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DHCP Clients getting Wrong Subnet Mask

By jimm ·
We have been having a weird set of issues come up recently. We have several subnets that we are getting reports of IP address conflicts errors from end users. We are assigning addresses via DHCP. The addresses (hardware, and IP) that are being listed as being in conflict are usually not the same. I initialy thought this could be a dhcp server issue, so I changed one subnet with issues to a different DHCP server with a differant scope of addresses. No change the issue still exists. Then errors started occuring on a subnet in a remote location, running a totally differant setup for dhcp(The main site uses Microsoft's DHCP Server propagating dhcp traffic across a Cisco 6509 Switch's subnets with helper addresses setup, and the remote site with a single subnet with a Cisco 4006 switch, and running DHCP Server on the 1750 Cisco Router at that location) Finally I found one lead but am not sure what it means. We just got in a new Fluke Optiview Network analyser, and I put it on each subnet with issues. The Optiview shows that on each subnnet about 6 PC's have the wrong subnet mask, It should be a class C subnet mask but is coming up as class A or B. I verified that these PC's were not set with static IP addresses, and if you release andrenew the ip address manually the correct mask appears, but within a couple days the incorect mask reappears. The only thing in common with these computers seems to be that they are running Windows 98, they are all of several differant brands, and also are running several differant brands of Network cards (3Com, integrated Dell, and Aopen) My thoughts are that since the subnet masks are not right the dhcp servers cannot tell that the IP addresses they are assigning are in use and they reassign the same address. Any ideas? Am I even on the right track here?

Jim Matuska

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It's funny you mention cisco

by LordInfidel In reply to DHCP Clients getting Wron ...

We have had that same problem here intermittentley (ms?).

We also use nt4 dhcp and cisco products (cataylst 2900's in the usr netwk).

This stopped happening however when we did 2 things.

1- we redid our scopes. Basically seperating them onto 2 different servers, 1 took the higher end and the other took the lower end.

2. Did away with wins. it no longer operates on our network.

95% of our nics our 3com's. But the one thing in common between all of these was the win98se os.

Ihave a feeling that there is just someting in their stack, compunded with they way the servers were handing out the leases and going over the catylsts. Somewhere along the line the ack got mangled.

One other thing we did, was set the lease period to only 24 hours.

But take heart to know that you are not the only one out there with this problem. It is actually wide spread.

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What is your IP network range?

by davidpmartin In reply to DHCP Clients getting Wron ...

About two years ago I was working on network an experienced a very similar problem. We had 6 different networks that we were using DHCP for, but it would only correctly feed out the IPs for the first two networks. Well much to my surprize here's what we found out:

You can set up superscopes with Microsoft's DHCP server, but you must be running SP4 or higher to do this. And another dirty little MS secret - Microsoft's DHCP servers are classless - meaning that you can only serve out distinct networks. What we were doing was mixing our superscope between class C and class B networks - and we confused the **** out of the DHCP server. When we went to issuing all class B addresses, our DHCP server worked just fine. Bottom line: You mustpass out you DHCP along classless lines if you want to take advantage of superscoping. If you are using class C addresses, that means a new network every 254 computers, With Class B's you have more flexiblity - but my guess is you don't have class Baddresses. Even though you must put in the subnet mask for the network - it really is meaningless to MS DHCP server.

Your cisco products should not have any influence on the DCHP server if you have IP helper turned on. Make sure you have the IPhelper address pointed to the specific DHCP server you want to use - you can specify an network, but it really doesn't help matters and it greatly increases broadcast traffic.

Hope this helps.

Dave Martin, CSS1, CCNP, CCDP, MCSE

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