DHCP IP address conflict using Wnr2200 as wireless access point in a Windows network

[DavidePetta]

Hi everibody,

I'm using a WNR2200 router only as a wi-fi access point to our lan, which has already a DHCP server.

The lan DHCP server is excluding some address in a range (e.g. from .50 to .100) since these addresses are used by some server to give them a static IP in our lan. WNR2200 too has its lan address in that range.

DHCP server in WNR2200 is disabled, so any device wi-fi connected should use the lan DHCP server, but in the last days some wireless attached device has an IP in that excluded range (e.g. a smartphone connects and gets the .74 IP), so sometimes we have an IP conflict.

We haven't changed anything both in the WNR2200 configuration and in the DHCP server configuration, but this wrong ip assignment is still happening also after a server and /or WNR2200 reboot.

How may I prevent the WNR2200 access point to assign a "forbidden" IP to a wi fi connected device?

Thank you in advance for your help.

[doraemon · Accepted solution]

Time for you to examine your network.

Where's the .74 coming from?

If you suspect that the wnr2200 is providing an IP even if its dhcp is disabled then turn off all routers/dhcp server and leave the wnr2200 turned on for you to see.

Connect a pc or a device to it, if it gets an IP then that's your culprit but then if not, check your network and retrace your connections.

[doraemon · Accepted solution]

Time for you to examine your network.

Where's the .74 coming from?

If you suspect that the wnr2200 is providing an IP even if its dhcp is disabled then turn off all routers/dhcp server and leave the wnr2200 turned on for you to see.

Connect a pc or a device to it, if it gets an IP then that's your culprit but then if not, check your network and retrace your connections.

[DavidePetta]

Thank you for your hint, it has been very useful:

I've turned off our DHCP server, but WNR2200 attached devices were still able to obtain an IP address (and of course these addresses were also in the "forbidden" range).

I've used the DHCP Find tool and I've discovered that there's another (unexpected) DHCP server in our lan: the ISP router.

Since we're not allowed to configure that router I'll call our ISP to make them turn off that DHCP service.

I hope that this will fix our problem.

[TheEther]

If you or the ISP is unable to completely disable the DHCP server on the router, then you may still be able to effectively neuter it.  Reduce its DHCP range of IP addresses down to 1 address, such as .2 to .2.  If the ISP router support DHCP address reservations, then you can map .2 to a non-existant MAC address.  This way, no device will ever receive an IP address directly from the ISP router.

[DavidePetta]

Ok, our ISP has disabled its DHCP server on the router.

Everything seems ok right now: no address conflicts, no forbidden address given.

We'll monitor our lan in the next weeks to watch for similar problems.

In the meantime, thank you all for your help!