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I just purchased a static IP package from my ISP, and was told the IP was something like 24.x.x.x. And they assigned the static IP to the modem.

So, if I plug my cable modem directly to my laptop, and google for "what is my IP", it indeed shows 24.x.x.x.

However, if I put the cable modem LAN cable in my router, and then connect my laptop through my router WiFi, and google for what my IP is, I get a completely different IP, something like 205.x.x.x.

Why is this happening? The whole goal of purchasing a static IP was that it was the same all the time, and I could just setup port forwarding as I see fit in my router. Can anyone please explain? At this point I'm not even sure the 205.x.x.x is a static IP address, and I don't believe so.

If it's that way, is there a way to make sure that even if I go through my router, that it gets the 24.x.x.x IP address?

The setup on my router is set to "Automatic Configuration DHCP". And while discussing with my ISP tech support they said that they could see the static IP actually assigned to the modem. What I wonder is, if I change the router setup to static IP, and specify it, what'd happen with the modem, if they are thinking/looking to assign the IP to the modem and not the router?

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thanks for the edits slhck – silverCORE Feb 4 '12 at 15:08

1 Answer 1

up vote 7 down vote accepted

The modem is operating in bridge mode. When you are plugged modem directly with the laptop, it is the laptop MAC address that they see, and that is the MAC address they statically assign the address to.

When you plug in the router, they see a different mac address, and assign a dynamic address to it as it isn't the one they have configured.

All you need to do is tell them you have a new device plugged into your modem and want them to update the MAC address (do this while the router is plugged in).

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makes sense Paul, the weird thing is i never had the router unplugged, so they should've seen the mac of the router and not the laptop, no? even if they saw both, wouldn't they have asked which one was the router? they did specifically mention that the modem was the one being assigned the static ip, although i dont really trust tech support much – silverCORE Feb 4 '12 at 8:25
the reason i mention this is because when i called to change from dynamic ip to a static one, i was behind the router either they saw multiple mac's and just chose to assign to either the modem or the laptop, or they explicitly assigned the ip to the modem.what do you think? – silverCORE Feb 4 '12 at 8:27
@silverCORE if the laptop was behind the router, then it is difficult to see how the MAC address of the laptop leaked through - any MAC addresses are limited to the local broadcast domain, so they shouldn't be able to see it. It might be that the MAC reserved for the IP was reset to the laptops MAC when it was plugged in directly, and it won't revert to the router one until some timeout. It is probably best to give them a call and see what they think. Are you power cycling the modem each time? – Paul Feb 4 '12 at 11:16
thanks for the clarifications Paul, I'll give em a call, but what you said makes total sense. – silverCORE Feb 4 '12 at 15:08
just to update the question: my ISP never specifically said I had to update the router configuration to Static IP instead of Auto configuration. I had my suspicions, but I had read in a couple of pages that even though I had static, the configuration had to be for dynamic. Anyways, after changing the router configuration and power cycling both router and modem, it is now the router who was the static IP. Thanks. – silverCORE Feb 4 '12 at 20:34

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