Laptop being assigned IPv4 address out of router's DHCP range

Question

I have a laptop connecting wirelessly to an E4200 Linksys router. The router is configured to use DHCP with a range from #.#.#.100 to #.#.#.254. However, the laptop is being assigned an IP address of #.#.#.80 which is below the specified DHCP range. I'm not sure how this is even possible. Also, the laptop has Internet connectivity.

Out of curiosity I performed a ping -a on my laptop's assigned IP address from another client and it's showing a completely different host name.

Any ideas on why the laptop is being assigned an IP address out of the DHCP range specified in the router?

could you have another dhcp server running in your subnet? – johnshen64 Mar 28 '12 at 19:28 — johnshen64, Mar 28 '12 at 19:28
Make sure you are not using a static address as well.. – gparent Mar 28 '12 at 19:31 — gparent, Mar 28 '12 at 19:31

score 7 · Accepted Answer · answered Mar 28 '12 at 19:29

7

Look at the ipconfig /all output. What is it showing as the DHCP server. Is it coming from the router/device you expect? Perhaps you have a rogue DHCP server somewhere.

answered Mar 28 '12 at 19:29

Zoredache

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Precise answer. And correct IMO. – Tim Mar 28 '12 at 19:35
The DHCP server is different from the other clients. I wonder how I can change the DHCP server to point to the correct one? – Bryan Roth Mar 28 '12 at 19:35
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The problem isn't the laptop, it's the rogue DHCP server. – David Schwartz Mar 28 '12 at 19:36
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You need to turn off the other DHCP server. You generally should never have two running on the same network unless you have performed special steps to setup clustering. Multiple DHCP servers on the same network causes badness. – Zoredache Mar 28 '12 at 19:36
@Zoredache The DHCP server that the laptop shows is actually pointing to one of the two DNS servers we have in the network. The other clients are pointing to the router as the DHCP server. – Bryan Roth Mar 28 '12 at 19:39
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@BryanRoth you can't choose which DHCP server your clients 'point to'. DHCP is a broadcast protocol - the clients shout out asking for an IP address and if a server responds, great. As others have pointed out, you need to turn off the unwanted DHCP service. – Chris McKeown Mar 28 '12 at 20:55
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@BryanRoth If it's pointing to your DNS server then you know where your rogue DHCP server is :-) – Sander Steffann Mar 29 '12 at 05:13

score 2 · Answer 2 · answered Mar 28 '12 at 19:30

Your laptop probably had previously the #.#.#.80 address from a previous connection (it was connected elsewhere, or you just reconfigured your current router). After reboot, it asked to renew the ip address to the Linksys, which have the same ip address than the previous router. The answer was lost or something gone wrong, so your laptop kept its previous address.

You have internet connectivity because both LAN have same settings (same gateway, and same DNS which is usualy the gateway).

Laptop being assigned IPv4 address out of router's DHCP range

2 Answers2