Cable modems - how do they work type question

[DeadSeaSquirrels]

Ok, I just had to deal with an issue today that really sucked, since it caught me by surprise. Basically I knew somebody who had two cable modems in the house (the good old days of paying more). Anyway, for on one of the cable modems I setup a router, which then shared the connection between 2 other computers. The other cable modem connected another single computer straight.

Well basically I came in today finding that the ones connected to the router didn't work anymore (well the internet didn't). They were getting a response from the router, and the router had an IP address, and DHCP was working and everything, but for whatever reason the internet wasn't working. So I think to myself, well maybe because the other computer is plugged in now and if they only have 1 IP address, maybe somebody got booted.

Well I unplugged the other computer, restarted the router and its adjacent modem, and everything worked. And it appeared as though I had solved it.

But what I hate is, I was wrong. But not really sure what really happened. I guess I should have known this already, but it just kind of didn't register with me. Anyway, each cable modem is what doles out IP address. I mean they don't themselves, but the cable companies keep track of the cable modem's MAC and I guess they correspond a certain MAC to an IP address. So every cable modem has a "right" to get an IP address.

I am not sure what happened but my solution worked, eventually the people I did this for called the ISP because they just couldn't swallow the idea that they only have one IP address, which leaves the one computer far away from the router SOL if the router is on. So we spoke to the ISP, we did some figuring out and they did something, and the modem that connected straight to the computer worked again? I don't know why, it didn't work in the first place, but now I feel like an idiot and the peope who I helped probably thought I was an idiot too. hrmm.

So how does all this stuff work anyway. Is there anything else I need to understand?

 

[SuperMachoMan]

The cable modem converts information from analog form to digital form and vice versa, that is all. It has nothing to do with network addressing.

If you two cable modems set up on different lines, then they were each transmitting data for separate IP addresses. One has nothing to do with the other.

You modem or router probably just needed to a reboot.

 

[gunrunnerjohn]

I don't know what the problem there is, but there is no issue in having more than one cable modem in a single location. I've had two hooked up here several times doing some testing. I have several friends and a couple of neighbors that have come for tech support, so I have them bring everything, including the modem. We get it all working properly, and they carry it home. Never failed yet.

 

[DeadSeaSquirrels]

What about this IP thing. Can I have two IPs with only one modem, I believe so, but if I'm wrong let me know. In the old days that is how they did it right? And then you'd still need to get a hub or something to split the line. But can you have only one cable wire coming in and split it and pipe it to two different modems and then have each modem go to separate computers.

I mean I wouldn't do that, that is ridiculous but I am just wondering in terms of possibilities. If I did that would I get two IPs, or would that simply not work?

 

[gunrunnerjohn]

You don't have two IP addresses with one cable modem normally, however with Comcast locally, you can pay them for additional IP addresses. If you had two IP addresses from your ISP, you could then use a switch/hub and split it to two systems. I have a friend that does this, and uses the one to go directly to his server, and the other to a router for the other machines.

Of course, you can have a router and have many private IP addresses, all sharing the same public IP address for the basic single IP address cable rate.

 

[drquest]

Originally posted by: DeadSeaSquirrels
What about this IP thing. Can I have two IPs with only one modem, I believe so, but if I'm wrong let me know. In the old days that is how they did it right? And then you'd still need to get a hub or something to split the line. But can you have only one cable wire coming in and split it and pipe it to two different modems and then have each modem go to separate computers.

I mean I wouldn't do that, that is ridiculous but I am just wondering in terms of possibilities. If I did that would I get two IPs, or would that simply not work?

To have two active cable modems in one home you'd need to pay for service 2 times, since you are getting twice the bandwidth... it's similar to having two separate phone lines sort of....

Each modem can generally support many IP addresses but most cable companies only give you one and most will allow you to get an additional IP for extra $. The best way would be to put the modem on a router of some type, Linksys, Netgear etc... let it take your one free IP and then you can have just about as many computers on the backside as you need. Plus being behind a NAT router has some advantages such as worms and Internet baddies would have a tougher time getting to you.

mrpeabody

 

[JackMDS]

Computer work binary. So two modem no good. However 2 in the power of 2 (binary)
Works.

So if you have 4 Modems (2 in the power of 2) you can suck the whole Internet into your LAN. :Q

:beer:

Late Night Humor.

 

[jonmullen]

watch out the network guru has had a few too many

 

[Kashan]

The modem gets an internal IP address, you wont see it, dont need to see it, and dont care about it. The actual cable modem is just a bridge, that is all it is. The modem forwards DHCP request, when your PC boots up, it sends out the discover, and sees the modem, the modem then forwards that discover message down the line to your ISP, which sends its Offer of, the modem forwards that etc down the line to your pc/router whatever gets its IP address.

The modems itself keeps a log of the mac address's it sees, if you are running a router, no matter how many PC's you have behind that router it will only see the router's mac. Most modems have a limit on the mac address's it can have in memory, Usually its 8, although some can hold more, and some less. So for instance, you have your modem, running to a hub, with 8 PC's attached to it, and a router attached to it. The modem can only have 8 MAC addresses in its arp table, so it disregards the 9th pc and that PC will NOT get an IP address(note: This is based of a COM21 Doxport1110 modem).

If you have two modems, and are only paying for 1 level of service, that 2nd modem will not work unless you have a Cable company that has no idea how to run their network **Cough** comcast **Cough** You get 1 ip address by default, Ive done network support for 4 different cable modem companies now, and its been like this at all 4. You can get an additional IP address for $5/month(usually the standard) but if you have a router there is no point. Also, another thing, is even if you do get a 2nd modem, make sure they run a 2nd dedicated line for that modem, if not that 2nd modem is gonna run like poopoo. Unless you are in a brand new complex.

Small office home office routers lock up constantly, they need to be rebooted every know and then. If you are running a router, this is how it works.

Your router gets the actual IP address, then the router just does NAT for an internal address of 192.x.y.z for your other machines. You only need 1 IP address if your running a router. Most cable modems can handle up to 8 ip address and 10 Megabits of throughput.

Ive worked for cable modems for the past 7 years, any questions drop one Ill do my best to check back.

Note: Im pretty drunk and high right now, so I hope this is right, I think it is, it sound right, but took me like 2 hours to type!

 

[p0lar]

Often, this is part of the DOCSIS standard as well, both for accounting and DHCP relay. You *CAN* have your cable modem pick up an address on the cable-modem0 interface, have a look at Cisco's uBR900series cable modems; however, many cable companies will only issue one DHCP lease per line (unless misconfigured).

What probably happened was they tightened down their policies down the wire one day, figuring that no one was actually [ab]using that particular exploit/policy and you got nailed by it. You called, complained, and they let you continue your [ab]use.

There's more to this than meets the eye, but it's not worth it for the purpose of this discussion!