When the number of devices on
a LAN becomes large (there is no fixed definition of precisely what large means), it is
more efficient to divide the LAN into multiple segments. This segmentation can be done
using a device called an Ethernet Bridge.
Ethernet Bridges and Switches An Ethernet Bridge is a LAN interconnection device
that operates at the Data-Link layer (Layer 2 of the OSI model). It may be used to join
two (or more) LAN segments to construct a larger LAN. It also regulates the traffic
between these segments by filtering traffic based on (source and destination) MAC
addresses in the traversing Ethernet frames; the bridge basically ???learns??? which MAC
addresses can be reached through each of its ports and constructs a table that maps a
list of (MAC) addresses to a port. It then parses incoming frames and forwards them
based on the content of this table. Broadcast frames (with all 1s in their DA field) will
be forwarded to all ports except the port they arrived on. A Bridge may also enforce a
security policy separating different workgroups located on each of the LANs. Bridges
were first specified in IEEE 802.1D.14
A Switch is essentially a bridge where the bridging??”examining the packet and
forwarding it??”is done using hardware (so forwarding frames is done very quickly).
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