the default DHCP configuration that is often used by most wireless access points is the 192.168.0.x address range may not mesh with the address range you are already using. For example, suppose your existing wired network uses the 190.160.25.x address range -- I'm just making up a number -- and you decided to add some wireless access points. If those access points were configured to assign addresses in the 192.168.0.x range, it would potentially cause communication issues on your network.

In this example, the clients that had been assigned the 192.168.0.x addresses would probably not be able to communicate with network hosts using the 190.160.25.x addresses. The reason is clients using the 192.168.0.x address range would think hosts with the 190.160.25.x addresses were on a different network segment, even if that were not the case. They would, therefore, need to use routing tables to figure out how to reach this segment.

Now that we have looked at the problems associated with the way wireless access points assign IP addresses by default, let's look at how to get around them. First, some wireless access points are more flexible than others, so not all of the techniques I will be discussing will work with all access points.

How to resolve an IP address conflict

The best way to prevent IP address conflicts and other communication problems is to decide beforehand what scope of addresses each DHCP server and each access point will manage. That way, you can allow each DHCP server and access point to assign IP addresses as needed, without having to worry about overlaps.

For example, on my own network, I use the address range 147.101.x.x. Again, I chose this address range at random. Since I have one DHCP server and one wireless access point, I configured the DHCP server to assign addresses ranging from 147.101.101.1 to 147.101.101.100. I then configured the wireless access point to assign addresses ranging from 147.101.101.101 to 147.101.101.200. That way, all of the IP addresses that could potentially be assigned fall within a common range, but there is no risk of addresses overlapping and causing an IP address conflict.

Why does it happen?

In spite of an administrator's best efforts, IP address conflicts sometimes occur. Conflicts can result from an administrative configuration error or from a user manually assigning an IP address to a device. Unfortunately, IP address conflicts are notoriously difficult to resolve. If you need to resolve an IP address conflict, you might start the troubleshooting process by looking at the entries on your DNS server. There's a chance the DNS server will list the host names of the devices with the conflicting addresses. These host names may provide clues as to the device type and location. Depending on the type of DNS server you are using, the DNS records may also be able to tell you whether the addresses have been assigned statically or dynamically. Third-party tools can also help to resolve IP address conflicts.

Once you locate the devices with conflicting IP addresses, resolving the problem is easy. If the IP address was manually assigned, you pick a different address that you know is available, or configure the device to use DHCP for IP address assignments.

If one of the devices is already receiving its address from a DHCP server, and you are not having problems with overlapping DHCP scopes, then resolve the conflict and either reboot the device or enter the following commands -- if the device is running Windows:

IPConfig /release

IPConfig /renew

The first command will detach the IP address. The second command creates a new IP address lease.

Wireless AP configuration considerations

When you configure an access point to assign a specific IP address range unique to your network, take a couple of other things into account. First, consider that you probably use a few static IP addresses on your network. You must define exceptions for any static IP addresses that are in use to prevent that address from being assigned. On my own network, for example, I have a DNS server that uses the address 147.101.101.34. This address falls within the range of addresses that my DHCP server is configured to assign. I, therefore, defined an exception, so the DHCP server would never assign 147.101.101.34 to a client. If my DNS server used the address 147.101.101.201, however, no exception would be necessary because the address falls outside of the DHCP scopes, which typically define a single physical subnet on your network on which DHCP services are offered and, therefore, would not be assigned by a DHCP server.

Another thing you need to take into account is if you configure an access point to the wireless devices on your network, you will also have to configure the access point to assign a DNS address to clients. If you neglect to explicitly assign a DNS server address, then the access point may not assign a DNS server address to clients as a part of the IP address lease. Depending on the make and model of the access point, it might instead provide clients with the IP address of your internet service provider's DNS server. If this happens, the client will be able to browse the internet, but it will not be able to perform DNS name resolution of resources on the local network.

As you can see, blindly installing a wireless access point can cause a number of problems. Since many access points also act as DHCP servers, it is important you decide ahead of time how the new access point will fit in with your existing IP address scheme.

Next Steps

Ten steps to troubleshoot wireless connection problems

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This was last published in July 2017

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