Problem: Duplicate IP address assignments on the same network

 

An IP address should be unique across your network. The Dynamic Host Configuration Protocol (DHCP) server cannot assign a single IP address to more than one client.

Under certain conditions, the DHCP server will attempt to verify that an address is not currently in use before it assigns it to a client. When the DHCP server detects that an address is being used when it should not be, it will temporarily mark that address as used and will not assign that address to any client. You can use the DHCP server monitor to view which IP addresses the server has detected are in use but were not assigned by the DHCP server. These addresses will have a USED status and an UNKNOWN_TO_IBMDHCP client identifier.

Here are some common reasons for this problem.

Multiple DHCP servers are configured to assign the same IP address.

If two DHCP servers are configured to assign the same IP address to clients, then it is possible for two different clients to receive the same IP address. One of the clients will receive the IP address from one of the DHCP servers, and another client will receive the same IP address from the other DHCP server. Multiple DHCP servers can serve the same subnet or network, but they should not be configured with the same address pool or overlapping address pools.

A client has been manually configured with an IP address which is managed by DHCP.

The DHCP server typically attempts to verify whether an IP address is currently in use before assigning it to a client. However, there is no guarantee that the manually configured client is currently connected to the network or available to respond when the DHCP server is verifying the IP address. So, the DHCP server might assign the IP address to a DHCP client. When the manually configured client connects to the network, you will have duplicate IP addresses on your network. IP addresses that are managed by DHCP should not be used to manually configure the network setup for a client. If a client needs to be manually configured with an IP address, that IP address should be excluded from the DHCP server's address pool.

 

Parent topic:

Troubleshooting DHCP

Related concepts
Managing leased IP addresses