This document discusses different types of network devices used to connect computer networks, including gateways, bridges, and routers. Gateways connect different types of networks and perform protocol conversion. Bridges operate at the data link layer and forward packets between network segments based on MAC addresses. Routers use routing tables and protocols to determine the best path between networks and forward packets accordingly. Dynamic routing allows routers to automatically update their routing tables based on network changes.

1. COMPUTER NETWORKS

2. TOPIC
GATEWAYS , BRIDGES AND
ROUTERS

3. CONTENTS
 Gateways
 Types of gateways
 Bridges
 Types of bridges
 Routers
 Types of routers
 Routing tables
 Dynamic and static routing

4. GATEWAYS
 Network element that acts as an entrance
point to another network.
 It is generally a workstation or server
 Two way path b/w networks
 Used to connect different networks
 Belongs to transport layer and application
layer of OSI model.
 Protocol conversion also done by gateways.
 gateways interconnects the heterogeneous
networks e.g. Microsoft windows NT server
to SNA (IBM’s system network architecture)

5. TYPES OF GATEWAYS
 Basically there are two types of gateways
(1) Transport gateway and
(2) application gateway
 The former is for transport layer and the later is for
application layer.
 Transport gateways connect two computers that use
different connection oriented transport protocol. It can
copy the packets to one connection to other,
reformatting them as need be.
 Application gateways understand the format and
contents of the data and translate messages from one
form to another e.g. an e-mail gateway could translate
internet messages to an SMS messages on mobile
phone. So it identifies the actual meaning of data.

6. BRIDGES
 Another device to connect LAN s
 Operates at the data link layer
 Provides packet filtering
 A Bridge examines each packet as it enters through
one of the ports. It first looks at the MAC address of the
sender and creates a mapping between the port and
the sender's MAC address. It then looks at the address
of the recipient , comparing the MAC address to the list
of all learned MAC address. If the address is in list, the
Bridge look up the port number and forwards the
packet to the port where it thinks the recipient is
connected. If the recipient's MAC address is not in the
list , the Bridge blocks the data from passing.
 Bridge can connect different network architectures like
Ethernet and Token Ring

8. TYPES OF BRIDGES
1> Transparent Bridge.
2> Translational Bridge.
3> Source-route Bridge.
Other than above there are also local bridges ,
remote bridges.

9.  Transparent Bridge :
- invisible to the other devices on the network.
- only performs the function of blocking or
forwarding data based on MAC address. MAC
address may also be referred as hardware
address or physical address. These addresses
are used to built tables and make decision
regarding whether a frame should be forward
and where it should be forwarded.

10.  Translational Bridge :
- Translational Bridges are useful to connect
segments running at different speeds or
using different protocols such as token
Ring and Ethernet networks.
- Depending on the direction of travel, a
Translational Bridge can add or remove
information and fields from frame as
needed.

11.  Source-route Bridge :
- Source-route Bridges were designed by
IBM for use on Token ring networks.
- derives the entire route of the frame
embedded within the frame.
- This allows the Bridge to make specific
decision about how the frame should be
forwarded through the network.

12. ROUTERS
 A router is a device that forwards data packets along
networks.
 A router is connected to at least two networks,
commonly two LANs or WANs or a LAN and
its ISP's network.
 Routers are located at gateways, the places where
two or more networks connect.
 Routers use headers and forwarding tables to
determine the best path for forwarding the packets,
and they use protocols such as ICMP to communicate
with each other and configure the best route between
any two hosts.
 Very little filtering of data is done through routers.

13. TYPES OF ROUTERS
 Broadband Routers
 Wireless routers
 Edge router
 Subscriber edge router
 Label edge router
 Inter-provider border router
 Core router
 Wired routers

14. ROUTING TABLE
 A routing table is a set of rules, often viewed in table
format, that is used to determine where data packets
traveling over an Internet Protocol(IP) network will be
directed.
 All IP-enabled devices, including routers and switches, use
routing tables.
 A routing table contains the information necessary to forward
a packet along the best path toward its destination.
 Each packet contains information about its origin and
destination.
 When a packet is received, a network device examines the
packet and matches it to the routing table entry providing the
best match for its destination. The table then provides the
device with instructions for sending the packet to the
next hop on its route across the network.

15. A basic routing table includes the following information:
 Destination: The IP address of the packet's final
destination
 Next hop: The IP address to which the packet is
forwarded
 Interface: The outgoing network interface the device
should use when forwarding the packet to the next
hop or final destination
 Metric: Assigns a cost to each available route so that
the most cost-effective path can be chosen
 Routes: Includes directly-attached subnets, indirect
subnets that are not attached to the device but can be
accessed through one or more hops, and default
routes to use for certain types of traffic or when
information is lacking.

16. Static and dynamic routing tables
 Routing tables can be maintained manually or
dynamically.
 Tables for static network devices do not change
unless a network administrator manually changes
them.
 In dynamic routing, devices build and maintain
their routing tables automatically by using
routing protocols to exchange information about
the surrounding network topology.
 Dynamic routing tables allow devices to "listen"
to the network and respond to occurrences like
device failures and network congestion.

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