I keep getting occasional IP address conflicts on about two hundred DHCP clients - half are Win9x, the other half 2000 Pro and XP Pro. It seems DHCP server is leasing an already-leased IP address. Leases are 24 hours. All devices have up-to-date service packs and updates.

What to do? Thanks.

Determine which IP address(s) is causing the conflict. Next time you see the message, make note of the IP.

Determine why. There is quite probably at least one network device that has that IP manually assigned. Take the system offline that gives the IP address conflict message. Ping the IP from another system. If you are lucky the ping will report back the DNS name of the network object

Then set that device to obtain an IP address automatically, or set a reservation or exclusion for that IP on your DHCP server

Thanks hwood for the advice. The static IP reservations I use do not fall within the available range/scope configured for the DHCP pool, so that kind of conflict is unlikely, but I will check anyways.

The IP address coflicts seem to always be this scenario: one client boots and successfully receives a lease. Within 24 hours another client will boot and receive the same IP lease, which of course causes the conflict and freezes each station, assuming both machines are powered on at the same time.

LBMiller:

Have you manually verified that each of your client systems are set to "Obtain an IP address automatically?" If formerly systems were setup with static IP addresses and then switched over to DHCP, it is fairly easy to "miss" one or two in the changover.

In seven plus years of working with ethernet networks I have never heard of a DHCP server giving out the same IP address twice in the same lease period. Every time I have heard of a similar circumstance, anywhere, eventually it was tracked down to at least one IP address being set manually, or more than one DHCP server being used and somewhere there were overlapping scopes of IP addresses.

Hi hwood,

Looks like your suggestion is the likely cause. I will manually verify TCP/IP settings on all devices in our network once and for all (take some time!). Thanks alot for the help, I'll post my results if I uncover anything unusual.

Another cause for duplicate IPs assigned by DHCP may be the WINS server. If your network is connected to other networks and you are using a WINS server, make sure the replications processes are going fine. If they don´t, then one of the WINS servers (not necessarily yours but another one in the network) may contain already old mappings which weren´t deleted in due time because, precisely, of a bad replication.

The consequence will be that where *your* DHCP thinks a specific IP number is free for use, there will be a WINS mapping pointing to that IP which wasn´t properly updated, and that will create the duplicate IP conflict.

Therefore, if you think your WINS is okay but suspect someone else´s (which is supposed to replicate with yours) not working properly, try to force your clients to use only your WINS by making your DHCP to assign it as both the primary and the secondary WINS server.

Hope this helps!

Andrés

To fix it just bring up the command prompt and type:

ipconfig /release

then type:

ipconfig /renew

The DHCP server will then give you a new IP address.

Andy, I didn't think about WINS, that's a good idea because last time I checked my protocol analyzer there were in fact packets from other devices/services/protocols/platforms outside of my WAN segment that reach us. I will checked/adjust the WINS settings on DHCP service. Then I will run my network monitor and catalog all visible devices and see about filtering all traffic where necessary, including tweaking our access-lists and software-based firewalls - I think your suggestion has gotten me started on another project. But of course before that I will fix the IP conflicts. Thanks alot to all for the help.