Modem assigning IP instead of router!

August 26, 2011 at 21:55:50
Specs: Windows 7
Can someone please help?

I got a new modem today (a Cisco DPC3825) and it seems to be presenting a problem. I used to be able to simply and quickly change my IP address, by changing the MAC address though my Netgear router. Now when I do that, I can see the IP address has changed in the router settings, but when I double check it using, the IP address does NOT actually change, but remains the same.

Any suggestions or advice to get around this would be greatly appreciated. I am not very techie, and have no idea what to do.

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September 21, 2011 at 07:38:01
WAN IP assignement has nothing to do with MAC address of your router . Its just a facility that your ISP may have provided . The IP adrress lease from ISP for same MAC might remain same for even 7 days or more .
If you are sure that you have not subscribed from Static IP from ISP , just recycle the power of the Cable Modem, it should do your purpose

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September 21, 2011 at 15:04:57
Actually Vishal, the MAC address of your interface not only can, but in many many instances does have everything to do with your external IP address.

I don't know about where you live, but here in North America, the majority of ISP's have their DHCP setup to continue leasing the same IP to the same customer based on MAC address. If your MAC doesn't change, you keep getting the same IP. They do this because it's the easiest setup for them to configure and maintain.

In those cases, changing your IP is as simple as spoofing your MAC. If you were to check, you'd find most, if not all, SOHO Router's have a setting that allows you to change (spoof) your MAC.

Some ISP's take it one step further and have customer connections restricted to two (or sometimes more) MAC addresses. If you change hardware a third time, your new one won't get TCP/IP settings because it's MAC is not registered with the ISP. The customer then has to contact their ISP and have them delete the existing MAC mappings for their connection. Most of these types of providers actually have a place for customers to change their MAC mapping on the ISP's website. Which is to say, delete the MAC of a device that's no longer connecting so you can then add the MAC of the new device.

I recently replaced the combo modem/router/access point my provider initially gave me when I signed on with them 5+ years ago. Instead of a combo unit, I asked for a modem only and got it. I then bought a SOHO Router and spoofed the MAC of my old combo unit on the replacement router so I'd get the same IP.

It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.

***William Henley***

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September 21, 2011 at 15:18:18
ms-kris there is no good legal reason why you are changing your mac address to get a different wan ip. odds are your isp reviewed their logs and put a stop to it with a static assignment.

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September 22, 2011 at 20:15:13
Hi Curt
Thanks for your update . I live in India.. As mentioned in my reply too, here also ISP tries to leases the same IP to same MAC address. Due to scarcity of IP addresses , it is very uncommaon that you get the same ip just after a restart.

Again thanks for your update.

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