When Metcalfe originally developed Ethernet, computers were connected to a
single copper cable. The physical limitations of a piece of copper cable carrying
electrical signals restricted how far computers could be from each other on
an Ethernet. Repeaters helped alleviate the distance limitations. Repeaters are
small devices that regenerate an electrical signal at the original signal strength.
This process allows an Ethernet to extend across an office floor that might
exceed the Ethernet distance limitations.
The addition or removal of a device on the Ethernet cable disrupts the network
for all other connected devices. A device called an Ethernet hub solves
this problem. First, each port on a hub is actually a repeater. Second, hubs let
computers insert or remove themselves nondisruptively from the network.
Finally, hubs simplify Ethernet troubleshooting and administration. As networks
grow larger, companies need to fit more and more computers onto an
Ethernet. As the number of computers increases, the number of collisions on
the network increases. As collisions increase, useful network traffic decreases
(administrative traffic actually increases because of all the error messages getting
passed around). Networks come to a grinding halt when too many collisions
occur.
Ethernet bridges resolve this problem by physically breaking an Ethernet into
two or more segments. This arrangement means that devices communicating
on one side of the bridge do not collide with devices communicating on the
other side of the bridge. Bridges also learn which devices are on each side and
only transfer traffic to the network containing the destination device. A twoport
bridge also doubles the bandwidth previously available, because each port
is a separate Ethernet.
Ethernet bridges evolved to solve the problem of connecting Ethernet networks
to Token Ring networks. This process of translating a packet from one LAN
technology to another is called translational bridging.
As Ethernet networks continue to grow in a corporation, they become more
complex, connecting hundreds and thousands of devices. Ethernet switches
allow network administrators to dynamically break their networks into multiple
Ethernet segments.
Initially, switches operated as multiport Ethernet bridges. But eventually, as the
cost per port decreased significantly, Ethernet switches replaced hubs, in which
each connected device receives its own dedicated Ethernet bandwidth. With
switches, collisions are no longer an issue, because connections between computer
and switch can be point-to-point, and the Ethernet can both send and
receive traffic at the same time. This ability to send and receive simultaneously
is called full duplex, as opposed to traditional Ethernet, which operated at half
duplex. Half duplex means that a device can receive or transmit traffic on the
network, but not at the same time. If both happen at the same time, a collision
occurs.
This is different from subnetting in a couple of distinct ways. First, Ethernet is
a Layer 2 protocol, and subnetting has to do with IP addressing (which is a
Layer 3 function). Second, IP addressing is a logical segmentation scheme, and
switching is a physical separation, because each end station has a dedicated
physical port on the switch.