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The internet setup at my home is as follows:

I have a cable modem (192.168.100.1) connected with a coaxial cable. The cable modem is connected to a wireless router (192.168.1.1) via an ethernet cable. I'm using internet on my laptop by connecting to the wireless network.

Entering the aforementioned IP addresses I can access both the cable modem and the router. Everything seemed fine but when I accessed the cable modem page (192.168.100.1) I noticed that there was a field there that said "IP Address: 10.199.0.77". When I enter this address in my browser I see the same page as 192.168.100.1. "10.199.0.77" is NOT my WAN IP address, which is x.x.34.56.

Why does my cable modem have multiple local IP addresses?

  • From what I know (from a German cable ISP): if your router is not provisioned you get a 10.x.x.x IP address (private IP range). After provisioning you get a real public IP. So can you double check when and where you read the IPs? – Werner Henze Feb 6 '15 at 10:45
  • @Werner Henze I read the WAN IP from the router's page (192.168.1.1) and the modem's page (192.168.100.1). As for 'when', I'm presently connected to the internet (i.e have a public IP) and I can still access my modem using both 192.168.100.1 and 10.x.x.x . – mrsinister Feb 8 '15 at 7:41
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    And on which page did you read the 10.x.x.x IP? What is the name of the page, what is the context, the text before the IP? This might help understand what the IP is used for. – Werner Henze Feb 9 '15 at 9:32
  • It's probably just natting from your ISP? 10.199.0.77 will be your inside local address and 192.168.100.1 is just your local network. – CustomX Feb 12 '15 at 15:05
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There is a difference in definitions on your LAN, the ISP WAN, and your Internet address.

  • Your LAN address is where your computers are.
  • Your ISP has a WAN. 10.199.0.77 is you on their WAN. Usually this would be transparent to you.
  • Your ISP translates their WAN address (you on their modem) to their internet address used by you x.x.34.56.
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    It's called double NAT, and it seems to be getting more common. Basically the cable modem has its own built-in NAT, and then the home router has an additional NAT. – Charles Burge Oct 30 '17 at 17:36
  • @CharlesBurge Exactly. Thank you for clarification. – CoreyJJohnson Oct 30 '17 at 17:45
  • @CharlesBurge so in this case it sounds like the ISP is using a CGN, so it sounds like the local router is doing NAT, and then the ISP is doing NATting as well on their end. Am I understanding that correctly? Rather than there being two NATs from the router and gateway doing both. – rb612 Aug 16 at 20:29
  • No, not necessarily. It probably just means that the residential cable modem has built-in NAT, with a public IP address on the outside and a private address space on the inside. If you attach your own NAT device to such a cable modem, then you're NATting twice. – Charles Burge Aug 17 at 1:04
  • @CharlesBurge It seems like it's very possible that maybe the IP address that OP is referring to is actually the "HFC IP address" which starts with 10. (see here for example). Hence there's no double NAT necessarily but rather that extra IP is just the IP given to the modem internally by the ISP, which is what CoreyJJohnson was hinting at. What are your thoughts on that possibility? It sparked my interest so I did a bit of research. – rb612 Aug 17 at 5:03

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