Cause: An unauthorized DHCP server has been configured by someone attempting to control or disrupt your network. The unauthorized DHCP server is assigning IP addresses and other configuration parameters that have already been assigned to other nodes by an authorized DHCP server. The result is that nodes are assigned duplicate IP addresses or incorrect configuration parameters. Incorrect configuration parameters can interfere with a node’s ability to communicate to the network in any number of ways. Incorrect parameters can even be used to cause a node to connect to a server that is controlled by an unauthorized user, thereby allowing the unauthorized user to take control of the client.

Solution: Find the unauthorized DHCP server and disable it or disconnect it from the network.

Cause: The Windows 95 DHCP client has a two-second timeout for the time between when it accepts an offer of an IP address in a message sent to the server and the time it expects an acknowledgment of that acceptance in a reply from the server. Other clients, such as Windows NT, have a four-second timeout.

Solution: Obtain the dchcpupd.exe patch from Microsoft that changes the timeout on Windows 95 clients from two seconds to four seconds. The patch is dated 2/12/96 and includes a file named VDHCP.386.

Cause 1: The mobile client’s lease is not released when the mobile client moves to a remote office. This can occur when the remote office is on a subnet different from that of the local office and the remote subnet is serviced by a different DHCP server.

Solution 1: Determine the lease time assigned to this client. If the lease is not released quickly enough, reduce the lease time. Otherwise, have the client manually release the old IP address before it leaves the local office.

Cause 2: The mobile client uses two leases at the same time because it cannot use the same lease and the same IP address at the new location.

Solution 2: Do one of the following:

Cause: One or more global client parameters were not configured properly in DHCP.

Solution: Verify that all the parameters assigned by DHCP are properly configured.

Cause: Clients are not releasing the leases when they are finished using them because the lease time is too long.

Solution: Reduce the lease time to a few hours so that leases can be made available to other clients that are waiting to use the IP addresses. Otherwise, you might need to purchase more IP addresses and configure more or larger address ranges to make more IP addresses available.

Cause: The IP addresses of the clients might change if you use DHCP continually over a period of time and the lease period is set to a reasonably low value.

Solution: Use static DHCP assignments when you want to use a specific IP address assigned to the client for identification and management.

Cause 1: The server is not physically linked to the client’s communications media or the server did not bind its IP protocol to the interface card, which shares physical media access with the client.

Solution: Check the server’s physical connections. Load inetcfg to ensure that proper binding exists.

Cause: The server's configuration for its local interfaces does not match the configuration within eDirectory for the same server.

Solution: Load inetcfg and check to see if the server has a legal IP address on each local subnet it serves. Also, use the iManager utility to ensure that each local subnet is properly configured.