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Bugs & Fixes: Bridge Over Troubled Modem

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#1 TheSentinel


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Posted 25 December 2011 - 11:34 PM

Bugs & Fixes: Bridge Over Troubled Modem

By Ted Landau, Macworld Dec 24, 2011 4:39 pm

In the beginning, I had a Comcast cable modem attached to my AirPort Extreme Base Station. All worked well. At least reasonably well. Most of the time.

But the time came when Comcast informed me that, if I wanted the higher speeds Comcast was promising, I needed to upgrade my modem to a DOCSIS 3.0 model. I readily agreed.

A Comcast rep offered me an SMC Wireless Gateway, which is a combination of modem and router. I hesitated at first, claiming that all that I really needed was a new modem. But the rep told me that, if I ever wanted to add Comcast voice (which, in truth, I had no intention of doing), I would need the Gateway. For some reason, I reluctantly agreed.

As the SMC Gateway is a router, it largely duplicated the functions of my AirPort Base Station. In fact, if I were so inclined, it would be possible to disconnect the Base Station and depend solely on the Gateway. I was not so inclined.

The upgrade process was not as simple as merely swapping the modem for the Gateway. Both devices use NAT to share a single IP address. This double NAT configuration is not a recommended configuration. To avoid this situation, the quick solution was to put the AirPort Base Station in "bridge mode." This meant that the Base Station would serve as a passive transfer point, leaving the main routing functions to the SMC device. Actually, I would still use the Base Station to route to my AirPort Express devices and other peripherals. But the Gateway would assign the IP addresses. To make this change, I launched AirPort Utility, accessed my Base Station settings and went to Internet -> Internet Connection. From the Connection Sharing menu, I changed "Share a public iP address" to "Off (Bridge Mode)."

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