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<sect1 id="Connectivity-Devices">
The network topologies above can use several types of connectivity devices.
Some are required (such as hubs for star networks) and othes provide
optional features; for example, a repeater can extend the length of a
bus network segment.
Repeaters connect network media to extend total length. The purpose of this
device is to eliminate the effects if attenuation. However, as is outlined in
the 'Overview' section of this document it can sometimes inadvertently add
'noise' to the network signal. Please see the 'Overview' for further details
on this device.
Hubs connect nodes and network resources in a network to a central point in a
star topology. It should be noted that the usage of these devices has largely
been eliminated as the development of 'switch' and general 'switching-fabric'
technology has delivered increased levels of speed and efficiency in network
communication. Switches and routers are two types of hubs.
Switches connect nodes in a private network to a central point in a star
topology. They send packets to nodes based on MAC address and provide
roughtly the same functionality as 'routers' but much more efficiently
and on a different level.
Bridges filter and move data between segments based on MAC address.
For example, an ethernet bridge is a device that controls data packets
within a subnet in an attempt to cut down the amount of traffic. A bridge
is usually placed between two separate groups of computers that talk
within themselves, but not so much with the computers in the other group.
A good example of this is to consider a cluster of Macintoshes and a
cluster of Unix machines. Both of these groups of machines tend to be
quite chatty amongst themselves, and the traffic they produce on the network
causes collisions for the other machines who are trying to speak to one
another. A bridge would be placed between these groups of computers. The
job of the bridge is then to examine the destination of the data packets one
at a time and decide whether or not to pass the packets to the other side of
the ethernet segment. The result is a faster, quieter network with less
collisions. Several bridges can work together to create even larger networks
of Ethernets using the IEEE 802.1 spanning tree algorithm. As this is a
standard, Linux bridges will interoperate properly with other third party
bridge products. Additional packages allow filtering based on IP, IPX or MAC
addresses. Please see the Bridge-HOWTO for further details as to their purpose,
usage and implementation.
A special purpose computer, hardware device or software package that
handles the connection between two or more packet-switched networks. Routers
spend all their time looking at the logical source and logical destination
addresses of the packets passing through them and deciding which route or
host to send them on to. Please see the 'Routing' section for further details
on this device.
A network device that combines bridge and router capablities.
The technical meaning is a hardware or software set-up that translates
between two dissimilar protocols/data formats, for example America Online
has a gateway that translates between its internal, proprietary e-mail
format and Internet e-mail format. Another, sloppier meaning of gateway
is to describe any mechanism for providing access to another system,
e.g. AOL might be called a gateway to the Internet.