DHCP Server uncontactable on SBS server

Posted on 2006-05-16
Last Modified: 2010-04-19

I was browsing through event veiwer and came accross this error under the system tab,

Unable to contact a DHCP server. The Automatic Private IP Address will be assigned to dial-in clients. Clients may be unable to access resources on the network.

I have also noticed that once an IP address is changed on one of the computers (for example network cable unpluged and pluged into a router to configure it) when the network cable is pluged back into the computer (for the main network) the DHCP server on SBS is not contactable and the NIC assigns itself the ip address above

Has anyone come accross this issue before?

Gavin McMillan
Question by:gavinandrewmcmillan
    LVL 8

    Expert Comment

    If ISA on the SBS server is not configured correctly it can prevent client PCs contacting the DHCP service.
    LVL 2

    Author Comment

    I do have ISA Installed, I am still working on configuring it, i am also having problems with FTP now too, which is being a hassel.

    Do you know of a good guide that can help me configure ISA to work easily with all the features needed?

    I had it configured and working well, but since upgrading to ISA 2004 have had problems.

    LVL 2

    Author Comment

    Well, it turns out that this is at least partially my fault.  You see, when I set up ISA 2000, I never let ISA build the LAT table for me - I always manually specified the internal address range I wanted.  So for an SBS using the default IP of, I would specify a LAT of to

    ISA 2004 varies from ISA 2000 in that it firewalls all network interfaces, including the internal interface.  My DHCP problem was that DHCP requests happen via the broadcast address of .255 - since my LAT entry ended at .254 - ISA blocked the traffic, so the DHCP Server never received the client request, and the client thus was unable to pull an IP via DHCP.

    The resolution for this problem is to:

    Go into your ISA management console, expand the configuration node and then select the Networks node, then edit the internal listing to state for example to otherwise ISA will block any broadcasts as they are performed on (which until changing it wasnt part of your internal network).
    LVL 74

    Expert Comment

    by:Jeffrey Kane - TechSoEasy
    Running the Configure Email and Internet Connection Wizard will accomplish much of this configuration.  And that's where you should start on an SBS, since it will integrate the configuration with other SBS services.


    Accepted Solution

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