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I currently have Time Warner Cable Wideband Internet service. This uses a SMC Cable Modem (Model SMCD3GN).

If I plug my MacBook directly into the cable modem via Ethernet and restart it, I get an IP address in the range.

If I plug my AirPort Extreme router into the cable modem via Ethernet and restart it, I get an IP in the range.

Why does Time Warner change the IP range depending on which device I plug in, and how does the cable modem know that a different type of device has been connected to the cable modem?

Also, I seem to get much better performance on the range than - I can only speculate that perhaps a different set of routers/switches is handling each range?

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Have you questioned Time Warner about it? – Ƭᴇcʜιᴇ007 Nov 11 '11 at 14:03
As I will be calling them again soon (about un-reliability of said Wideband service) I will attempt to ask. Unless I can get bumped up to a higher level tech their call center reps don't seem to know much. – Darren Newton Nov 11 '11 at 14:05
The cable modem knows because it sees a different MAC address. If you clone the MAC address from one device on the other, does the IP follow? If yes, then you are certain that their edge router is doing it based on MAC addresses. The change in ranges is a bit more odd, usually a router will only have a set range and those are typically controlled by region. It is possible one of those is meant for a region much farther away from you and something is misconfigured on TW's end and you are getting some odd routing as a result. See what a traceroute yields from IPs to a single destination. – MaQleod Nov 11 '11 at 16:07
up vote 2 down vote accepted

Okay, I got an answer from a Time Warner rep on the DSL Reports forums. Basically, TWC does look at the Mac address of the device that connects, but they don't care what type of device it is. The IP is leased to a MAC Address for approximately 72 hours.

My AirPort router had been given the range address based on it's MAC.

When I plugged in my MacBook TWC saw a new MAC address and assigned it a range IP because it was available in the IP pool owned by TWC.

When I switched back to the AirPort, TWC saw its MAC and said "Oh Hai! You're back!" and gave me the address again because it had not been released yet.

I unplugged the AirPort for a few days and then plugged it back in and got a range IP.

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Who knows why they do it but they can determine what device you are plugging in (or have a reasonable guess at least) by the MAC address which does trace back to a hardware manufacturer/maker and certainly assign an IP address based off that.

It would be interesting to see what happens if you plugged a Linksys router in.

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