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Why does my home network only work with one computer at a time?

If there is a conflict with your IP address, it could result in only one of the machines on your network working at a time.

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The network consists of a cable modem, a hub/router, CAT5 cable in the walls, and two computers. If I disconnect one computer at the hub, the other one works. If I disconnect that one and connect only the other computer, it works. But, when I connect both neither works. I suspect a problem with the two computers getting the same address. Is this likely? How do I trouble shoot this kind of problem?

I suspect pretty much what you’ve indicated: a problem in the IP address assignment. But exactly what problem depends on a few details. Details we can look at.

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Hub or router

First, let’s make sure that the “hub/router” is in fact a router. That’s important because most ISPs (and, notoriously, most all cable ISPs) will give you one and only one IP address. If you just put in a hub, then both computers are competing for the single IP address. Typically the first one to request the IP address will win and be able to work.

The solution there is to get a router. A broadband router will take the single IP address for itself, and then hand out multiple local IP addresses to all the computers connected to it. It’ll do some magic (called NAT, for Network Address Translation) to make sure each computer’s conversations on the internet are kept straight. Another side effect of using a NAT router is that you are very effectively firewalled and protected from several different internet threats.

Looking at the IP address

So let’s say you have a router, or your ISP will hand out multiple IP addresses. Or both. Let’s examine what your machines are seeing.

I’ll assume Windows 7 at this point, but much of this applies to pretty much any version.

Open a command prompt, and type “ipconfig /all”, followed by Enter. You should see something very much like this:

Windows IP Configuration

        Host Name . . . . . . . . . . . . : AskLeo
        Primary Dns Suffix  . . . . . . . :
        Node Type . . . . . . . . . . . . : Mixed
        IP Routing Enabled. . . . . . . . : No
        WINS Proxy Enabled. . . . . . . . : No

Ethernet adapter Local Area Connection:

        Connection-specific DNS Suffix  . :
        Description . . . . . . . . . . . : 3Com 3C918 Integrated Fast Ethernet Controller (3C905B-TX Compatible)
        Physical Address. . . . . . . . . : 00-C0-4F-79-84-6A
        Dhcp Enabled. . . . . . . . . . . : Yes
        Autoconfiguration Enabled . . . . : Yes
        IP Address. . . . . . . . . . . . : 192.168.1.104
        Subnet Mask . . . . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . . . . : 192.168.1.1
        DHCP Server . . . . . . . . . . . : 192.168.1.1
        DNS Servers . . . . . . . . . . . : 206.124.128.1
                                            206.124.128.3
        Lease Obtained. . . . . . . . . . : Tuesday, July 13, 2004 6:19:55 PM
        Lease Expires . . . . . . . . . . : Wednesday, July 14, 2004 6:19:55 PM

We’re first going to look at the IP address assigned. This listing, from my own machine, shows an IP address beginning with 192. That’s one of the address ranges reserved for private networks. Along with “Dhcp Enabled” being “Yes”, this implies that I’m behind a router, and that the router has assigned me my IP address.

If instead you see “0.0.0.0” or an address that begins with “169.”, then Windows was unable to get an IP address. “0.0.0.0” is no IP address, and a “169.” is an address that Windows will simply make up in order to have an address – but most things won’t work.

If you’re behind a router and you don’t get an IP address, then you need to confirm your physical connection, and more likely your router’s configuration. DHCP, the IP-assignment protocol, can be turned off on routers; make sure it’s on.

If one of your machines get an internet address beginning with something other than 192 or 169, say “156.” instead, and the second machine fails to get an IP address at all, then I’d bet that the first machine is connected directly to the internet, and that router either isn’t a router at all, or its DHCP has been turned off.

An interesting test is to go to both machines and note the IP addresses they’re using. On the machine that can’t connect to the internet, in the Command Prompt, type “ping ” followed by the IP address of the working machine. Chances are that will work, and indicates that the physical connections are all working. In fact there’s even a good chance that pinging in the other direction, from good to bad, will work if the failing machine has had a 169. address assigned to it.

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Comments

  1. I’ve also seen this problem occur when one of the computers is plugged into the uplink port.

  2. I’ve got a 4 port Linksys router that sits between my broadband cable modem and a linksys gigabite 5 port workgroup switch that has all the computers in our home hooked up to.

    Everything works fine except one computer. It has internet access all right; and everyone in the workgroup can see the computer in network neiborhood, but nobody can ping it or share files with it.

    The DHCP feature of the Linksys router has given it an IP address but no one can reach the computer.

    Can you please help me out with some concrete advice!!

    Ive also tried resetting and restarting all the computers as well as the router and workgroup switch several times.

    Unfortunately the printer is on this computer!!!

  3. I would rather not have a router if I didn’t have to, but just use a crossover cable from nic1 to nic2, and make the host computer have the net connection via usb/cable modem (Mot5100). The host always works and the kids (nic2) never has. Also having the host run XP and the nic2 run 98 is good for using older periferals and software. Could THIS be the problem?

  4. If I understand what you’re describing, it could work. You’d need internet connection sharing turned on for the internet connection.

    But a router’s still my recommendation :-).

  5. How can I address a computer behind a router? I know the GW address assigned by my ISP to the router. Router has now assigned 192.. addresses to my computers connected to it. From my office, I want “remote desktop” sharing to my home. I have XP on both systems. What address will I use to get to the computer behind the “router”. I have the valid tcp/ip address of the router and the private address (NAT) 192.. assigned by the router to the computer. I do not want to buy a software such as “PCAnyWhere” to achieve this.

  7. Hi, I have PC anywhere, I have ports 5631-5632 open on home and work, I can connect to the computer at work hat has DNS 192.168.1.100, but not the other, which also has the host running. I can’t see the other computer at all.
    I also have a Linksys router & Win XP same router, and XP on all, How do I connect to the other computer DNS 192.168.1.104??
    Thanks for any help, Nan

  8. hi,If a network is shared between two or more computers then a router is the best choice..
    Look for the right Ip address assigned by the router if do not have a right ip address the DHCP services are not enabled ..
    The router assigns address using this DHCP services..
    how to access the service:
    1.start > run > services.msc look for DHCP enabled
    double chick and start this service.
    2.start > run > regedit
    in registry editor ,
    HKEY_LOCAL MACHINE [+]
    SYSTEM[+]
    CURRENT CONTROL SET[+]
    SERVICES[+]
    DHCP [Click this and on right side ]
    RIGHT click the DEPEND ON SERVICE
    click MODIFY
    [tcpip afd netBT]should be the value..
    note: take back for the registy by file > export> “file name”

  9. Leo, Hello, and thanks for the information on setting up your ip address, well, thats waht we asked you at the time, anyway” now i really need to know how i can get my website links that are broken deleated? there are several broken links that go to our url(www.ksjfnetradio.com) there are several out there that should not be there, well please TUNE IN and reveiw our radio station at http://www.ksjf-netradio.com click on TUNE IN at the website and your player will begin the live stream , then reply soon. thanks alan ajmes c/o ksjfnetradio.com P.O. Box 3134 Salina,kansas 67402-3134 USA

  10. If you mean links on other websites … you’ll have to contact the other websites and ask them to change the links. If you mean the search engines, then you’ll simply need to wait for their indexes to be updated.

    I would, if you can, keep the old domain and have it redirect to the new one.

  11. I have 2 computers on a linksys main computer with xp and a laptop with 2000. they are connected but i can not share files or printer or the dialup on my main computer.please help?

  12. Hi Leo,

    For the past few months my cable ISP (AOL via NTL Cable) has not been assignimg an IP address and the result is I have been working on a 169.xxx auto ip. My problem is the ISP is adamant that they are providing a service since I can connect to the internet using one PC and therefore they are doing nothing about it. My problem is, without an ip address assigned, I cannot configure my internet router. Is there a way around this and also, would I be right to take this further with the ISP with the understanding that I am getting a substandard service?

  13. Your situation’s a little unclear to me, but I believe you should still be able to cofigure your router. Do you get an IP address if you connect your computer directly to the Cable modem? If so, what kind of IP is it? (192.x.x.x?) Some cable modems act as routers themselves, in which case the IP address is assigned to them.

  14. hi leo am behind rounter on LAN and i assigned static ip 10.1.1.12 to my laptop..i want to stream media over internet using media encoder and,but i am not sure what ip i should provide to other person to access my stream over internet ..would it be my router ip address or my isp ip address..i tried my isp ip adress but its not workin and i already open the port am using for streaming but when i used my isp address but i cant stream in net ..

  15. hi leo, firstly many thanks for your reply – its truely appreciated. When I connect my PC directly to the cable modem, I get the autoconfiguration IP address 169.254.xx.xx. If i do a ipconfig /release and renew, I get a message effectively saying ‘DHCP server not allocating IP address – contact your network admin’.

  16. Yeah, you need to go kick your ISP. Assuming you’re physically properly connected directly to the cable modem, it sounds like they’re droppping the ball.

  17. Once again, Leo many thanks for your help. You are truly a gem. I would really appreciate if you could help with one more thing on this subject – when I call the ISP they are adamant that all is OK. What can I say to them from a technical perspective to explain my problem so that they understand the issue without straying? Is there a correct terminology to describe what is happening here so the ISP can be directed straight to the problem area? I feel that since I am an amateur, they are pulling the wool over my eyes.

  18. Did the ISP provide the cable modem? I expect they did, and they should be making it right by you.

    It’s not really that technical … The computer seems to think the cable is plugged in properly, or it wouldn’t self assign an IP address. If their cable modem acts as a router, then it’s not assigning an IP address as it should. If the cable modem does not act as a router, then the IP address is not being assigned from upstream.

    If they’re unwilling to help, you might need to find someone who can come in and have a hands-on look at the situation. It certainly could be a simple cable modem configuration issue, for example, but there’s no way to know from here.

    Good luck!

  19. Leo, what can I say? I know a thank you is not enough. Instead maybe its more appropriate if I say to the rest of your readers that they are truly in safe, wise and highly knowledgeable hands when they come to you for advice and that I speak from personal experience. So readers – lets all buy Leo a Latte !!!

  20. Hi and hello, can anyone help me, i have an internet cafe problem on one of my pc, I have 10 pc connected on internet thru a switch all of them working fine when suddenly all of them wont work on the internet, i’d try connecting one by one when i get to the 5th pc my internet wont work i disconnect the 5th pc i hooked up only 1 to 4 and 6 to 10 of my pc my internet works, is there a posibility that my 5th pc has a bug or lan card problem? help me plss..

  21. My isp gives me 2 personal ip addresses to use. I have a linksys router set-up right now using only 1 of the 2 ips. I wish to use the second ip to increase my bandwidth with multiple pcs. Do I need to add a switch to make use of the second ip or can I turn my router into a hybrid switch/router? How can I get use of my second ip?

  22. Your bandwidth is NOT related to how many of your IPs you use. Using the second one will not make anything faster at all. (I’ve been allocated 5 here at home, but use only one. There’s no reason for me to use any of the others yet. Certainly not speed.)

  23. Thanks for the reply Leo, I understand now that having both ips will not increase speed, my isp max is up to 7Mbps(High Speed Extreme), but never even come close to that from most web sites and sharing this between 3 pc’s has never been a problem, even when they are all running high demand internet activities.. But I still want to designate the other ip seperately. There are some advantages to having a second ip in place, perhaps not speed. Original question – Do I need a switch to do this or can I use my Linksys router(WRT54GS) to take advantage of the second ip?

  24. I don’t think the router will do it. You’ll probably want the broadband connection to come in to a switch or hub to “split” the raw internet connection, and then from there one conenction to your router configured as today for one IP, and then another connection to another device with the other IP. Good luck!

  25. Makes sense, I will look into getting a switch if I go that way. I am also looking into third party firmware for my router, I may get lucky.

  26. Hi, I have a problem that sound the inverse of the original quesiton of the thread. I have a laptop which I want to use on my home network and office network but each time a move it it seems that I need to reinstall the workgroup to allow the file sharing. IS this the rule, that you can only set your laptop to accept and belong to 1 workgroup? thanks

  27. Hi, My laptop is assigned DHCP ip in office and i use static ip at home for internet. In office my outlook and internet both are working fine. But i am not able to connect to other file server. I don’t know why? although all the server ping from my system. Can you help me out?

  29. I have a LAN connection, and I use it in my desktop computer. When i plug lan cable in my laptop, internet does not run. How can i run it on my laptop also?
    thanxxxx

  30. I have the following probleme with my PC.
    I have a PC with a wireless card.
    I can connect to a wireless network but sometimes I did not receive any IP or I receive an IP ho starts with 169.At the same wireless network I can conect any time with a laptop and I do not have any probleme with the IP.
    Please , tell me haw do I resolve the probleme
    My mail is [Email Address Removed]
    I wait for anserws
    thak you

  32. I need to clarify something about IP’s and speed vs. bandwidth.
    A single computer will only get one IP no matter how many you subscribe to, and therefore no matter how many IP’s you have a single computer will NOT get any speed increase.
    To use multiple IP’s, you need to use a switch– a hub might work but trust me here and just pay the extra few dollars for a switch. If you know how to configure it properly, a router can do the same thing, but it means disabling the router functions so it will basically operate like a switch (only it costs a lot more).
    Each device connected to the switch will draw a different IP (as long as you subscribe to enough IPs, of course). Most ISP’s limit your bandwidth by IP, so you might be able to get FULL bandwidth to EACH computer using this method.. up to the physical limit of the modem of course.
    This means that in many cases if your ISP says they give you 7meg down and 256k up, having two computers each with it’s own IP will usually give you the 7meg down 256k up on EACH machine, but NOT double the bandwidth on either one.

    The easiest way to test if your ISP limits you by IP or by total bandwidth across the modem is to hook 2 comps up to a switch, and run an upload speedtest on each computer seperately, note the results, then run the test again on both computers at the same time to compare the difference.

  33. I have an ISP that cliams they must charge extra for using more then one PC. If I connect a s/w both pc get an IP. which I assume they will charge for if I stayed connected, But if I use a NAT router why would they care 256k is 256k that is all the router will get with DHCP connection and a IP provided to it. Will they look past the router to the internal IP’s and charge me for spliting the bandwith I already pay for.?? Thanks

  34. —–BEGIN PGP SIGNED MESSAGE—–
    Hash: SHA1

    It’s possible to tell, but only if they monitor your traffic and look for
    random clues like machine names in email headers. Depends on how intrusive they
    want to be. They will be able to tell that you have a router pretty easily
    though. Personally, when I was in your situation I used a router and things
    were fine.

    Thanks,

    Leo

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  35. When I run the “ipconfig /all” command, it will open for a second and then disappear! Why is this happening????

    I’m trying to solve a problem with my laptop losing its internet every 5-7 days or so. I just can’t understand what is causing this to happen! I thought I’d check out what you suggested but the “ipconfig /all” window won’t stay open!

    Thanks,
    Gina

  36. When I run the “ipconfig /all” command, it will open for a second and then disappear! Why is this happening????

    I’m trying to solve a problem with my laptop losing its internet every 5-7 days or so. I just can’t understand what is causing this to happen! I thought I’d check out what you suggested but the “ipconfig /all” window won’t stay open!

    Thanks,
    Gina

    This is for Gina and others who go to RUN and type in the IPCONFIG command. I was having the same problem ipconfig would open a window for a second – then close. I went thru start, all programs, accessories, command prompt and it worked fine:).

  37. I have a home network with about 6 computers I went to add a new dell XP pro computer and I am unable to get network access. Network line has been tested as the lap top works there no problem. All network setting are like the others as far a the set up wizzard but still does not get IP. Firwall is off, no zone alarm so ???

    I done all the options in IE6 and still no IP tried release and renew nothing.

  38. Good information for me too, but How can I Open a command prompt? I’m blocked in the first step.

    Command prompt is typically in Start Menu – All Programs – Accessories – Command Prompt

    Leo
    15-Apr-2012

  39. Based on information, I Have the IP address: 192.168…..and Dhco Enabled: Yes. but when i click on my wireless router and insert the wireless network key, it does not connect. While my wireless internet shows very strong signals and I have 2 printers connected to this internet wirelessl.

  40. I have the ip address and Dhcp Enabled is Yes but I can not connect to my on wire less Router, how should be the setting if internet option and fire wall to make the connection possible?

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