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In the case where an IP address is assigned to a cable modem and utilized by computers on the home network through a router connected to that cable modem, is it technically incorrect to claim that a TCP/IP connection was established with this WAN IP address, since it was assigned to the cable modem and TCP/IP connections can be established only between programs running on computers or other devices like tablets and so forth?

In other words, if somebody claims they established a TCP/IP connection with an IP address assigned to a cable modem, would it be technically correct to tell them that this is not possible since a cable modem cannot do that?

Links to standards or literature that confirms this highly appreciated.

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What makes you think a cable modem isn't running any software? Hell, there are managed switches and motherboards out there that allow IP connections to them... Also, you have to consider visibility in case of techniques like NAT. –  Michael Kjörling Sep 22 '13 at 20:09
    
<pedantry>If you really want to be correct, a connection cannot be established with an IP, it can only be established with the hardware to which that IP has been assigned. An IP is just a number.</pedantry> –  terdon Sep 22 '13 at 20:11
    
You can definitely establish a connection to a cable modem... It is a network device and most have internal web servers for configuration. –  BrianAdkins Sep 22 '13 at 20:12

4 Answers 4

up vote 2 down vote accepted

Cable modems typically function as bridges and not as hosts. This means the traffic passes through unmodified on Layer 2 or higher.

I guess one might use the phrase "connected to his cable modem" to mean "connected to the device directly behind the cable modem." but it is technically inaccurate the way most people would usually mean it.

Nine times out of ten a cable modem's ethernet interface is connected to a router (or a single computer), and that's what has the IP and that's where it's possible to make a TCP/IP connection.

If a cable modem has a built-in router, the actual "cable modem" part of the device is a bridge, bridging to an internal router.

Some cable modems have management or diagnostic webservers accessible via TCP/IP on the LAN side (192.168.100.1 is typical), but you usually cannot get to that from the WAN side.


DHCP, the protocol that assigns an IP address, uses UDP. I don't think TCP is involved at all. So if no other network traffic has happened since DHCP gave whatever device on the other side of that modem an IP, no TCP connections are established. UDP is part of TCP/IP so some "TCP/IP" traffic did happen though. Unless your cable ISP assigns the customer a fixed IP and the IP is already programmed into the device behind the cable modem. Then this would not happen.

TCP/IP connection can only be established between programs running on computers or other devices like tablets and so forth?

There are many devices that talk TCP/IP. Equipment at your ISP's central office can certainly intercept your traffic ("captive portal" type of thing - Comcast download warnings or Verizon authentication failure "interstitial" page, for example) and connect to whatever device is behind the modem. So it is possible.

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I don't know what's typical, but I do know that one of the first things I figured out how to change on my Zyxel ADSL modem was its networking mode, changing it from NAT gateway (which is the default configuration) to bridge mode. The one I had before that didn't offer NAT at all; if you wanted NAT, you hooked up a NAT router of your own to one of its LAN ports. Even when the modem is configured in bridge mode, my ISP is able to connect to it (I have confirmed this in talking to their support department). It probably (hopefully!) uses RFC 1918 space to do so, but it may be non-IP as well. –  Michael Kjörling Sep 22 '13 at 20:22
    
Okay, so in this case the IP would be assigned to the router actually but even if that was the case it is still incorrect to claim that a TCP/IP connection was established with the IP address. Right? –  Dean Kuga Sep 22 '13 at 22:37
    
Please see edits. –  ultrasawblade Sep 23 '13 at 0:49

Even after putting aside the fact that cable modems can establish TCP connections, no, such a claim is not incorrect.

What is incorrect is your assumption that the phrase "connection with an IP address assigned to a cable modem" automatically means "connection to the cable modem itself". Those are two different things.

The other end does not know, and does not care, whether the cable modem generates TCP packets by itself, or just rewrites the headers on packets received from your computer.

And as far as the other end of the connection is concerned, the claim is completely true: there is a connection between an (IP address, TCP port) pair belonging to the other end, and an (IP address, TCP port) pair belonging to the cable modem.

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To the device on the public side of the NAT gateway, it looks like a TCP connection was established to a certain TCP port on the public IP address of the NAT gateway box. There's no way for it to know that the box it connected to is a NAT gateway, and is translating and forwarding the connection to a host on the private side of the NAT.

I suppose from a TCP perspective, the TCP endpoint is a two-device cluster. The NAT gateway and the host behind it together form the TCP endpoint.

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You are asking two different questions, here.

Is it tecnically incorrect to claim that TCP/IP connection was established with an IP address?

As @terdon pointed out in a comment to the question, a TCP/IP connection is made to something that has been assigned a given IP address. If you want to be technically correct, that is very different from establishing a connection to "an IP address".

The IP address is like a phone number, whereas the device is the phone that is hooked to a phone line that has been assigned that particular phone number.

if somebody claims they established a TCP/IP connection with an IP address assigned to a cable modem would it be technically correct to tell them that this is not possible since a cable modem cannot do that?

No. What makes you think a cable modem doesn't run any software that can accept (or for that matter initiate) such a connection? A cable modem isn't just a bunch of transistors, dumb ICs, capacitors etc., it's a small computer in its own right, running software. What that software does is up to the manufacturer.

For one, my own modem (which strictly speaking is an ADSL modem and not a cable modem) can be configured in either bridging or gateway (NAT) mode, along with having a whole slew of other configuration options as well. That would be impossible if it was just a "dumb" modem that sat between two electrical interfaces and translated signals from one form to another.

There are even managed switches and (mostly if not exclusively server) motherboards out there that allow you to make IP connections to them for monitoring and configuration purposes.

You also have to consider the effects of visibility. If someone connects to an IP address and the host at that IP addresses (which may be a cable modem, home router, personal computer, printer or what have you) forwards the connection to a different IP address, to the one who connects it looks like they are communicating with the IP address they established a connection to, even if the traffic is forwarded by that host to another IP address. This is the basis for how NAT works, for example.

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