Data Communications and
Computer Networks: A
Business User’s Approach
Third Edition
Chapter 8:
Local Area Networks: Internetworking
Objectives
•List the reasons for interconnecting multiple local area
network segments and interconnecting local area
networks and wide area networks
•Identify the functions and purposes of the various
interconnection devices that have been used over time
•Define a hub and describe the situations in which a
hub is used
•Describe how a transparent bridge learns
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Objectives (continued)
•Describe what a switch is and how it differs from other
devices
•Describe the types of situations in which using a
switch is advantageous
•Describe what a router is and how it differs from other
devices
•Describe the types of situations in which a router is
used
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Introduction
•Sometimes necessary to connect local area network to
another LAN or to wide area network
•Local area network to local area network connections
often performed with bridge-like device
•Local area network to wide area network connections
usually performed with router
•Switch - can be used to interconnect segments of a local area
network
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Why Segment or Internetwork Local
Area Networks?
•To separate / connect one corporate division with
another
•To connect two LANs with different protocols
•To connect a LAN to the Internet
•To break a LAN into segments to relieve traffic
congestion
•To provide a security wall between two different types
of users
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Hubs
•Hub - interconnects two or more workstations into a local area
network
•When workstation transmits to a hub
•Hub immediately resends data frame out to all connecting
links
•Hub can be managed or unmanaged
•Managed hub possesses enough processing power that it can
be managed from a remote location
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Bridges
•A bridge (or bridge-like device)
•Can be used to connect two similar LANs, such as two
CSMA/CD LANs
•Can also be used to connect two closely similar LANs, such
as a CSMA/CD LAN and a token ring LAN
•Examines destination address in a frame
•Either forwards this frame onto next LAN or does not
•Examines source address in frame
•Places this address in a routing table to be used for future
routing decisions
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Bridges (continued)
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Transparent Bridge
•Does not need programming
•Observes all traffic  builds routing tables
•This observation is called backward learning
•Each bridge has two connections (ports)
•Routing table associated with each port
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Transparent Bridge (continued)
•Bridge
•Observes each frame that arrives at port
•Extracts the source address from the frame
•Places that address in the port’s routing table
•Transparent bridge is found with CSMA/CD LANs
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Transparent Bridge (continued)
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Transparent Bridge (continued)
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Transparent Bridge (continued)
•Transparent bridge can also convert one frame format
to another
•Note that some people / manufacturers call this kind of
bridge a gateway or router
•Bridge
•Removes headers and trailers from one frame format
•Inserts (encapsulates) headers and trailers for second frame
format
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Remote Bridge
•Capable of passing data frame from one local area
network to another when
•Two LANs are separated by long distance
•There is a wide area network connecting the two LANs
•Remote bridge takes the frame before it leaves the first
LAN
•Encapsulates the WAN headers and trailers
•When packet arrives at destination remote bridge 
that bridge removes the WAN headers and trailers
leaving original frame
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Remote Bridge (continued)
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Switches
•Combination of hub and bridge
•Can interconnect two or more workstations
•Like bridge, it observes traffic flow and learns
•When a frame arrives at a switch, the switch
•Examines destination address
•Forwards frame out the one necessary connection
•Workstations that connect to
•Hub  on shared segment
•Switch  on switched segment
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Switches (continued)
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Switches (continued)
•Backplane of a switch is fast enough to support
multiple data transfers at one time
•A switch that employs cut-through architecture is
passing on the frame before the entire frame has arrived
at the switch
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Switches (continued)
•Multiple workstations connected to a switch use
dedicated segments
•Very efficient way to isolate heavy users from the network
•A switch can allow simultaneous access to multiple
servers, or multiple simultaneous connections to a
single server
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Switches (continued)
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Isolating Traffic Patterns and Providing
Multiple Access
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Isolating Traffic Patterns and Providing
Multiple Access (continued)
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Isolating Traffic Patterns and Providing
Multiple Access (continued)
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Isolating Traffic Patterns and Providing
Multiple Access (continued)
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Full-Duplex Switches
•Allows for simultaneous transmission and reception of
data to and from a workstation
•This full duplex connection helps eliminate collisions
•To support a full duplex connection to a switch, at
least two pairs of wires are necessary
•One for the receive operation
•One for the transmit operation
•Most people install four pairs today, so wiring is not
problem
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Full-Duplex Switches (continued)
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Network Servers
•Network servers provide necessary storage for LAN
software
•Usually focal point for network operating system
•Increasingly, network servers are functioning as
bridges, switches, and routers
•By adding appropriate card, a server can assume
multiple functions
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Routers
• Router - device that connects a LAN to a WAN or a WAN
to a WAN
• Router:
1. Accepts outgoing packet
2. Removes any LAN headers and trailers
3. Encapsulates necessary WAN headers and trailers
• Because router has to make wide area network
routing decisions  router has to dig down into the
network layer of the packet to retrieve network
destination address
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Routers (continued)
•Routers are often called “layer 3 devices”
•Operate at the third layer, or OSI network layer, of the
packet
•Often incorporate firewall functions
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Routers (continued)
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LAN Internetworking in Action:
A Small Office Revisited
•Recall In Action example from Chapter Seven
•A small office with 20 workstations in one room and
15 workstations in another room were connected to a
server via 100BaseTX
•One hub was kept in a closet near the 20 workstations
while a second hub was near the server
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LAN Internetworking in Action:
A Small Office Revisited (continued)
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LAN Internetworking in Action:
A Small Office Revisited (continued)
•Now Hannah wants to connect the LAN to the Internet
•She adds a router next to the server and connects it to
the hub
•She connects the router to a high-speed telephone line
such as a T-1 service
•She will also have to program the router to perform IP
addressing and firewall functions
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LAN Internetworking in Action:
A Small Office Revisited (continued)
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LAN Internetworking in Action:
A Small Office Revisited (continued)
•Now network usage is so high that Hannah must
consider segmenting the network
•She decides to install a database server near the
original server and replace both hubs with switches
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LAN Internetworking in Action:
A Small Office Revisited (continued)
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Summary
• Interconnecting multiple LAN segments as well as
LANs and WANs
• Interconnection devices
• Hubs
• Transparent bridges
• Switches
• Routers
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