Windows XP Home Networking: Building Network Bridges
Published: April 22, 2002
By Sharon Crawford, Windows XP Expert Zone Community Columnist
Editor's Note: Past articles by members of the online community are archived for your use. The information may become outdated as technology changes. For the most current information, please search the Web site or post a question in the newsgroups.
A couple of months ago, we needed to add several computers downstairs to our main network, a domain running Windows 2000 servers that is discretely hidden away on the top floor of our house. We didn't have wireless cards in abundance and I drew the line at drilling even more holes in the ceilings for running additional Ethernet cable. So we decided to take advantage of the bridging capability built into Windows XP. Called the Network Bridge, this is a feature of the Network Connections folder that allows a computer with multiple network adapters to act as a bridge, connecting different local area network (LAN) segments. A Network Bridge is required, for example, to connect different networking technologies such as a wired Ethernet segment and a wireless 802.11b segment.
We decided to connect the computers downstairs to each other using Ethernet cable. Then, by adding an additional, wireless network card to one computer, we could use that computer to connect to the upstairs network and act as a bridge. In this way, all the computers downstairs could connect to the upstairs network and the Internet through the wireless connection. Sure beats threading new CAT5 cable through the walls, and it's a lot cheaper than hiring an electrician to do it for us.
You don't need to be connecting two networks to get value from bridging. Bridging is also useful for connecting a laptop with a built in 802.11b (wireless) card to a cabled network when you don't have a wireless access point—add a wireless card to any other computer on your network and it can bridge the wireless onto the cabled network.
Bridging Requirements
To connect several computers, install a network card in each and run CAT5 network cabling to connect all of them to a hub (and thus each other.) The computer that will be used for bridging will need both a regular network card for the cabled connection and a wireless network card that will make the connection to the wireless access point (WAP) or gateway on the second network—in our case a Linksys WAP-11 connected to the main upstairs network. When the two networks have been bridged, the downstairs machines will be assigned IP addresses by the DHCP server and will have access to the Internet.
The bridging computer can be running either Windows XP Professional or Windows XP Home Edition. If you use the Home Edition, the bridging computer won't be a member of the domain you connect to but you can use your credentials as an authenticated user on the domain to map hard drives on the domain and thus get access.
Procedure for Creating a Network Bridge
After the network interface cards (NIC) are installed and connected, go to the bridging computer and open Control Panel, click Network and Internet Connections, and then click Network Connections.
Note: To create a network bridge you must select at least two network connections that are not being used by Internet Connection Sharing (ICS) or the Internet Connection Firewall (ICF).
In the Network Connections folder, a connection for each network adapter is displayed. Click the connection for the wireless NIC. In the Details pane, you should see that it's successfully connecting to your network. In Figure 1, you'll see that the one used in this example has been assigned an IP address by our DHCP server and the address is in our network's range.
Clicking the other connection for the Ethernet adapter shows that it has an IP address beginning with 169—this is an automatic private address assigned to computers that are not connected to the Internet.
Next click both network connections (if you have several segments, hold down the Ctrl key and click all the connections that correspond to all the LAN segments you want to bridge), right-click one, and then click Bridge Connections.
After the bridge configuration is complete, you'll end up with a Network Connections window that looks like the one below.
Troubleshooting a Wireless Bridge
Here I'll address a common problem that users of wireless cards are encountering when attempting to create a network bridge. Your wireless card may seem to be working, but the bridge is not created. Or the wireless NIC looks like it's unplugged, as shown in the figure below.
If the bridge appears to be made but doesn't actually pass network traffic, it could be that your wireless NIC doesn't support promiscuous mode, even though, when Windows XP queries the card, it reports that it does. (A description of how the bridging technology works and a description of promiscuous mode can be found in Windows XP Bridging and Media Support for Home Networking.)
Unfortunately, this appears to be a problem with many wireless cards on the market right now. There is a solution to this problem, which is to manually force the NIC into Compatibility mode. The Knowledge Base article, Bridge May Not Work with a Non-Promiscuous Mode Network Adapter, that discusses this problem lists three cards that have to be forced into Compatibility mode. Even if your card is not listed, the fix is the same for all of them. You need to set the wireless adapter to ForceCompatibilityMode. First, open a command window:
1. | Click Start, click Run, type cmd, and then click OK. |
2. | In the command window, type netsh bridge show |
3. | Note the number assigned to the wireless adapter and type netsh bridge set a 1 e where you substitute the number displayed in the previous step for the number 1 in this step. 
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4. | To double check the wireless card is correctly set with Force Compatibility Mode enabled, type the netsh bridge show a command again. |
You can also contact the NIC manufacturer or check their Web site for more information about possible updates for your card.
Bridging Using the ICS or ICF Computer
As mentioned earlier, to make a bridge you must select network connections that are not being used by Internet Connection Sharing (ICS) or the Internet Connection Firewall (ICF). However, if the computer has three network cards, one of them can be the connection to the Internet with the other two used for bridging. To make a bridge this way—using the private ICS connection and another network connection (other than the Internet connection), the bridge must be made before enabling ICS.
Use the Network Setup Wizard to create the network bridge and to enable ICS. The Network Setup Wizard will look for the connection to the Internet, configure Internet Connection Firewall if you wish, and bridge multiple network adapters connected to your home network.
Sharon Crawford is a former editor now engaged in writing books and magazine articles. Since 1993, she has written or co–written two dozen books on computer topics. Her books include Windows 2000 Pro: The Missing Manual, Windows 98: No Experience Required, and Windows 2000 Professional for Dummies (with Andy Rathbone).
