SMC Networks ADSL Barricade SMC7204BRA - Manual (Page 125)

Configuring Bridging
126
On the receiving network, a LAN protocol such as Ethernet takes
over, helping the packet reaches its destination.
When the bridge does not recognize a packet's destination hardware
ID, it broadcasts the packet through all of its interfaces – to each
network it is attached to.
Note: Bridges vs. Routers : The essential difference between
a bridge and a router is that a router uses a higher-level
protocol (such as IP) to determine how to pass data.
IP data packets contain IP addresses that specifically
identify the destination computer. Routers can read this
information and pass the data to the destination
computer, or determine which next router to send the
data to if the destination is not on a connected network.
Bridges cannot read IP information, but instead refer
to the hardware ID of the destination computer, which
is also included in data packets.
Hardware IDs are unique numbers that manufacturers
assign to each piece of hardware they sell. A bridge
learns to recognize the hardware IDs accessible
through each of its ports. When it receives a packet,
the bridge simply forwards the packet through the port
it associates with the given hardware ID, or through all
its ports if it does not recognize the ID. The hardware
ID is often referred to as the Media Access Control
(MAC) address. Routers are considered more
intelligent and flexible devices than bridges, and
often provide a variety of security and network
administration services based on the IP protocols.