One very common DHCP related issue is the assignment of an unexpected IP address. For example, suppose that your DHCP server was configured with an IP address scope of 192.168.0.1 to 192.1680.50. You would expect network hosts to be assigned IP addresses in this range. Now, suppose that a workstation on your network appeared to be having problems communicating with network servers. You issue an IPCONFIG /ALL command to view the workstation’s IP address configuration. Instead of the expected address range, the workstation has been assigned an address beginning with 169.254.

So what happened? If a host on your network is unexpectedly assigned an address beginning with 169.254, you can rest assured that the address was not assigned by your DHCP server. What actually has happened, is that the workstation has failed to contact a DHCP server. When this occurs, the workstation will assign itself an IP address using a Windows feature known as Automatic Private IP Addressing (APIPA).

Microsoft built Automatic Private IP Addressing into Windows as a way of helping those who have very small networks. For example, if you were to create a small Windows network, you would not have to manually configure IP addresses even if there were no DHCP server on the network. APIPA would automatically assign a unique class B IP address to each machine on the network. This is great for small home networks but completely inappropriate for larger networks.

If a workstation resorts to using an APIPA assigned address, it is because its requests for an IP address have gone unanswered. There are several possible causes for this problem. Assuming that the other computers on the network are able to acquire an IP address from your DHCP server, you can rule out the DHCP server as the cause of the problem.

More than likely, the issue is related to the networking hardware installed in the workstation that is having the problem. For example, the Network Interface Card might be assigned an incorrect driver. Another possible cause of the problem is that the patch cable is not plugged into the Network Interface Card, or is not connected to a switch on the other end.

Of course, just because only one computer on the network is having trouble obtaining an IP address doesn’t completely rule out the server as the cause of the problem. If other workstations are successfully obtaining IP addresses, then you can be sure that the server is working properly. However, it could be that the server has run out of IP addresses that it can assign to clients. You can easily tell if this is the problem by comparing the size of the DHCP address scope to the number of devices on your network that request IP addresses from the DHCP server.