Helping people with computers... one answer at a time.

I have a cable modem with USB and ETHERNET connections, and I've connected a computer to each. Both seem to work, and I get different IP's on each machine. But when I talked to the cable company they told me it's not re commanded and that it can even seriously harm the modem! Sometimes the machines get the same default gateway address and when this happens some things don't work. Is all this logical? Is it allowed to have 2 IPs?

I'm a little suspicious about the threat of "serious harm" to your modem, and I actually expect the gateway address to be the same.

But whether or not it should work, or whether or not you're allowed to have two IP addresses ... that's up to your ISP.

I'll start with the modem. Normally those that supply two different connection types, such as USB and ethernet, expect that you'll use one or the other, but not both at the same time. Some may, but I wouldn't count on it. In any case, it'd have to be a very poorly designed modem if trying to use both caused it damage. At worst I'd expect one or the other to simply not work.

"... whether or not you can have two IP address is totally up to your ISP."

Now, whether or not you can have two IP address is totally up to your ISP. They may arbitrarily limit it, typically to one, and usually charge more if you need more IPs. It's also possible that attempting to use two on the same connection can cause confusion, especially for incoming connections. Again, that's totally at the whim and configuration of your ISP. In general, though, unless you're paying for more than one IP, you can probably only use one.

Now, the gateway IP address is fairly simple. That's just the next computer or router upstream from yours that is the "gateway" to the internet. When you communicate to another computer on the internet, the packets you send are physically sent to that gateway as their first step out onto the internet. It's surprising to me that you got two different gateway addresses, but certainly not impossible.

My real recommendation in the situation you describe is to bypass all these questions and work with a more common solution that's designed specifically for what you're attempting to do: a NAT router. Routers are not that expensive, and basically sidestep all of your questions. They're designed to take that one IP address and allow you to share it with two, or more, computers.

It's a much simpler solution.

Article C2525 - January 19, 2006 « »

Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

January 21, 2006 9:14 AM

Can you tell me specifically if the Motorola SB5100 SURFboard Cable Modem will be affected?

January 24, 2006 1:25 AM

Hi Leo thank you for answearing my question! helped me figure out some stuff about my cable connection. keep up the good advicing :)

January 25, 2006 8:33 AM

Ah, disregard that first comment, I bought a wireless router yesterday and am doing fine on that path thanks.

April 17, 2006 9:25 AM

I was equally bothered by the fact that the Surfboard 5100 could be used with both the Ethernet and the USB connection, but the modem somehow is equipped with the ability with two gateways, thus making the two computers unable to communicate. There are two ways to work around this problem:

a) Disconnect the cable - No internet, but you can share files printers etc... The modem will work like a DHCP router(single gateway) across both port. See the configuration pages:

The SURFboard cable modem can be used as a gateway to the Internet by a maximum of 32 users on a Local Area Network (LAN). When the Cable Modem is disconnected from the Internet, users on the LAN can be dynamically assigned IP Addresses by the Cable Modem DHCP Server. These addresses are assigned from an address pool which begins with and ends with Statically assigned IP addresses for other devices on the LAN should be chosen from outside of this range.

b) It is a treacherously slow experience, due to many lost packets(like a hundred to one) is to use VPN. I've somehow had better experiences with Linux(Puppy distro) than Windows XP. Note that you must disable your Firewall and configure your router(if your ethernet connection is not connected to a hub) for the correct ports.

It is best to get a router (40-100$) and connect everything through Ethernet. Besides it is much faster.

Alan Nitz
May 13, 2006 5:02 PM

i had the same problem, but my other computer said that the usb was bad or somthing and then proceded to open up a crap load of internet explorer windows, on the other hand my ethernet one runs fine.

May 22, 2006 8:09 AM

One thing I noticed on my Connections with a motorola sb5100 is that, even though my computer can be connected thru either usb or ethernet, my ISP authenticates the hardware address of the Ethernet card (to which it is currently registered) and so i cannot use the usb right now! (this maybe overcome by asking the isp to change my hardware address to authenticate my usb address and manually set my ethernet address to the same). But finally i still wont be able to connect two computers at any time since I am alloted only one IP address! So a router is absolutely necessary!
Can someone help me clarify my doubt here? Can I connect my ethernet modem to a hub and the rest of the lan also to that hub and use my internet? I have only one IP address alloted for me!

May 22, 2006 8:11 AM

Something similar has been mentioned here: .... Though i am not sure if it works!

July 30, 2006 1:19 PM

I'm trying to connect 2 computers to a Motorola sb1500, one by USB and the other by LAN via a router, however, the LAN connection won't work. Is this because the original connection was via USB? I can't connect both via the router as only one has an ethernet card so I have to keep swapping the USB over - any help is appreciated!

September 2, 2006 8:13 AM

Dear Paul,

The motorola SB 5100 requires that you connect the router to the cable modem (both ends RJ-45-Ethernet), in ur scenario through a cross over cable. What you r doing is using a srtaight thro standard cable. Post it whether this solves ur problem or not.

January 7, 2007 7:09 AM

Im trying to set up my own network with a lap top to my computer. The computer is connected via usb through its modem. When installing the new router the connection is ethernet, as soon as the usb is disconnected and the ethernet plugged in I no longer have internet connection and cant install the router properly. All the lights on the router are working barring the internet light. Do you have any suggestions as to whats going on?

April 6, 2007 9:07 PM

At the moment i have 2 computers sitting next to each other with a surecom EP-808X Switch, and a SURFboard SB5101 Cable modem, I have a Eternet cable going into the switch and two ethernet cables going from each computer into the switch, i can connect on each computer. BUT! I have to dissconnect one because i can only use one I.P. Otherwise they will cancel each other out, how can i fix this problem, i have tried to connect to my modem to add more I.Ps connect but can't find a tab that applies that.
:D Thanks mail me

Leo A. Notenboom
April 8, 2007 9:42 AM

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You'd have to talk to your ISP to get more than one IP on the internet.

It's much easier to replace your switch with a router and share the one IP you

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October 18, 2007 12:30 PM

I cant get an ethernet cable to work with my pc. im trying to connect to a motarolla 5120 but only usb will work .anyone have any suggestions?

October 24, 2007 9:26 AM

listen my usb connection is deleated and how do you get back the usb connection and i want it install back on my computer help me please

Allan Cameron
September 10, 2008 3:54 AM

I tried this tonite on my ADSL modem to 2 computers
At first it didnt work but then Vista computer asked If I wanted to diagnose/fix and then it worked far so good!!

USB-man :)
October 24, 2008 6:48 AM

i've recenlty bought everything required for "Xbox 360 Live" - "which connects you to the internet through computer" , but i think i've bought the wrong ethernet cord, its an "USB cord" and i'm wondering, can i connect it to my "Computer and my Model6100" and still get internet, And i've bought an 5 port ethernet router.

Jabril hopkins
March 14, 2009 7:49 PM

I have a cable modem with USB and ETHERNET connections, is it possable(because i can not seem to correctly use the usb slot on the modem) to connect the computer and a xbox 360 and run the internet on both while using the cable and usb slot?

May 25, 2009 8:59 PM

I have a SpeedStream 5200 router. Before I had two computers connected, one through both USB and ethernet. I just switched computers though and now the one connected through USB is having major issues. I'll get about 5-10 seconds of normal internet, then drop down to near nothing for about a minute, and it'll cycle like that. It's horribly slow and frustrating. What could be causing this?

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