DHCP & subnet assigns

[Pogi]

What would/could cause a Win2K Server running DHCP service to assign subnets to clients that is not the subnet the server is on? In other words, say the Win2K server is on subnet 255.255.255.0 but the subnets it assigns its clients is 255.255.0.0.

The network in question has a router to handle broadband net connections and the clients are a mix of WinXP and Win2K Pro.

Any help is appreciated. Thanks!

[Sandman72]

Are you sure the scope options are set properly? Is it in an Atice Directory environment? If so is it an authorized dhcp server? Anything else running or capable of providing DHCP?

[kato2274]

Originally posted by Sandman72
Anything else running or capable of providing DHCP?

this is probably where to look. make sure you don't have 2 DHCP servers running. the second could be something like a broadband router, a linux box etc.

you could stay after hours and shut DHCP down on the win2k server then try to release and renew some clients. if they pick up an ip at all, then start looking for the other device that's running DHCP.

[SpongeBob]

i have seen missconfigured routers do this... give out DHCP also if the routing table is wrong or you have passing of "win's" traffic or the like type feratures turned on, then sometimes a rouge router will pass the broadcast message to the wrong DHCP server. (if each subnet has it's own)


Most likely you got a rogure server / device. Search and destry mission is in order.

Packet sniffer on the switch where it occurs is good idea, Perfmon in win2k or 3rd party... do 10 release and renews while logging all traffic, the filter for DHCP packets. open them up and look at the ip's.

You'll know if 10 mins where it is gettign the ip from!

[CeeBee]

1. Do a ipconfig /all on a client computer and see if the DHCP server is the one you suspect.
2. It is possible that the scope was created when the subnet mask for the server was 255.255.0.0 and then the subnet mask was changed to 255.255.255.0. In this case the DHCP will still provide the old mask to the clients. In this case the best solution is to re-create the scope.

[SpongeBob]

Originally posted by CeeBee
1. Do a ipconfig /all on a client computer and see if the DHCP server is the one you suspect.
2. It is possible that the scope was created when the subnet mask for the server was 255.255.0.0 and then the subnet mask was changed to 255.255.255.0. In this case the DHCP will still provide the old mask to the clients. In this case the best solution is to re-create the scope.

hehe... DUH...

/me smacks forehead!

I should have thought of that! Good Job!

ipconfig /all shows WHO your DHCP server is.

still cant believe i didnt think of that 1st. kudos!!


Sometimes the obvious is oblivious!!

[Pogi]

Originally posted by CeeBee
It is possible that the scope was created when the subnet mask for the server was 255.255.0.0 and then the subnet mask was changed to 255.255.255.0. In this case the DHCP will still provide the old mask to the clients. In this case the best solution is to re-create the scope.

I'm thinking this is what had happened. I've taken over as the admin of this network, only problem is I'm only in the building maybe twice a week. I've done a workaround by assigning static IPs to the clients. Next time I'm in the building I'll try recreating the scope.

Correct me if I'm wrong, but won't the original scope have to be blown away first before I build a new one?

[CeeBee]

Originally posted by Pogi
Correct me if I'm wrong, but won't the original scope have to be blown away first before I build a new one?

No, not as long as you make changes in the existing one so that the assigned IP's from the 2 scopes don't overlap. But it might take less time to just write down the few options that you assign (unless you have some address reservations), delete the old scope and create a new one.

[Rô©kHøµñÐ]

Might check any layer-2 switches for Broadcast control options like automatic gateway.

Had an issue on a few of our HP4000's assigned the default gateway to the value of the clients IP address.

Might also want to force the DHCP clients to send a release command on shutdown through server options.