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- So, I’ve got a cable modem now ((Motorola SB4200) and in another thread a while ago I mentioned how even though I have ZoneAlarm on all the time, I never get hit by P2P broadcasts anymore, or… -any other kind really. When I had the dialup, I’d see warnings regularly. Somebody mentioned that there’s a lot the cable company may be doing on their end to block this stuff… I haven’t noticed any strange behavior on the part of the PC, and I don’t have any anti-virus programs but the online scans turn up empty.
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- So why does the Xmit light on the cable modem go on every 1-2 seconds? It’s done this ever since the guy first hooked it up. It even does this when the PC is turned completely off, and neither the network card or the BIOS is set to wake on lan…?
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Generally, what is happening is that your ISP is (and I know my explanation isn’t 100% true but it’s close) pinging your modem to ensure there is a signal.
My DSL modem is the same.
My ISP is making sure there is a modem to communicate with. In most internal networks this isn’t the case, meaning a server in a business is set up to recieve then transmit information only when asked but in an ISP environment, their servers need to know that you are active.
I don’t believe this to be the case with modems, but I don’t recall. It’s been a while since my networking classes, 1997 and 1998.
Since cable, DSL and wireless are on all the time, they need to keep that communication open. It is basically sniffing for your modem, easy explanation but not entirely true I imagine.
I think I am on target with this, if anyone has anything to add or correct, the floor is now yours.
Hmmm… my Motorola (unknown model) cable modem and Comcast service don’t do any evident pinging – when all of the computers are asleep, the TX light stays dark.
It could be that THIS PARTICULAR modem, though, doesn’t flicker for “internal” ping requests.
Hmmm… does my modem have an IP address that I don’t know about? I don’t have a clue how cable modems work, but I was under the impression they weren’t in and of themselves an IP device.
Bathisar
Any modem, be that cable, DSL, dial-up or wireless are always assigned some kind of IP address when they connect.
Some are temporary IP addresses like with most modems or routers. Mine happens to have a static IP because I have it set up that way.
Depending on your operating system you should be able to access your assigned IP address during any given session. On Win95/Win98 you go to the DOS command prompt and type in “winipcfg” (without the quotes) and on NT and Win2K you type in “ipconfig” (again without the quotes) and it will let you know what your assigned IP address is. This is regardless of the modem or router you use.
I am not sure about XP as I have little knowledge in XP’s workings…'spose I should get educated on it as I am sure that I will be thrown into that soon.
Nitpick: Modems don’t get assigned IP address. Other things like the network card or Linksys router you plug into the modem get an IP address. But modems are layer 1 devices and really couldn’t care less about addresses or packets or anything like that. If the box you call a “modem” really has an address, it’s doing more than just being a modem.
Of course, this doesn’t mean that the company providing the media for your internet access [this doesn’t have to be your ISP] doesn’t have some way to check for the presence of your modem, or that your modem isn’t checking periodically for a connection – both of which would cause your TX light to flash.
Also ipconfig and winipcfg will tell you the addresses of network cards in your machine and not the address of any router you might be connected to. [Which you might be able to discover using tracert.]
Servo
For all purposes, modems ARE assigned an IP address, in technical terms your “connection” is assigned an IP address but you can’t connect to the 'net without some kind of IP address but my explanation was a simplification of what is going on with his/her modem.
With my father’s cable connection, his modem is assigned an IP address. Adelphia records the MAC address and when you make changes it must be rebooted (when I installed his router to connect their home network) for at least 3 hours for their servers to release the MAC address and the IP address to reassign a new IP address to that MAC address when it comes back up. (not sure if that makes sense, been a long night.)
To the outside world all my computers are viewed as the same IP address externally but the connection is seen as a specific IP address. I run a NAT network so my 10.0.0.x is not seen to the rest of the world but my 63.x.x.x address is…this is my gateway address to the internet.
ipconfig and winipconfig give you the internet IP address that is assigned to you if you aren’t on an internal network or on a router. However a strict modem will get an IP address as it has no passage or gateway to the net except that which is assigned by the ISP…does that make sense?
Man I need some sleep.
I realized that you might have been oversimplifying, which is why I labeled my post a nitpick.
No. The only things that get IP address are devices that understand TCP/IP, and modems [by definition] do not. For example, I have a cable modem. Its ethernet interface is connected to my Linksys switch/NAT device/router and it also connects to the cable system just like my cable box does. Its only job is to take the bits it sees on the ethernet interface and get them onto the cable. It does this by modulating my data onto a specific frequency reserved for my cable company’s internet service [as opposed to the frequencies reserved for cable TV]. There’s no IP address here. The modem doesn’t know what TCP/IP is, it just knows, “I take this bit from this cable and I put it over here on this cable.”
If you were to look up my public IP address on my cable company’s DHCP servers you would see the MAC address of my Linksys, not any internal address my modem may have. [And it may have one, but it’s inconsequential to me and has nothing to do [directly] with my internet traffic.]
I don’t know exactly how Adelphia hands out its IP address, but if it uses a standard cable modem then they’re assigning an IP address to the device that’s connected to the ethernet interface of the modem. Not to the modem itself.
I understand what you’re saying but it’s, technically, not correct. In the case of no router then it’s the ethernet card in the computer that is being assigned an IP address. [And this is the address that ipconfig will report.]
Imagine this situation: I purchase a package from my DSL ISP that includes three dynamically assigned IP address. I connect my phone line to my DSL modem, and I connect a pure switch [no routing, no NAT, no nothing] to the ethernet interface of the modem. Then I connect three computers to the switch and, magically, they can all connect to the internet. Where are the IP addresses? They’re in each of the three computers. It’s the same thing with your strict modem situation only with two more computers.
With simple networks you can think of the modem as being assigned the IP address, but with anything even slightly more complicated it just doesn’t work.
Servo, where I am (I use comcast) the modems are indeed assigned an IP address. They have an IP address on a different subnet and range than your computers are assigned so the head end equipment can monitor them. I know they use SNMP (Simple Network Monitoring Protocol) to get information about my link status using the modem’s IP address
DougC the Xmit light flickering is probably caused by some idjit doing a “port scan” looking for an open port to hack into. Since most of your ports are off, if you are set up right (i.e., not using Windows XP default “all on” setting), then nothing happens and Zone Alaram won’t catch the ones to “off ports”.
Also, most ISPs do block you from running web and ftp servers on the standard ports (and will block you on non-standard ports if they catch it). So presumably they are doing some scanning or monitoring of their own.
MannyL,
You’re right, but that IP address is immaterial to your internet connection. It’s just there for management purposes. If you went to my web server I wouldn’t see that IP in my log files, but the one assigned to your computer [or Linksys router, or whatever].
I might also argue that that little box you have is more than just a modem. Modems modulate and demodulate, and that’s it.
You are probably seeing broadcasts and multicasts on the cable segment you are on. ARP requests are the biggest ones. This was really bad when Code Red/Nimda was starting out before my ISP blocked inbound port 80. Depending on the provider, you will also see DHCP requests from other machines in your neighborhood and OSPF routing advertisements from your ISP’s routers (bad ISP).
And regarding IP addresses on the cable modems… Most cable modems have 3 interfaces. A managed ISP side address (usually 10.x.x.x network) that they use to upload DOCSIS configs, etc., an internal customer IP address (depends on brand, but most I’ve seen are 192.168.1.100), and a third bridging interface that doesn’t have an IP address and just forwards traffic only destined for the MAC address of the machine(s) connected to the cable modem.
It may be immaterial but you said that the Modems don’t get an IP address, not that Modems don’t have an IP address that is on the same network as you.
It is possible that the LED is flashing due to the traffic on the internal (cable modem management address ) that is bleeding over. I remember reading an article that many routers and modems would flash the led in tune to the bits flowing and someone could monitor that to see your network data.
The problem here is that your “cable modem” is actually a modem, plus some other parts. It’s really those other parts that have the IP address. But we refer to the whole little box, lazily, as a “modem.”
Modems convert between digital data and analog waveforms that is all they do. This is the function that is common to all modems, it is the definition of the word “modem,” and it is what I was referring to in my posts. I’ll admit I should have been more clear on this point.
My problem was with this statement:
Modems are NOT assigned IP addresses, the computer [more accurately, the device you are using with a TCP/IP stack] is assigned an IP address.
Because any IP address a “modem” may have for management or configuration purposes is unimportant to a person trying to understand the basics of home networking, I left out that aspect of some “modems” to avoid confusion.
Just to clarify what I was asking outside the scope of the OP: I wasn’t looking for the “simplified” answer, and I know everything I need to about my local network administration. I was curious about the Comcast cable modem itself, i.e., how it communicates with the servers and whether it has its own IP address or some other technology – back when I had ISDN, for example, the modem wasn’t an IP device in its own right.
FWIW, I can get to my router over the 'net using my account name (my Comcast doesn’t use MAC addressing) acct#.warren.mi.comcast.net or something like that. I don’t do it often, so I don’t know. BUT it’s easier to remember (I suppose) than some dynamic IP address (which has been static for some reason since Comcast took over @Home).