|  | 7.5 | Effects of Layer 2 Devices on Data Flow
| 7.5.1 | Ethernet LAN segmentation

There are two primary reasons for segmenting a LAN. The first is to isolate traffic between segments, and to achieve more bandwidth per user by creating smaller collision domains. Without LAN segmentation, LANs larger than a small workgroup would quickly become clogged with traffic and collisions, and would deliver virtually no bandwidth. The addition of devices like bridges, switches, and routers segment the LAN (shown) into four collision domains.

By dividing large networks into self-contained units, bridges and switches provide several advantages. A bridge, or switch, diminishes the traffic experienced by devices on all connected segments, because only a certain percentage of traffic is forwarded. Both devices act as a firewall for some potentially damaging network errors. They also accommodate communication between a larger number of devices than would be supported on any single LAN connected to the bridge. Bridges and switches extend the effective length of a LAN, permitting the attachment of distant stations that were not previously permitted.

Although bridges and switches share most relevant attributes, several distinctions still do exist between them. Switches are significantly faster because they switch in hardware, while bridges switch in software, and can interconnect LANs of unlike bandwidth. A 10 Mbps Ethernet LAN and a 100 Mbps Ethernet LAN can be connected by using a switch. Switches can support higher port densities than bridges. Some switches support cut-through switching, which reduces latency and delays in the network, while bridges support only store-and-forward traffic switching. Finally, switches reduce collisions and increase bandwidth on network segments because they provide dedicated bandwidth to each network segment.

Segmentation by routers has all of these advantages and more. Each interface on the router connects to a separate network, so insertion of the router into a LAN creates smaller collision domains and smaller broadcast domains. This occurs because routers do not forward broadcasts unless programmed to do so. However, the router can perform bridging and switching functions. The router can perform best path selection. The router can be used to connect different networking media, and different LAN technologies. Note the router in the teaching topology is connecting Ethernet, Token Ring and FDDI LAN technologies - segmenting the LAN, but doing much more. Routers can connect LANs running different protocols (IP vs. IPX vs. AppleTalk) and can have serial connections to WANs.