What do switches do?
A switch is a device that segments networks into different
subnets. Segmenting the network into different subnets keeps one
network from overloading with traffic. Therefore, a switch forwards
all data in the data layer and sometimes the network layer as it
filters the data. A switch allows a connection to be established
and it terminates a connection when there is no longer a session to
support.
Prior to switches there were hubs. Hubs also connected multiple
independent (connected) modules in a network, but they were not as
efficient as switches. Since most switches work in Layer 2, and not
in Layer 1 like a hub, they are better at filtering data. A switch
looks for Ethernet MAC addresses, keeps a table (the bridge
forwarding table) of these addresses, and navigates the switch
between ports. The switch prevents collisions and gives full
bandwidth to each connection at the switch port. Switches save
bandwidth by only sending traffic to destinations that have
traffic. When a switch switches Ethernet frames, they monitor the
traffic for the response from that frame and see what device, on
what port, responds to that flooded frame. There are different
types of switches that range from "dumb" switches that lack
manageability and can monitor only 4-8 ports to "managed" switches
that can get statistics on switch traffic, monitor connections, and
hard-code up to 96 port speeds and duplex. Then again, there are
"chassis-based" switches with blades or cards that perform not just
switching, but routing and intrusion detection too! If you are
interested in this, look into Cisco's Catalyst 6500 Series. Another
type of switch preferred by large enterprises is the "Layer 3
Switch," because it has the functionality of a router .