The word topology refers to the arrangment of computers and wiring in a network. The four most common topologies are the bus, the star, the ring and the mesh. Each topology has its own advantages and disadvantages.
In a bus topology, each node (computer, server, peripheral etc.) attaches directly to a common cable. This topology most often serves as the backbone for a network. In some instances, such as in classrooms or labs, a bus will connect small workgroups. Since a hub is not required in a bus topology, the set-up cost is relatively low. However, this topology's wiring scheme is unstructured-meaning without a central point of concentration-making it difficult to troubleshoot.
A star topology has connections from each node to a central network hub. The failure of one connection will not usually affect the others. And, since all traffic passes through the hub, the hub becomes a central point for isolating network problems and gathering network statistics. The disadvantage to the star topology is that it requires a large amount of network cable. The star topology is the most common topology in local area networks.
A ring topology features a closed loop. Data packets travel in a single direction around the ring from one network device to the next. Each network device acts as a repeater, meaning it regenerates the signal. If one device fails, the entire network goes down.
A mesh topology provides multiple links between devices. A true mesh topology would have a link between each device on a network. So a network of six devices would have 15 connections. True mesh networks are seldom implemented because of the exponential growth in connections. A more common implementation of a mesh will provide multiple links in a few strategic locations, like between servers. The advanatages to a mesh system include fault tolerance and increased bandwidth.
Most real world networks combine these topologies. For example having a star network in each building, but connecting the buildings together with a bus network.