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Comcast allows the home user to buy a second IP address. I bought a Cisco switch (I don't know whether it's Layer 2 or Layer 3) based on some advice I got. One port of the switch is connected to my router, which in turn serves my LAN, and another port is connected to a standalone computer. A third port is connected to my modem. I confirmed that both the router and the standalone computer get different external-facing IP addresses.

This setup has worked for me for months. However, I recently got a speed upgrade from Comcast and now only one node in the switch works at a time. They're sending a technician out to "disable a trap."

My question is, how is this setup able to work in the first place?

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Sounds like your router is working in modem mode and you have been restricted to a single public IP. If you enable NAT and DHCP on the router, your standalone computer and LAN will all recieve internal IPs from the router.

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  • When you say "router," do you mean my modem? The standalone computer isn't connected to the router, it's connected to the switch. This sounds like you thought I said something like "I have a router behind a modem and my computers connected to my LAN aren't getting internal IP addresses," which is not at all my issue.
    – user25366
    Nov 7 '13 at 21:09
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The cable modem must encapsulate the ethernet frame entirely, or just modify it to work on the cable network, so that their router knows the MAC addresses of your computers and addresses traffic to your house that way. I know this is possible because I had one of the first cable modems and it was connected to a hub to two computers with two IP addresses (so, there wasn't even any switching going on).

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  • So the switch -- or in your case, the hub -- doesn't have a notion of its own MAC address, right? It just exposes each MAC address of each device connected to it? Then, I imagine, Comcast picks a MAC address and assigns it an IP address?
    – user25366
    Nov 7 '13 at 21:16

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