Those of us with separate modems and routers (some Internet connections have the two functions combined into a single device) are likely to focus on the router and ignore the modem. But, like a router, a broadband modem is a computer with a web based user interface. And, like any device with a web interface, the modem has an IP address.
You might think that to address a modem by its IP address, you would have to connect a computer directly to the Ethernet port of the modem. In fact, I have seen instructions for a cable modem that said to do just that. But this is not always the case.
A few years ago, when dealing with an ISP outage, I stumbled across the fact that my RCA modem was addressable at 192.168.100.1. I connected a computer directly to the modem and could see assorted technical details about the modem and its connection to my ISP. Not that my ISP cared.
After some more outages, I got tired of connecting a computer directly to the modem and tried
from a computer on the LAN. The request had to pass through the router, and, pass it did. From a computer with a local IP address of 192.168.4.5, I was able to communicate with the modem.
This is an interesting gray area.
On the one hand, from the perspective of the router, the modem is the outside world. Requests to the modem leave the WAN port.
On the other hand, the modem has an IP address that is reserved, by definition, for internal use only. No IP address that starts with 192.168 is allowed on the public Internet.
So, you might expect a router to detect the internal-use-only IP address, and block it from leaving the WAN port. But, that's not the way they roll (I tested with multiple routers).
I will never forget talking to a tech support person at my ISP (those days are over, its all computers now) who swore that 192.168.100.1 could not possibly be my modem (smug, felt I).
I made a note of this magic IP address and went on with my life. For years. Until my ISP raised the monthly rental fee beyond the point of reason.
I was not going to spend almost $100 a year, year after year, for an old modem with a retail value of maybe $50. So, I bought and installed a new Arris/Motorola modem.
One of the first things I did, was try the magic IP address. Sure enough, the new modem responded to 192.168.100.1.
Shortly afterwards, I was visiting someone and tried to access their Zoom modem with 192.168.100.1. It worked too.
At a third location another Arris/Motorola modem also responded to 192.168.100.1. Interestingly, this modem also responded to 192.168.0.1. However, the links to other web pages re-directed the browser back to 192.168.100.1.
Modems from three different manufacturers respond to 192.168.100.1. Could there be a standard that I am not aware of? After the Edward Snowden revelations, is it unreasonable to wonder if this is a back door?
Feel free to try this at home and report your results in the comments below.
Update: Feb. 19, 2015
I have now verified that a Linksys cable modem also responds to 192.168.100.1.
One item on the Zoom modem home page was a "CM address" of 10.11.22.33 (I have changed the last three bytes). Wondering if this was a valid IP address (and guessing that CM meant Cable Modem) I entered it into a web browser. Sure enough, the modem responded to this IP address too. Like IP addresses that start with 192.168, those that start with 10 are also reserved for internal use only.
This being a Defensive Computing blog, I suggest taking note of the normal state of affairs. Then, when things inevitably go wrong, you have a point of reference.
Perhaps make screen shots of the assorted modem web pages. If there is an error log, copy/paste the messages in the log to save them. Like the Windows event logs, some messages are surely normal.
You may also want to check the signal levels and compare them to documentation from the modem manufacturer.
And, while on the subject, it's also good to make note of the various lights on the modem when all is well. The top one is solid green, the next one is blinking yellow, etc. Here too, the idea is know what normal is, in preparation for something going wrong.
What else is there to say about a modem?
Turns out, they too can have vulnerabilities. More on that, next time.
Update Feb 23, 2015: See follow-up: Using a router to block a modem
Update: Feb 25, 2015. DSLReports has a list of modems and their private IP addresses. Most modems in their list use 192.168.100.1 but some use 10.0.0.1, 10.1.10.1 or 192.168.0.1.