Cable Modem IP Different Than My Public IP

[Piyono]

My Hitron CDA-35 cable modem (available only through ISPs; purchased from Rogers) reports a WAN and gateway IP addresses that bear no resemblance to my public IP.

The modem's IP addresses are in the public address space; why would the ISP give me one IP to connect to them and then another to connect to the public internet?

Out of curiousity I looked up the modem's IP and it is listed as belonging to DoD Network Information Center in Columbus, Ohio.

I haven't done much more research on the matter but the records appear to be current as of 2020-12-1, at least on the sites I used. I suppose it could be that the DoD sold a bunch of IP addresses and the records were not updated.

 

[sechs]

CGNAT?

I wouldn't worry about the IP information. Even if the owner hasn't changed in years, I've seen them be wrong.

 

[ArAfGo]

As I start to poke around in other tech posts/threads I ran into this thread and I am quite perplexed by a tech thought process that what IP address is showing in a given context is unimportant? Now that use of the vocabulary "context" isn't quite right, but I didn't want to use configuration.

Still, what IP information that is being shown to the rest of the entities on the Net is not some "Do Not Worry" type of situation, in my view.

I wonder if any one else agrees with me?

 

[sedrosken]

Because in this case CGNAT is making it so that the IP the modem is getting is different from the IP being shown to remote hosts -- CGNAT is the way ISPs are still getting mileage out of IPv4 and its limited addressing.

The way it works is basically like how your home network is behind your router but can access the internet through your router, except one level up. You wind up sharing a public-facing IP with several customers in this scenario. In this case, his modem's IP is like the local IP of your computer, and the public IP is like what your router would have on its WAN interface were it to not be under CGNAT.

It's very common for mobile phones especially as you're never expected to need to forward ports through it, and it's usually Good Enough™ for most home users as they unlike us power users have no use for port forwarding as they don't self-host anything.

The reason we say "don't worry about it" is because the public-facing IP is the carrier's problem at that point, in the event of, for example, a DMCA notice. You can ask for your own IP if your ISP defaults to CGNAT, but this usually comes at an additional cost.

 

[ArAfGo]

My goodness, sedrosken, you're good at this summary stuff. Second one in such a short period of time.

Thank you.

Just thinking; you should work in public relations.