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The first type of network topology is called a bus, in which nodes are strung together in a line, as shown in Figure 2-1. The key to understanding how a bus topology works is to think of the entire network as a single cable, with each node “tapping” into the cable so it can listen in on the packets being sent over that cable. If you’re old enough to remember party lines, you get the idea.

In a bus topology, every node on the network can see every packet that’s sent on the cable. Each node looks at each packet to determine whether the packet is intended for it. If so, the node claims the packet. If not, the node ignores the packet. This way, each computer can respond to data sent to it and ignore data sent to other computers on the network.

FIGURE 2-1: Bus topology.

If the cable in a bus network breaks, the entire network is effectively disabled. Obviously, the nodes on opposite sides of the break can continue to communicate with each other, because data can’t span the gap created by the break. But even those nodes that are on the same side of the break may not be able to communicate with each other, because the open end of the cable left by the break disrupts the proper transmission of electrical signals.

In the early days of Ethernet networking, bus topology was commonplace. Although, for most networks, bus topology has given way to star topology (see the next section), many networks today still have elements that rely on bus topology.