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incorrect IPs and subnet being handed out but only sometimes

Posted on 2008-10-16
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Last Modified: 2012-05-05
This is a wierd one and keep in mind that it only happens on occasion. The setup is for a hotel and conference area.

Basic Setup:
Cisco 1841 router setup up as DHCP server to hand out IPs between 10.0.0.40 -254 (255.255.255.0 Subnet)
From Router ---> Dell 3048 switches to split to different rooms and Cisco 1121 APs.
The router, switches and AP's all have static IPs set in the 10.0.0.1-39 range

I have triple checked that all the APs are NOT setup as a DHCP servers and their "role in the network" are set as Access Points.
 
The issue is sometimes when guests (or even I) connect to an AP they are being assigned an IP of 10.0.0.19 (or ony one of the IPs below the routers DHCP scope) and a subnet of 255.255.255.224 !?!

Any ideas on where to begin troubleshooting this?
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Question by:huntleyj
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by:that1guy15
ID: 22732415
Someone is connecting a router/access point to your network that has DHCP enabled. I would assume since this is a hotel a guest has a small home router that is configured with DHCP and when they connect it to your network it starts passing out IPs. They are most likley using it to get wireless access to a computer or access for multiple computers through one connection
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by:huntleyj
ID: 22732516
Possible. Next few times it happens I'll look at the DCHP bindings on the router and record the MAC addresses and look for a common MAC Address. That's if it's the same guest.
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by:that1guy15
ID: 22732583
If it is always the same block of IPs then it could be an employee or a guest that stays frequently. Even though the 10.0.0.0 range is very common you would usually see the assigned ips all over the board.
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by:huntleyj
ID: 22732636
What I might do is change the whole IP scope to something less common, say 10.113.0.x . A bit of a pain but if it fixes it it will be well worth it.
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that1guy15 earned 500 total points
ID: 22732842
You can do that but it will not stop rouge DHCP servers from passing out IPs. But you will know when someone gets the wrong ip because they will not be able to communicate with anything.
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