A segment is a specially-configured subset of a larger network. The boundaries of a network segment are established by devices capable of regulating the flow of packets into and out of the segment, including routers, switches, hubs, bridges, or multi-homed gateways (but not simple repeaters). Network designers create segments to physically separate related computers into groups.
This grouping can improve network performance and security. In Ethernet networks, for example, computers send many broadcast packets onto the network, but only other computers on the same segment receive them. Network segments and subnets serve similar purposes; both create a grouping of computers. The difference between a segment and a subnet is as follows: a segment is a physical network construction, whereas a subnet is simply a higher-level software configuration.