Bus - Early Ethernet LANs used
a bus topology. A bus is a cable (coaxial) that is run through an
area, and workstations and servers tap into the cable to get access to the
network. The bus is a single length of cable that is terminated at
each end. The bus design was prone to problems since a break
anywhere in the bus could bring the entire network down. Although
there are still many bus-based networks in service, new installation
almost always use the extended star topology. Figure
shows
an example of a bus type topology.
Star - The star topology is
the most common and is the one used with Ethernet hubs and switches.
The hub or switch is the center of the star, and cables radiate out to
nodes like spokes on a wheel. The biggest advantage of the star over the
bus is that if a cabling connection to any node fails it does not
effect the others. Two or more switches can be connected using
backbone cable (usually fiber optic) which creates an extended star.
The extended star is the most common topology used in today's
networks. Extended stars are also used to create WANs. 
Ring - The Token Ring and Fiber
Distributed Data Interface (FDDI) network architectures use the ring
topology. Token ring is commonly implemented as a star with a
central concentration device known as a multi-station access unit or
MAU and is technically a star ring. FDDI is a true ring used
primarily to interconnect other types of LANs. FDDI is actually a
dual ring with one acting as the primary and one as backup in case
the primary fails. The ring topology is also found in WANs. FDDI can
be used to create a WAN, although is it not commonly done. Another
very widely used WAN technology that is based on the ring topology
is Synchronous Optical Network or SONET. Figure
shows a
ring topology.
Mesh - The mesh is typically a WAN
topology used to interconnect multiple LANs for redundancy of
communications paths. They can also be used in LANs to provide
redundant links between routers and switches. With a "full
mesh", each site has a link to every other site. With a partial
mesh each site has a link to more than one site but not to every
site. Routers typically interconnect sites. Figure
shows a
partial mesh topology between LANs at four different locations or
sites. Note that every site is not directly connected to every other
site but every site has at least two connections to other sites in
order to provide redundancy. This partial mesh uses four links
between three sites so every site has an alternate route to get to
another site if a WAN link goes down. The minimum number of links
would be three: where LANs B, C, and D could all link back to LAN A,
which would create a star topology. The maximum full mesh would
require 6 links.