Answering Some Common Windows XP Home Networking Questions

I've covered lots of different topics for the Expert Zone, but the ones that seem to garner the most feedback and interest are the networking columns. Clearly, this is an area that still causes problems for people. And not just home users—I regularly receive questions from IT pros and power users as well. I can't begin to cover every question that arises, but I'd like to take the top three issues I've seen recently and try to shed some light on them.

In this column, I'll cover how to use Remote Desktop when the Remote Desktop host computer is behind a router, how to configure and connect a wireless router (gateway) when adding it to a network with a wired router, and discuss how to work with automatically configured IP addresses.

Use Remote Desktop for Computers Behind Routers

I repeatedly read about issues with Remote Desktop in the newsgroups. How do you use Remote Desktop over the Internet when there's router (often called a gateway) at the other end? The router performs Network Address Translation (NAT), which makes all of the PCs on your local network behind the router appear to have the same IP address, the address of the router.

Remote Desktop uses TCP port 3389 for its connection, so there are several pieces to the puzzle you need to configure.

1.	Enable Remote Desktop on the Remote Desktop host PC. See Set Up Your Computer for Remote Desktop and my earlier column on tuning Remote Desktop for slow links.
2.	If you're using Windows Firewall in Windows XP Service Pack 2 (SP2), it will be automatically configured when you turn on Remote Desktop. If you're using a third-party firewall, consult the documentation for that product to allow incoming traffic to TCP port 3389.
3.	Configure the router to forward traffic for TCP port 3389 to the IP address of the Remote Desktop host PC. This will be different on every version of router software, but all involve roughly the same steps.

This is how I configure my XiNCOM Dual WAN router:

1.	Log on to the router using the Internet Explorer interface.
2.	Enable a virtual server for Remote Desktop, as shown in Figure 1. Figure 1
3.	Point the endpoint of the virtual server to the IP address of my PC.
4.	Determine the IP address of your router. This can be difficult if you don't have a fixed IP address and most people don't. However, many routers now support dynamic DNS services such as TZO.com and DynDNS.org, which let you use DNS to find your domain even when your address changes at the discretion of your ISP. If your router doesn't provide built-in support for dynamic DNS, most of the available services have client software that you can run on your PC to manage the process. Figure 2
5.	On the client PC you want to connect with, open the Remote Desktop Connection Wizard shown in Figure 3 and enter the IP address or the DNS name of your router, not of your Remote Desktop host PC. Figure 3

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Configure a Network with Multiple Routers

As more of us set up home networks with both wireless and wired devices, our networks become increasingly complicated. One question I've run into repeatedly in the newsgroups is how to configure and connect a wireless router (also known as a wireless gateway) when adding it to a network that uses a wired router.

The first answer, actually, is to not buy a wireless router if you already have a wired router—get a wireless access point (AP). They generally cost about the same as a router, but are specifically designed to do what you need. But for those of you who already have a router, here are some basic configuration steps.

1.	Ideally, only one router should act as a Dynamic Host Configuration Protocol (DHCP) server. You can choose which one, but generally it should be the one directly connected to the DSL/cable modem. Disable the DHCP server on the second router.
2.	Or you can carefully configure them both to act as DHCP servers, but use different address ranges of the same address prefix defined for the subnet. For example, I could configure the wired router to hand out DHCP addresses in the range from 192.168.0.50–192.168.0.99, while configuring the wireless router to serve out addresses in the 192.168.0.100–192.168.0.149 range, as shown in Figure 4. The catch here is that the default address range for the two routers may be different, especially if they came from different manufacturers. If they are, you'll need to change them so that they're both part of the address range being used on the home network subnet. Figure 4
3.	Configure the second router to be a bridge, not a router. Not all routers will do this, however. Check the software for the two routers you have, and if only one of them can act as a bridge, set it up to do bridging and connect the other one to the cable/DSL modem. My D-Link wireless router won't act as a bridge, so I would have to connect it to the DSL modem.

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Why Automatically Configured Addresses Can Cause Problems

It's annoying when a computer that used to work properly suddenly stops connecting to other computers on the network and to the Internet. If the number of messages found on community forums is any indication, this happens all too often. It's more of a problem with wireless, but I've seen it with wired networks too. Although there can be several reasons why a particular computer suddenly stops connecting to other computers, one explanation has to do with automatically configured IP addresses.

In Windows XP, the default configuration is for your network connections to automatically obtain an IP address configuration. This requires that a device acting as a DHCP server is available on the network to automatically assign IP addresses to DHCP client computers. DHCP server functionality is often built into home routers (wired or wireless) and other types of Internet gateway devices (IGDs). However, if the computer is unable to contact the device acting as a DHCP server or it is slow to respond, your Windows XP–based computer will decide there isn't a DHCP server available on the network and will automatically assign an Automatic Private IP Addressing (APIPA) address. APIPA addresses all start with 169.254. followed by the rest of the address, as shown in Figure 5.

Figure 5

This is actually a good thing for a home network that doesn't have a router, because it automatically assigns IP addresses from a common address range. But it's a bad thing if you're using a router, because the computer with an APIPA address won't be able to communicate with other computers on your network that are not using an APIPA address or use your router to access Internet resources.

What should you do if this happens to you? First, make sure that the device acting as the DHCP server is actually up and running. It's often a good idea to unplug the power to the device, wait 10 seconds, and then plug it back in. This restarts the device, clears all its memory and registers, and generally makes it happier.

If the computer getting the APIPA address is using a wireless connection, make sure you're where you can get a good quality wireless connection and that the wireless client has successfully authenticated with your wireless router or wireless AP. If it's a wired connection, make sure your cable is solidly plugged in on both ends and that the indicator lights are the right color (usually green, but yours may be different.) Then open Network Connections.

1.	Click Start, click Control Panel, click Network and Internet Connections, and then click Network Connections. The Network Connections dialog box is shown above in Figure 5.
2.	Right-click the network connection with the problem, and then click Disable.
3.	After the connection is listed as disabled, right-click it again, and then click Enable.

If you have good connectivity to the device acting as the DHCP server, and it is sending out addresses, your problem should be fixed.

When running Windows XP with SP2, choosing Repair for a wireless connection in the procedure above will do both steps for you automatically. Earlier versions of Windows XP didn't fully disable the connection and often had problems as a result.