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DHCP clients changing network segments

By paul ·
My company has a class b ip address which is divided into class c subnets for use by different sites (eg - site 1 gets x.x.1.*, site 2 gets x.x.2.* - each site has more than 1 subnet - My own has 3). All use the same subnet mask (255.255.255.0)
My DHCP server serves all 3 subnets without problem, with 3 seperate scopes. Each other site has its own DHCP server.
When a laptop client leases an address at another site and visits here it cannot access the network.
I assume that it requests a renewal from its known DHCP server, doesn't get a response so broadcasts a request. My own DHCP server gets the request, but declines since the originator has an IP address in the same subnet but with a scope handled by another server - hence no new address. (is this assumption correct?)
Manually releasing and renewing the address works OK (but renewing alone doesn't), but this is unsuitable since regular users don't have priveleges to do this.
The problem only affects NT4 machines and only "one-way" (moving back to another site works OK).
Question is - what can be done (without major changes) and why does it work OK "the other way"? I suspect it is a network hardware problem (routers/switches), but can't be sure. More details available if required.
TIA
Paul

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DHCP clients changing network segments

by mishratron In reply to DHCP clients changing net ...

Unless the ipconfig is blocked then give each laptop a script with
these commands
ipconfig /release
ipconfige /renew
exit

this should help speed up your users.

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DHCP clients changing network segments

by paul In reply to DHCP clients changing net ...

As already stated - this is unacceptable as under NT4 regular users do not have permissions to release the IP address.

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DHCP clients changing network segments

by mishratron In reply to DHCP clients changing net ...

Unless the ipconfig is blocked then give each laptop a script with
these commands
ipconfig /release
ipconfige /renew
exit

this should help speed up your users.

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DHCP clients changing network segments

by paul In reply to DHCP clients changing net ...

As already stated - this is unacceptable as under NT4 regular users do not have permissions to release the IP address.

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DHCP clients changing network segments

by deltree In reply to DHCP clients changing net ...

Your issue sounds similiar to mine.
check out http://www.techrepublic.com/forumqa/thread_detail.jhtml?thread_id=54602
Only our laptop users are using W95
When a laptop client initializes on a different subnet, TCP/IP trys to renew its lease. It broadcasts a DHCP request. The server sees an IP address that it doesn't recognize and responds with a dhcpNACK message. This is by design. When the client receives this packet, IP is suppose to reinitialize and go through the entire lease processes as if it never had a lease.
DHCP discover
DHCP offer
DHCP request
DHCP ACK
IP binds

I am at a loss as to why our clients are not following this process.
The DHCP server logs the event saying that it sent a dhcpNAK.
But on the client IP is still bound to its address from another subnet.
The only way it seems to work is to manually release and renew the IP address using winipcfg.
I thought this was a W95 problem only, but it doesn't look like it is isolated to an OS looking at your issue.
There is a reg hack to auto release IP address info on shutdown as the other respondants have indicated.
I've tryed it on my network and see the same results.
You might try it on your network.

I also suspect that our switches might be preventing the dhcpNACK from getting to the client. However, this doesn't really make sense to me assuming a NACK and ACK are the same format.

There is an interesting response on my thread regarding updating winsock from 1.1 to 2.0. This will be my next troubleshooting step.

Good luck!
email me if you find a solution other than the reghack. peterg@pnwbank.com

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DHCP clients changing network segments

by paul In reply to DHCP clients changing net ...

Thanks for the comments - unfortunately none of it really applies - the reghack is a win95 only (and users can't release the IP address under NT). I find this only happens with NT PCs.
We recently had a network change around which forced PCs onto adifferent VLAN (hence IP subnet) - Win95 and Win2k PCs when rebooted picked up new addresses with no problem. NT4 workstations had to have the addresses released and renewed manually. I guess this is the same problem.

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DHCP clients changing network segments

by bmccool In reply to DHCP clients changing net ...

It's possible that the router(s) between the sights are forwarding BOOTP requests (RFC1542)
Clients always try and renew, then will broadcast for any DHCP server.

Just a guess but it sounds like they might still think they are on the other segment.
this could be checked by ipconfig /all, if this shows an IP address of 0.0.0.0 then my theory is full of it, if it still shows the other subnet BINGO.

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DHCP clients changing network segments

by paul In reply to DHCP clients changing net ...

You're completely right - except that the BOOTP requests only seem to get forwarded in one direction. Move from site A to site B - get new IP address, no probs. Move back (B->A) the problem happens. I really need to eliminate any potential server config problem or client issues before passing onto someone who knows routers.

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DHCP clients changing network segments

by paul In reply to DHCP clients changing net ...

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