Troubleshooting the WLAN

• Tags:
• WEP,
• Wireless,
• IP,
• Signal Strength

Brien M Posey

Published: 19 Nov 2002 12:02 GMT

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DHCP configuration issues
Another tricky problem that can prevent you from successfully interacting with a wireless network is a DHCP configuration error. The DHCP server that you connect to can play a major role in whether you are able to use a wireless network.

Many of the newer access points have an integrated DHCP server. Typically, these access points assign the 192.168.0.x address range to clients. Often, DHCP access points will not accept connections from clients to which they have not issued an IP address. This means that clients with static IP addresses or clients that might have somehow acquired an IP address from another DHCP server could be unable to connect to the access point.

The first time I installed an integrated DHCP server access point onto my network, I decided to allow the access point to assign IP addresses to my wireless clients. However, my network uses the 147.100.X.Y address range. This meant that although wireless clients were able to communicate with the access point and were able to acquire an IP address, they were unable to interact with the rest of my network because of the IP address range mismatch.

There are two solutions to this problem:

• Disable the access point's DHCP services and allow the wireless client to lease an IP address from a normal DHCP server.
• Override the IP address range by configuring the DHCP address scope with your own block of IP addresses.

Either solution will work, but you'll have to work within the limitations imposed by your access point's firmware. Many access points will allow you to use only one solution or the other, not both.

Multiple access point problems
Suppose for a moment that two access points are in use, both with the default settings. If this is the case, both access points are assigning clients' IP addresses in the 192.168.0.X address range. The problem is that the two access points are completely unaware of which IP addresses the other access point has leased. So it's only a matter of time before there are duplicate addresses on your network.

The solution to this problem is to define a unique scope of addresses for each access point. By doing so, you'll prevent IP address overlaps.

Watch out for client lists
Some access points contain an allowed client list, which can be the root of wireless configuration problems. The allowed client list is a list of MAC addresses of permitted wireless clients. This is a security feature that's designed to prevent unauthorized users from connecting to your network. Normally, the allowed address feature is disabled by default. However, if a user has accidentally clicked the Enable button, the allowed address list will be enabled but won't contain any MAC addresses. This means that no wireless clients will be able to connect to the access point, regardless of any other configuration settings.

I've also seen the allowed address list become a problem when multiple access points are in use. Many administrators incorrectly assume that just because they enter the allowed addresses into the list, the addresses are then globally permitted to access the network. However, in most cases, this simply grants the users permission to access the network through the designated access point. If you want users to be able to go through other access points, you'll usually have to configure those access points separately.

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Go to section: Troubleshooting the WLAN Page 2: Verify WEP and SSID Page 3: DHCP issues

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