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I have a WRT54G router that is set up to serve out addresses via DHCP. It correctly serves up addresses to every other machine on the network, including another PC, my macbook when connected via wireless, my wife's notebook, and our printer. However, whenever I attach my macbook to the router via an ethernet cable, the address it is given via DHCP is wrong.

My local network is set up as 192.168.1.*.

However, when my macbook connects with an ethernet cable, it is given the IP 192.168.29.*.

Currently, I have the macbook set up with a manual IP address, and all seems to be working fine.

Any ideas on what could be causing this?

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What address range does your Airport connection on the macbook get? – Matthew Schinckel Jul 22 '09 at 3:41
up vote 15 down vote accepted

Are you sure there isn't a second DHCP server on the network?

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This is a good point - sometmes DHCP can be provided from unexpected directions. As a test, try powering off everything else except your router and your macbook, then force a DHCP renewal. – Bevan Jul 22 '09 at 8:28
Good suggestion. I think there might be a way to see which DHCP server provided the address. – Matthew Schinckel Jul 24 '09 at 0:14
Turns out that my linksys hiport was plugged in backwards and was trying to dole out IP addresses instead of my router. – Barry Jul 27 '09 at 14:39

Is your router setup for two different subnets?

You can 1) Check the subnet mask or 2) Just plug another device into the same physical port where you plugged your MacBook and see if it's also 192.168.29.*

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Do you have a "static dhcp" reservation setup for the MAC (ethernet) address of your MacBook?

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On both of my ADSL Wireless routers (one US Robotics, one NetGear), there is a way to review which DHCP leases have been granted - a list of MAC addresses, the assigned IP address, and the life of the lease. My Netgear router calls this "Attached Devices".

When you review this with your LinkSys WRT54G, does your Macbook show up?

This will allow you to identify whether the router is supplying the odd IP address, or whether it's coming from elsewher.

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I had a similar issue. I had plugged in my Vonage adapter to my router using the "PC" connection instead of the "WAN" connection. The "PC" connection was acting as a DHCP server on the network and assigning IP requests to devices, even to devices that were connecting via the Router's wifi connection! How crazy is that?

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The answer to the question is simple: user error. My linksys hiport was connected incorrectly and was trying to dole out IP addresses instead of just acting as another device on the network.

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