A network segment is a portion of a computer network where in every device communicates using the same physical layer. Devices that extend the physical layer, such as repeaters or network hubs, are also considered to extend the segment. However, devices that operate at the data link layer level or higher create new physical layers and thus create rather than extend segments.
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Ethernet segments
In the context of Ethernet networking, the network segment is also known as the collision domain. This comprises the group of devices that are connected to the same bus, and that can make CSMA/CD collisions with each other,and sniff their packets in promiscuous mode. It also includes devices connected to the same hub, which also can have collisions with each other. In modern switch-based Ethernet configurations, the physical layer is generally kept as small as possible to avoid the possibility of collisions. Thus, each segment is only composed of two devices, and the segments are linked together using switches and routers to form one or more broadcast domains. They don't go on forever.
Token ring segments
Computers connected to the same Media Access Unit
Token bus segments
Computers connected to the same token bus.
Other uses of the term 'segment'
The term network segment is sometimes used to refer to the portion of a computer network in which computers can access each other using a data link layer protocol (e.g., in Ethernet, this would be the ability to send a Ethernet packet to others using their MAC addresses). In this case, it is synonymous with broadcast domain. Occasionally, the term refers to a subnetwork.
