DHCP and IP addy question

My problem just seems weird to me. On our DHCP server when it hands out a certain IP address (always the same #) It randomly conflicts with other IP addresses. It claims the MAC address is the same even though it is not. I found this strange so I tried multiple computers using that IP address and I even manually used it and it keeps doing this.

Anyone know why just 1 IP address on a dhcp server would act like this? At the moment we have had to disable that ip address from being used.

Find more posts tagged with

Free for TechExams community: Cybersecurity salary guide

Compare cert salaries and plan your next career move

Button

Comments

Silver Bullet

What is acting as the DHCP Server. If it is a Microsoft server, then you can enable Address Conflict Detection.

loxleynew

Silver Bullet wrote: »

What is acting as the DHCP Server. If it is a Microsoft server, then you can enable Address Conflict Detection.

Yes still on server 2003. Will Address Conflict Detection actually add bad ip addresses to a do not use list? Also is it common for dhcp servers to have bad IP addresses? I cannot even understand why it would.

thanks for reply.

rwwest7

address conflict detection will attempt to ping an IP address before it assigns it. It sounds like you have a static somewhere on your network.

tiersten

loxleynew wrote: »

On our DHCP server when it hands out a certain IP address (always the same #) It randomly conflicts with other IP addresses.

The existing IP addresses are DHCP ones as well?

loxleynew

tiersten wrote: »

The existing IP addresses are DHCP ones as well?

Yea it always randomly chooses another ip address it says it conflicts with and they are all on the same dhcp server. Obviously it doesn't though because the other computer it says it conflicts with has it's own completely different ip.

As for the static ip, maybe we do have a computer out there with a static ip of that # but if we did why would it randomly choose computers to conflict with instead of listing a mac address not on the dhcp server?

Thanks.

Silver Bullet

I'm not sure that I'm deciphering your problem completely... but, you may have a rogue dhcp server on your LAN??

sprkymrk

loxleynew wrote: »

Yea it always randomly chooses another ip address it says it conflicts with and they are all on the same dhcp server. Obviously it doesn't though because the other computer it says it conflicts with has it's own completely different ip.

As for the static ip, maybe we do have a computer out there with a static ip of that # but if we did why would it randomly choose computers to conflict with instead of listing a mac address not on the dhcp server?

Thanks.

I think the "conflict" is always with the same IP address and mac address, the reason it appears to be differnet IPs is because once the conflict occurs, it assigns a new address to the client. This seems to be the case since when you exclude that IP from being allocated via DHCP the problem disappears. You need to check your switches MAC tables to find the conflicting MAC address, and trace it down to whatever host/appliance might be hard coded. Or just do a continuous ping against that IP and unplug cables one at a time until you get no reply...

I hope I understood your situation correctly, let me know if I missed something. Good luck.