This section provides the following troubleshooting information for DHCP:
If an IP host does not operate the same as when it was manually configured, verify that the parameters assigned by DHCP are the same as those when the host was manually configured.
If a node is intermittently inoperable, verify that the node is not using the same IP address as another IP host. If a duplicate IP address exists, verify that there is only one DHCP server for the subnet. Also verify that the IP addresses assigned by the DHCP server are not being used by manual nodes.
If DHCP hosts cannot obtain a DHCP lease when required, verify that enough leases exist to accommodate all hosts that use DHCP. If there are too few leases, obtain more IP addresses and configure more leases or reduce the lease time to a few hours. This ensures that more leases are made available to other clients that are waiting to use the IP addresses.
If a Windows 95 client cannot acquire a lease and responds with the messageUnable to obtain an IP network address the client requires a longer timeout. This problem might occur when the client and DHCP server are separated by one or more routers. To increase the timeout for Windows 95 clients, obtain a patch from Microsoft. The patch is dated 2/12/96 and includes a file named VDHCP.386. The patch itself is named DCHCPUPD.EXE.
If the number of leases available for clients decreases when you are using mobile clients, verify that the mobile clients' lease is released when the client connects from a remote office or that the mobile client can use the same lease and the same IP address at the new location.
The following list describes the most common operational errors that occur.
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 clients' 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---Use one of the following solutions:
Cause---One or more global client parameters were not configured properly in DHCP.
Solution---Verify that all 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---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 the Directory 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 check that each local subnet is properly configured using the DNS/DHCP Management Utility.
When a host is powered on, it is leased an IP address for a period of time, depending on the configuration settings of the subnet from which the address is assigned. If the machine is moved to another network while the original IP address lease is still valid, the user must release the lease. Other situations might also require that a lease be released, such as the use of a laptop computer in different locations of a given network.
To manually release and renew a DHCP-assigned IP address in Windows 95, complete the following steps:
Select Start, then Run.
Type winipcfg and press Enter.
The IP Configuration dialog box is displayed.
Click Release All.
The IP Address, Subnet Mask, and Default Gateway fields should display no addresses.
Click Renew All.
New addresses should appear in the IP Address, Subnet Mask, and Default Gateway fields.
Click OK to close WINIPCFG.
To manually release and renew a DHCP-assigned IP address in Windows NT, complete the following steps:
Select Start > Programs > MS-DOS Command Prompt.
From the DOS prompt, execute the command
ipconfig /release
A message is displayed indicating that the assigned IP address has been successfully released.
From the DOS prompt, execute the command
ipconfig /renew
A message is displayed indicating the new IP address that has been assigned.
To review DHCP settings,
From the DOS prompt, execute the following command to review DHCP settings:
inconfig /all