Windows XP Bridging and Media Support for Home Networking

Article
09/11/2009

Introduction

Windows XP supports a wide variety of networking technologies to connect computers in a home network. In a home with multiple computers that have different types of network adapters, such as a few computers on an Ethernet hub and a few computers that use wireless, attempting to connect them together can be a challenge.

Each set of computers that cannot be directly attached together because they exist on separate hubs or cabling systems or are separate technologies define a LAN segment.

Consider, for example, a home network consisting of four computers:

Two computers equipped with Ethernet adapters and connected to the same Ethernet hub.
Two computers equipped with phoneline adapters.

This configuration defines two LAN segments: one LAN segment for the Ethernet-equipped computers and one LAN segment for the phoneline-equipped computers.

It is possible to have multiple LAN segments using the same networking technology. For example, a home network consisting of five Ethernet-equipped computers:

Three of the computers use modern Ethernet adapters and connect to a common Ethernet hub.
Two of the computers use older Ethernet adapters and connect together using a coaxial bus cable.

It may not be possible to plug the coaxial bus cable into the Ethernet hub. Therefore, there are two separate Ethernet-based LAN segments.

To connect the computers on separate LAN segments together, there are two solutions:

Create multiple network segments and connect them with routers. In this solution, each LAN segment becomes a network segment, also known as a subnet. A network segment is a portion of a network that shares the same network identifier and is bounded by routers. With multiple network segment, nodes on separate network segments send packets to a router, which forwards the packets to the destination node.
Create a single network segment using a bridge. In this solution, the separate LAN segments are bridged together to create a single network segment. With a single network segment, neighboring 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.

These solutions are described in greater detail below.

Multiple Network Segment Solution

The multiple network segment solution works well for larger networks such as organization networks or the Internet. However, it introduces the following complexities:

Address assignment. With multiple network segments, each network segment must have a unique network identifier and each host must have a unique host identifier on each network segment. This configuration can be automated using the Dynamic Host Configuration Protocol (DHCP), however, home users should not have to configure a DHCP server.
Routing. If there is a single router, routing is relatively simple. All hosts on the separate network segments use the single router as their default router (also known as a default gateway). However, if there are multiple routers, the routing becomes more much complex(the routers must be configured with static routes or use a routing protocol to communicate network segment identifier location information.
Name resolution. By using local broadcasts, nodes can resolve the names of neighboring nodes on the same network segment. To resolve the names of nodes on other network segments, the name must be resolved by a name resolution server such as a Domain Name System (DNS) or Windows Internet Name Service (WINS) server. Home users should not have to configure either a DNS or WINS server.

Single Network Segment Solution

The single network segment solution does not scale to the size of an organization network or the Internet. However, it reduces the complexity of configuring your home network. To see the contrast, a single network segment solution minimizes or eliminates the following complex configuration issues of a multi-network segment network:

Address assignment. With a single network segment, there is a single network segment identifier. If your home network is not connected to the Internet, you can use the Automatic Private IP Addressing (APIPA) feature of Windows XP, Windows 2000, Windows Millennium Edition, or Windows 98 to automatically assign a common network segment identifier and a unique host identifier for each host. If your home network is connected to the Internet using the Internet Connection Sharing (ICS) feature of Windows XP or Windows 2000, the ICS computer acts as an automated DHCP server, allocating a unique host identifier from a common network segment identifier.
Routing. If your home network is not connected to the Internet, there are no routing considerations. If your home network is connected to the Internet using an ICS computer, routing configuration of the ICS computer and all the hosts is automated by enabling ICS.
Name resolution. Because your home network is a single network segment, all the nodes on the network segment can use local broadcasts to resolve the names of all the nodes on your home network. If your home network is connected to the Internet using ICS, the ICS computer automates the configuration of DNS for all the nodes on your home network and forwards DNS name resolution requests to DNS servers located on the Internet.

The correct solution for most multi-LAN segment home networks is the single network segment solution.

Windows XP Network Bridge

Windows XP includes the Network Bridge, a feature of the Network Connections folder that allows a computer with multiple network adapters installed to act as a bridge, connecting multiple LAN segments together to form a single network segment.

With the Network Bridge, there is no need to purchase an additional hardware-based bridge device. However, the computer running Windows XP must have network adapters installed for all the LAN segments that it is connecting together.

How to Manage Bridged Connections

Creating a single network segment using the Network Bridge is easy. Simply install the various network adapters for the LAN segments on the bridge computer. Then, 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 in the bridge computer.

To bridge LAN segments of the connections together, hold down the Ctrl key and click all the connections that correspond to all the LAN segments you want to bridge together. Then, right-click one of the selected Local Area Connection objects and click Bridge Connections, as shown in Figure 1.

Figure 1: Bridging two connections

Figure 1: Bridging two connections

The message "Please wait while Windows bridges the connections." is displayed. After the bridge configuration is complete, the connections that were selected now appear under the Network Bridge group, as shown in Figure 2.

Figure 2: Result of bridging two connections

Figure 2: Result of bridging two connections

To remove a connection from the bridge, right-click a connection in the Network Bridge group, and then click Remove from Bridge.

There can only be one bridge on a computer, but it may be used to bridge as many different network connections as the computer can physically accommodate. There can be multiple bridges on your home network.

Types of Networking Connections Supported for Bridging

The Network Bridge supports all networking technologies whose device driver installed in Windows XP advertises itself as Ethernet. This includes the following types of networking technologies commonly used in home networks:

Ethernet (10 Mbps, 100 Mbps, and Gigabit Ethernet).
IEEE 802.11b wireless (also known as Wi-Fi).
Phoneline-based connections.

Additionally, the Network Bridge supports connections using IEEE 1394.

How it Works

To create a single network segment from multiple LAN segments, the Windows XP Network Bridge uses the following technologies:

Layer 2 bridging.
Layer 3 bridging.

Layer 2 Bridging

Layer 2 bridging in the Network Bridge is an implementation of transparent bridging. With transparent bridging, the Network Bridge places the network adapters of the bridge in a special listening mode known as promiscuous mode. In promiscuous mode, the network adapter processes all frames received. Normally, the network adapter only processes specific frames received.

By processing all frames received on all interfaces, the Network Bridge learns which nodes are on which LAN segments by tracking the source address of received frames. The Network Bridge maintains a table of node addresses and the adapter from which the node is reachable. When a frame is received, its destination address is checked against the bridge table and if found, is forwarded without change to the LAN segment that contains the node with the destination address. If the destination address is not found, the frame is copied out all adapters except the adapter from which the frame was received, an operation known as flooding. All broadcast and multicast traffic is flooded.

The end result of the Network Bridge's operation is that nodes on separate LAN segments can send frames directly to each other while broadcast and multicast traffic is received by every node. These are the same conditions that exist when all the nodes of the network segment are connected to the same LAN segment. Thus, the Network Bridge transparently connects multiple LAN segments and makes it logically appear as the same LAN segment.

The flooding operation of transparent bridges can cause problems when multiple bridges are configured to form a loop. One problem is a forwarding storm where a frame with an unknown destination address is forwarded endlessly between bridges. To prevent these problems, the Network Bridge implements the industry standard IEEE 802.1D Spanning Tree Algorithm (STA) to determine which network adapters on the bridges can forward frames. The result of the STA is that bridges automatically configure themselves so that you have a loop-free bridged environment at all times. There is no configuration for STA for the Network Bridge, it is enabled by default and is self-configuring.

Layer 3 Bridging

For network adapters that do not support promiscuous mode or network technologies that do not support a broadcast-based transmission method, the Network Bridge acts as a Layer 3 bridge. With Layer 3 bridging, TCP/IP hosts on different LAN segments are transparently connected by the bridge computer.

Layer 3 bridging differs from Layer 2 bridging because the frame is changed as it is forwarded by the bridge computer. Layer 3 bridging differs from routing because a Layer-3 forwarding table, not a routing table, is used for the forwarding decision.

For Layer 3 bridging, the Network Bridge also provides special handing of DHCP packets so that nodes on Layer 3-bridged LAN segments can obtain a DHCP configuration from an ICS computer.