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A broadcast domain is a grouping of
collision domains that are connected by Layer 2 devices. Breaking up a
LAN into multiple collision domains increases the opportunity for each
host in the network to gain access to the media. This effectively
reduces the chance of collisions and increases available bandwidth for
every host. But broadcasts are forwarded by Layer 2 devices and if
excessive, can reduce the efficiency of the entire LAN.
Broadcasts have to be controlled at Layer 3, as Layer 2 and Layer 1
devices have no way of controlling them. The total size of a broadcast
domain can be identified by looking at all of the collision domains
that the same broadcast frame is processed by. In other words, all the
nodes that are a part of that network segment bounded by a layer three
device. Broadcast domains are controlled at layer three because
routers do not forward broadcasts.
 Routers
actually work at layers one, two, and three. They, like all layer one
devices, have a physical connection to, and transmit data onto, the
media. They have a layer two encapsulation on all interfaces and
perform just like any other layer two device. It is layer three that
allows the router to segment broadcast domains.
In order for a packet to be forwarded
through a router it must have already been processed by a Layer 2
device and the frame information stripped off. Layer 3 forwarding is
based on the destination IP address and not the MAC address. For a
packet to be forwarded it must contain an IP address that is outside
of the range of addresses assigned to the LAN and the router must have
a destination to send the specific packet to in its routing table.
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