General cable modem questions

videobruce · Aug 18, 2006

I have had DSL for years and never had any experiance with cable modems. I have a few generic questions about them. The one in use is the Motorola SB5100 on what was Aldelphia, now Time Warner cable. There is also a Linksys router in the mix.

1. Adding a broadband amp (US made Winegard 40-1000MHz) seems to kill the modem. Is it because of excessive signal or is there a need for coverage below 40 MHz? Mind you, this isn't one of those cheap no-name Chineese made wonders. It is only a 10db gain, so I doubt it is a excessive signal issue. I would say there is around +5-10 dbmV of signal with the amp.
2. I was told (by TW support) that you have to leave the configuration of your NICs' to "Obtain a IP address automatically". Is this true? I perfer to assign each conputer with a static IP address. With DSL, I have no issue here, so I don't understand why cable would be different. What does the output of the router have to do with what the modem sees??

Hope all of that makes sense.

gingo · Aug 18, 2006

videobruce said:
2. I was told (by TW support) that you have to leave the configuration of your NICs' to "Obtain a IP address automatically". Is this true? I perfer to assign each conputer with a static IP address. With DSL, I have no issue here, so I don't understand why cable would be different. What does the output of the router have to do with what the modem sees??

It doesnt matter, if you're using a router they will have private addresses anyway. Althought I'm not exactly sure why you would want to configure static addresses instead of using DHCP that the router provides.

SavageBasher · Aug 18, 2006

gingo said:
It doesnt matter, if you're using a router they will have private addresses anyway. Althought I'm not exactly sure why you would want to configure static addresses instead of using DHCP that the router provides.

It is much easier to assign static ip's if you do any file sharing or remote desktop.

klingens · Aug 18, 2006

Cable works from a few MHz to roughly 890 MHz,so yes <40MHz is important: without it, no upstream.

videobruce · Aug 18, 2006

Althought I'm not exactly sure why you would want to configure static addresses instead of using DHCP that the router provides.

So I know where each computer is and it's one less thing that has to happen at boot time since the decision was already made. Isn't it the computer that is asigning the address to the NIC?

gingo · Aug 18, 2006

videobruce said:
So I know where each computer is and it's one less thing that has to happen at boot time since the decision was already made. Isn't it the computer that is asigning the address to the NIC?

The addresses handed out by your router's dhcp usually have a long lease time (you can configure this) so it isnt getting a new address everytime you boot up anyway.
Using static IP's is completely fine though, I was just wondering if there were any specific reasons why you needed to.

videobruce · Aug 18, 2006

if there were any specific reasons why you needed to.

Other than what I posted, no.

The addresses handed out by your router's dhcp

On the home page for this router there is a "automatic DHCP" I thought that was for the WAN side and for cable internet, just as the choice for PPPoE is for DSL. Actually it even says that in the side bar for the router (brief description).
So I'm confused here.

CGR · Aug 18, 2006

Here is the problem with Cable that I have found with Comcast... You can manually enter the IP they give you, however it needs to get the IP frist through dhcp. The problem is they probably dont want to give you a static IP address and if they try to send you a new IP or update the one they gave you then you will find they cant and after so many tries they will lock the account.

I finally just gave up arguing with them and setup my firewall to use DHCP to get the address. Didnt want my rig to be leasing the address.

For an extra $20/month they told me I could have a static IP address. So I was ecstatic about that... BUT... It turns out that it weasnt a true static IP address. All they did was assign me an addres with DHCP then set that address aside so I would always get that address. Only problem was I could only get it through DHCP I was unable to manually have it configured. Biggest scam I have heard of....

I would go back to a slower DSL connection in a heartbeat if I had it available

videobruce · Aug 18, 2006

If 'Comcast' and 'Compaq' would merge would that give us 'Comtrash'?

I'm told elsewhere that it's the MAC # of the orginal computer, not the modem that their HE looks for. Any insite here?

I would go back to a slower DSL connection in a heartbeat if I had it available

Luckly, I don't have to, I already have it. It's nice living IN a city where everything is close. As far as slower, their "High Speed" is 380kbps up and down
(actualy, 300kbps down and 320 up)!

I'll live with my 360 kBps down and around 90 kBps up DSL.

gingo · Aug 18, 2006

videobruce said:
Other than what I posted, no. On the home page for this router there is a "automatic DHCP" I thought that was for the WAN side and for cable internet, just as the choice for PPPoE is for DSL. Actually it even says that in the side bar for the router (brief description).
So I'm confused here.

I have been talking strictly about DHCP for your internal computers. Your ISP will give you one IP address that is going to be the external or WAN address of your router, typically this changes every so often unless you pay your ISP for a static address.
The DHCP functions that your soho router does is for your internal computers only, it has nothing to do with your outside address.

gingo · Aug 18, 2006

videobruce said:
I'm told elsewhere that it's the MAC # of the orginal computer, not the modem that their HE looks for. Any insite here?

Yes, when you first setup your modem for use with the cable company they record the MAC of the computer you are using to access their webpage. At least thats what ive seen with Comcast and Cox.
This is the purpose of the MAC clone feature in your router.

videobruce · Aug 18, 2006

Don't they have to know the MAC of the router to establish the connection?

gingo · Aug 18, 2006

videobruce said:
Don't they have to know the MAC of the router to establish the connection?

Nope, they need the MAC of your modem though.

xtrmeocr · Aug 20, 2006

99.9% of the time amps are a bad idea

all they are going to do is add noise and that is NOT what you want.

xtrmeocr · Aug 20, 2006

gingo said:
Yes, when you first setup your modem for use with the cable company they record the MAC of the computer you are using to access their webpage. At least thats what ive seen with Comcast and Cox.
This is the purpose of the MAC clone feature in your router.

only the mac of the cable modem is recorded.

all cable modems learn the MAC of the device connected to it and the default config file only allows for 1 device. to clear the learned address table all you need to do is reboot the modem, and then you may connect any other device.

Enablingwolf · Aug 20, 2006

Adelphia pushes the signal to you.

I use the service myself.

For the 5100 type in http://192.168.100.1/signal.html

It should be in the range of 36dB. Which I am told is the range they look for, actually I am in the sweet spot. The power level should be in the range of -5 dBmV.

They are really good on the internet side if you call them. I hate calling about the tv, but the web side is actually pretty good. They can can talk to your modem and get you ready for whatever you need. You might want to escalate to Tier2 support. Just tell them the general idea of what your doing so the next level will know. Plus it is how they bump up to the next level.

archangelgabe · Nov 11, 2006

gingo said:
I have been talking strictly about DHCP for your internal computers. Your ISP will give you one IP address that is going to be the external or WAN address of your router, typically this changes every so often unless you pay your ISP for a static address.
The DHCP functions that your soho router does is for your internal computers only, it has nothing to do with your outside address.

I had a further quetion on a similar matter. I just got broadband and the modem only allows for connection to one computer. I have a laptop and a desktop. I wanted to hook up my laptop once in a while to the internet, but it wouldn't let me. (I heard it checked the mac address to make sure you're not hooking the modem up to another computer.) I then changed the mac address on my laptop to the one on my desktop then hooked the modem up to the laptop. Still no connection.

What other things do broadband routers check to prevent other computer from using the connection. How can i get around this, or is a router the only way. (it'd be an investment for only a few uses.) [edit: I'm looking up how to make a crossover cable. I'm not famillar with the order.]

I'm currently looking up a crossover network. (just for 2 comps) But then my desktop would have to be running.

TempliNocturnus · Nov 11, 2006

archangelgabe said:
I had a further quetion on a similar matter. I just got broadband and the modem only allows for connection to one computer. I have a laptop and a desktop. I wanted to hook up my laptop once in a while to the internet, but it wouldn't let me. (I heard it checked the mac address to make sure you're not hooking the modem up to another computer.) I then changed the mac address on my laptop to the one on my desktop then hooked the modem up to the laptop. Still no connection.

What other things do broadband routers check to prevent other computer from using the connection. How can i get around this, or is a router the only way. (it'd be an investment for only a few uses.) [edit: I'm looking up how to make a crossover cable. I'm not famillar with the order.]

I'm currently looking up a crossover network. (just for 2 comps) But then my desktop would have to be running.

That sucks, they are using MAC authentication now. I believe they banned my MAC several months ago and all I did was change it. You got roadrunner right?

All I can think of is you spoofed your MAC wrong and it's not actually spoofed. I use a program called MacMakeUp when I need to spoof my MAC. Or you could be spoofing the MAC for the incorrect ethernet adapter (make sure it's not the wireless one if you're using copper to connect). And make sure you don't have a static IP set.

Just google ethernet crossover and click images to find out the correct order for a crossover.

greyharte · Nov 12, 2006

hhhmmm ... i have comacast ... i have my own linksys cable modem and linksys router ... when i registered my modem w/ comacast all they needed was the model and the mac. in my router i dont worry about ip's or dhcp ... i set up for mac filtering allowing ONLY my 2 comps, my tivo and my router to access the network. most major services use dynamic ip addressing ... and if you ever have to reset your router to default you loose all your settings anyways ... which means you have to re-assign your ip's ... using dhcp allows the router to assign for your network and your provider to assign for your modem ... i have no issue file sharing w/ this method and w/ mac filtering the network is pretty solid and secure

Adragontattoo · Nov 12, 2006

more then likely the cable modem has assigned the IP to PC A, when you disconnect it and then hook up PC B it doesnt have a spare IP to give it. Try unplugging the Cable modem and then plug it back in when the second PC is connected.

The router actually comes in very handy as they usually have a built in firewall as well. Wide open connections are nice till the port scanners find you unprotected.

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