Сегмент компьютерной сети: определение и типы

Portion of a computer network

A network segment is a portion of a computer network. The nature and extent of a segment depends on the nature of the network and the device or devices used to interconnect end stations.

According to the defining IEEE 802.3 standards for Ethernet, a network segment is an electrical connection between networked devices using a shared medium. In the original 10BASE5 and 10BASE2 Ethernet varieties, a segment would therefore correspond to a single coax cable and all devices tapped into it. At this point in the evolution of Ethernet, multiple network segments could be connected with repeaters (in accordance with the 5 4 3 rule for 10 Mbit Ethernet) to form a larger collision domain. With twisted pair Ethernet, electrical segments can be joined together using repeaters or repeater hubs as can other varieties of Ethernet. This corresponds to the extent of an OSI layer 1 network and is equivalent to the collision domain. The 5 4 3 rule applies to this collision domain. Using switches or bridges, multiple layer 1 segments can be combined to a common layer 2 segment, i. e. all nodes can communicate with each other through MAC addressing or broadcasts. A layer 2 segment is equivalent to a broadcast domain. Traffic within a layer 2 segment can be separated into virtually distinct partitions by using VLANs. Each VLAN forms its own logical layer 2 segment.

A layer 3 segment in an IP network is called a subnetwork, formed by all nodes sharing the same network prefix as defined by their IP addresses and the network mask. Communication between layer 3 subnets requires a router. Hosts on a subnet communicate directly using the layer 2 segment that connects them. Most often a subnetwork corresponds exactly with the underlying layer 2 segment but it is also possible to run multiple subnets on a single layer 2 segment.