Computer Engineering Department
• Internetworking devices are products used to connect networks.
• As computer network grows in size and complexity, so do the
internetworking devices used to connect them.
Broadly, following are the Internetworking devices:
• Networking devices are used to construct networks.
• Example: A local area Network (LAN) may need to cover more
distance that its media can handle effectively. In this case, you
can use a repeater to regenerate the signal.
• Internetworking devices are used to connect networks.
• Example: If you want to link a LAN into an internet, you
need an internetworking device as a router or a gateway.
• An internet is an interconnection of individual networks.
• A repeater (or regenerator) is an electronic device that operates
on only the physical layer of the OSI model.
• A repeater installed on a link receives the signal before it
becomes too weak or corrupted, regenerates the original pattern,
and puts the refreshed copy back on the link.
• A repeater does not actually connect two LANS; it connects
two segments of the same LAN.
• A repeater forwards every frame; it has no filtering capability.
• A Hub is a multiport repeater. It is normally used to create
connections between stations in a physical star topology.
• Bridges operate in both the physical and the data link
layers of the OSI model.
• Bridges can divide a large network into smaller segments. They contain
logic that allows them to keep the traffic on each segment separate. When a
frame (or packet) enters a bridge, the bridge not only regenerates the signal
but checks the destination address and forwards the new copy only to the
segment the address belong.
• A bridge operates in both the physical and the data link layers.
• As a physical layer device, it regenerates the signal it receives.
• As a data link layer device, the bridge can check the physical
(MAC) address (source and destination) contained in the
• A bridge has filtering capability. It can check the destination
address of a frame and decide if the frame should be
forwarded or dropped. If the frame is to be forwarded, the
decision must specify the port.
• A bridge does not change the physical (MAC) addresses in a
• A bridge has a table used in filtering decisions.
Types of Bridges
• To select between segments, a bridge must have a look-up
table that contains the physical addresses of every station
connect to it. The table indicate to which segment each
• The address table must be entered manually, before a
simple bridge can be used.
• Whenever a new station is added or removed, the table
• Installation and maintenance of simple bridges are time-
consuming and potentially more trouble than the cost
savings are worth.
Types of Bridges
A multiport bridge can be used to connect more than two LANs.
• A transparent, or learning, bridge builds its table of station
addresses on its own as it performs its bridge functions.
• If a transparent bridge is added or removed from the
system, reconfiguration of the stations is unnecessary.
• A transparent bridge must meet following criteria:
1. Frames must be forwarded from one station to another.
2. The forwarding table is automatically made by learning
frame movements in the network.
• Transparent bridges work fine as long as there are no
redundant bridges in the system.
• Bridges are normally installed redundantly, which means that
two LANs may be connected by more than one bridge. In this
case, if the bridges are transparent bridges, they may create a
loop, which means a packet may be going round and round,
from one LAN to another and back again to the first LAN.
• To solve the looping problem, the IEEE specification requires
that bridges use the spanning tree algorithm to create a
• A spanning tree is a graph in which there is no
• In a bridged LAN, a loopless topology means a
topology in which each LAN can be reached from
any other LAN through one path only (no loop).
• It is not possible to change the physical topology
of the system, but we can create a logical topology
that overlays the physical one.
• Another solution to prevent loops in LANs connected by
bridges is source routing.
• In this method, the source of the packet defines the bridges
and the LANs through which the packet should go before
reaching the destination
• Routers have access
to network layer
addresses and contain
software that enables
them to determine
which of several
addresses is the best
for a particular
• Routers operate in the
physical, data link,
and network layers of
the OSI model.
• Routers relay packets among multiple interconnected
networks. They route packets from one network to any of a
number of potential destination networks on an internet.
• Gateways potentially operate in all seven layers of the OSI
• A gateway is a protocol converter. A router by itself transfers,
accepts, and relays packets only across networks using similar
• A gateway can accept a packet formatted for one protocol (e.g.
AppleTalk) and convert it to a packet for another protocol (e.g.
• A gateway is generally software installed within a router.
The gateway understands the protocols used by each
network linked into the router and is therefore able to
translate from one to another.