Segment (network)

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A network segment is part of a larger, coherent network that is virtually and / or physically separated from the rest of the network.

There are various reasons for dividing a network into several network segments: In larger networks this is useful for distributing the load of the broadcasts . In the case of interfaces between Local Area Network (LAN) and Wide Area Network (WAN), for example the Internet , separation makes sense for security reasons.

The term segment is used in a computer network at different OSI levels for different delimitations.

  • On OSI layer 1, it denotes the common physical medium to which computers are connected and is synonymous with collision domain .
  • On OSI layer 2 in bridges , the networks at the two interfaces are also referred to as segments. Similarly, ports on the switch that lead to multiple MAC addresses are referred to as segment ports.
  • On OSI layer 3 for IP, the term segment is synonymous with an IP subnet or with a broadcast domain.

Segments, Layer-1, Collision Domain

Within a common segment on OSI Layer 1, several computers are connected in a common collision domain analogous to physical bus topologies . With Ethernet, this type of connection is via coaxial cables or hubs .

A segment can only connect exactly two participants ( point-to-point ). This form is used for wiring using twisted pair cables or fiber optic cables . A segment with a point-to-point connection is often referred to as a link segment.

Repeaters or hubs can be used within a segment. Different layer 1 segments, however, are connected to each other via coupling elements such as bridges, switches or routers to form layer 2 or layer 3 segments.

Ethernet with bus topology, two layer 1 segments connected via router

Segments, Layer-2, Broadcast Domain, Segment Ports

Bridges and switches combine the connected networks into a layer 2 segment, which in turn can consist of any number of collision domains. Similarly, ports on the switch that lead to multiple MAC addresses are referred to as segment ports. Since Layer 2 broadcasts are transmitted to all ports in the same L2 segment, the term broadcast domain is also used here.

Two segments, connected by a switch, together form a broadcast domain

Segments, layer 3, IP networks and subnets

On Layer 3, entire TCP / IP networks or subnets are sometimes referred to as segments. In other words, stations with the same broadcast domain form a common subnet, which is normally congruent with an L2 segment. Different segments have to be linked via routers.

See also