Take it to the bridge

Bridging fulfilled an obscure enterprise networking need in the past, but it is now very relevant to domestic networking setups. Many households and small businesses often have a wired net­work with an Internet connection - no great surprises there - but now they may also have a smattering of Wi-Fi notebooks, plus a PDA or two, and maybe some Bluetooth phones.

Under most other operating systems, if you wanted to get these different networking devices chattering away to each other, you'd need to fork out for a network bridge or similar. And, in the case of a Wireless Network, you'd need to buy an access point. Now, thanks to Windows XP, there's no need to fork out for more expensive hardware - you can let XP act as a software bridge instead.

Access all areas

The great thing about bridging in Windows XP is that you can use to connect just about any network, no matter what networking media is used. In this context, media means the method of connection, so there are two types: wired and wireless. Just as there are several types of wireless technology, there are still several wired networking technologies around, including UTP/CAT 5, coaxial thin Ethernet and home phoneline networking or HPNA (Home Phoneline Networking Alliance). The software bridge in Windows XP can glue all these wildly differing media together.

Here's the scenario: you have two PCs linked with CAT 5 cable, another two linked with thin Ethernet, a laptop with Wi-Fi facilities and a Bluetooth PDA. You want somehow to connect these different network segments and let them share an Internet connection.

You could, of course, create multiple network segments and connect them with routers using IP routing. In this solution, each LAN segment becomes a subnet. With multiple network segments, nodes on separate network segments send packets to a router which then forwards the packets to the destination node. Packet forwarding is required because different protocols are used for different types of media. Setting up IP routing can be a tad tricky.

What a packet

Or you can create a single network segment using a bridge. Here, the separate segments are bridged together to create a single network segment. Neighbouring nodes on separate LAN segments send packets directly to each other and bridges forward the packets to the destination node on the appropriate LAN segment.

The software network bridge merely requires you to fit appropriate NICs (network interface cards) for all the different types of media into one PC and then use Windows XP to bridge those network connections. Windows XP's network bridge can manage your LAN segments, creating a single subnet for the entire network.

No configuration or costly hardware is needed and things like IP addressing, address allocation, packet forwarding and name resolution are greatly simplified in a single subnet IP network.

Only one bridge may exist on a Windows XP system, but it can be used to bridge as many different connections as the computer can physically accommodate. The main drawback of a software bridge com­pared to a hardware one is that the bridge PC has to be on all the time in order to work.

Bridge building

By default, the Network Setup Wiz­ard automatically creates a bridge when multiple network adapters are found on a Windows XP computer. However, the Network Setup Wizard won't bridge a network adapter that's connected to an external DSL or cable modem.

Manually creating a single network segment using the Network Bridge is easy. Install the various network adapters for the LAN segments on the bridge PC. Open Network Connections from Control Panel. Within the Net­work Connections folder, you should see a connection under the LAN and High Speed Internet group for each network adapter installed on the bridge PC.

To create a bridge, hold down the key and click on all the connections that you want bridged. Then right click on one of the selected Local Area Connection objects and select Bridge Connections. The self-explanatory message "Please wait while Windows bridges the connections" will then appear (see this screen shot).

After the bridge configuration is complete, the connections that were selected now appear under the Network Bridge group (see here). You must be logged on as an administrator or a member of the Administrators group in order to do this. To remove a connection from the bridge, right-click on the Network Bridge group and then select Remove from bridge.

If you use Internet Connection Sharing or the Internet Connection Firewall, that particular adapter must not be bridged. But if the bridge PC has three NICs, one of them can be the connection to the Web, while the other two are used for bridging. To make a bridge this way, using the private ICS connection and another network connection (other than your Internet connection) the bridge must be made before enabling ICS. A note for the curious: your network adapters no longer have anything in their network properties. For the most part, all the information and settings are now in the properties of the network bridge. If you want to change any properties, you'll have to do it through the Network Bridge properties option.

Many bridges to cross

To create a single network segment from multiple LAN segments, the Windows XP network bridge uses Layer 2 or 3 bridging. Layer 2 bridging - that is, the data link layer - places all the network adapters of the bridge in a special listening mode known as Promiscuous Mode.

Normally, a network adapter only processes specific frames received, but in Promiscuous Mode it processes them all. Thus the network bridge is able to learn which nodes are on which LAN segments by tracking the source address of the incoming frames.

For those network interface cards that don't support Promiscuous Mode or a broadcast-based transmission method, the Network Bridge acts as a Layer 3 bridge. Layer 3 is the network layer where the IP protocol lives. It differs from Layer 2 bridging because each frame is changed as it is forwarded by the bridge computer. Note that some Wi-Fi cards lie and tell you that they're in Promiscuous Mode when they're not, which can cause mysterious network problems. The solution here is to force Compatibility Mode. Open a command prompt and enter:

netsh bridge show adapter

then hit ENTER.

Note the number assigned to the wireless adapter. Now type the following, replacing "N" with the previously displayed number

netsh bridge set a N e

then hit ENTER

You can check that Compatibility Mode has been enabled by repeating the first command.

Bridge over troubled waters

Ironically, in the eyes of many network admins, bridges can cause "network floods" and are banned. Flooding can occur when multiple bridges are configured to form a loop - for example, when several notebooks have an Ethernet port as well as Wi-Fi and both connections are bridged.

This condition can cause a "forwarding storm", where a frame with an unknown destination address is forwarded endlessly between bridges. To prevent this, the Windows XP bridge implements the IEEE 802.1d spanning tree algorithm (STA) which should automatically and transparently configure itself to prevent flooding.

That's the theory, at least. In practice, despite the inclusion of STA in the Windows XP bridge, network flooding is a problem that's particularly prevalent in university networks where well-heeled students have notebooks with Ethernet and Wi-Fi and bridging is turned on. This can cause severe disruption, sometimes rendering portions of the network non-functional for all users.