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. ![](./75_files/image004.gif)
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.
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