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

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 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.

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 XP at this point, but much of this applies to WIndows 2000 and Windows NT, and there are equivalent steps for Windows 9x.

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.