I’ve connected a computer to each. Both seem to work, and I get different IP’s
on each machine. But when I talked to the cable company they told me it’s not
re commanded and that it can even seriously harm the modem! Sometimes the
machines get the same default gateway address and when this happens some things
don’t work. Is all this logical? Is it allowed to have 2 IPs?
I’m a little suspicious about the threat of “serious harm” to your modem,
and I actually expect the gateway address to be the same.
But whether or not it should work, or whether or not you’re allowed to have
two IP addresses … that’s up to your ISP.
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I’ll start with the modem. Normally those that supply two different
connection types, such as USB and ethernet, expect that you’ll use one or the
other, but not both at the same time. Some may, but I wouldn’t count on it. In
any case, it’d have to be a very poorly designed modem if trying to
use both caused it damage. At worst I’d expect one or the other to simply not
totally up to your ISP.”
Now, whether or not you can have two IP address is totally up to your ISP.
They may arbitrarily limit it, typically to one, and usually charge more if you
need more IPs. It’s also possible that attempting to use two on the same
connection can cause confusion, especially for incoming connections. Again,
that’s totally at the whim and configuration of your ISP. In general, though,
unless you’re paying for more than one IP, you can probably only use one.
Now, the gateway IP address is fairly simple. That’s just the next computer
or router upstream from yours that is the “gateway” to the internet. When you
communicate to another computer on the internet, the packets you send are
physically sent to that gateway as their first step out onto the internet. It’s
surprising to me that you got two different gateway addresses, but certainly
My real recommendation in the situation you describe is to bypass
all these questions and work with a more common solution that’s designed
specifically for what you’re attempting to do: a NAT router. Routers are not
that expensive, and basically sidestep all of your questions. They’re designed
to take that one IP address and allow you to share it with two, or more,
It’s a much simpler solution.