A network segment is a portion of a computer network that is separated from the rest of the network by a device such as a repeater, hub, bridge, switch or router. Each segment can contain one or multiple computers or other hosts. The type of segmentation differs according to the type of device used. For example, a bridge separates collision domains, while a router separates both collision domains and broadcast domains. Each network segment supports a single medium access protocol and a predetermined bandwidth. The more hosts that are on a network segment, the more divided this bandwidth is. Crowded network segments lead to a condition known as congestion, which results in degraded performance.

Each network segment can have its own hub or switch. In most cases a contiguous range of IP addresses will be assigned to each segment.

Perhaps the greatest advantage of having multiple segments rather than having all hosts on a single, large segment is that it can increase the amount of traffic that a network can carry. A major consideration in designing segmentation to maximize network capacity is to put computers that do not normally communicate with each other on different segments.