If you use DHCP servers to automatically configure the TCP/IP settings for workstations in your organization, a DHCP failure can lead to a major disruption in service. After all, if a workstation is not able to acquire an IP address, then it will have no way of accessing any of the resources on your private network or on the Internet. In this article, I will discuss some techniques that you can use to troubleshoot DHCP server failures.
Inappropriate Address Assignment
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).
Common DHCP Server Problems
If multiple workstations are experiencing problems with leasing IP addresses, then the problem is most likely related to the DHCP server itself. If you suspect that the DHCP server is the cause of the problem, then you might start off by doing some ping tests to verify that the DHCP server is able to communicate across the network.
If the DHCP server is able to communicate with other computers on the network, then I recommend verifying that the DHCP server has an IP address that is compatible with the scope that the server is configured to assign addresses from. For example, if the DHCP server’s scope consists of addresses from 192.168.0.1 to 192.168.0.50, then the server will not actually be able to assign those addresses unless the server itself has been assigned a static address in the same subnet range, such as 192.168.0.0 or 192.168.0.51.
IP Address Conflicts
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.
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Inappropriate Address Assignment
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).
Common DHCP Server Problems
If multiple workstations are experiencing problems with leasing IP addresses, then the problem is most likely related to the DHCP server itself. If you suspect that the DHCP server is the cause of the problem, then you might start off by doing some ping tests to verify that the DHCP server is able to communicate across the network.
If the DHCP server is able to communicate with other computers on the network, then I recommend verifying that the DHCP server has an IP address that is compatible with the scope that the server is configured to assign addresses from. For example, if the DHCP server’s scope consists of addresses from 192.168.0.1 to 192.168.0.50, then the server will not actually be able to assign those addresses unless the server itself has been assigned a static address in the same subnet range, such as 192.168.0.0 or 192.168.0.51.
IP Address Conflicts
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.
Full Information here
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