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Node A (Computer) Node B (Computer)
Modem Modem
Telephone line
Digital signal Digital signal
Analog signal
Figure 6.21 MODEM
6.5.8 Router
Routers are used for connecting various networks. A router may be used to interconnect several networks, possibly
having different topologies. Also, a router may be used to connect several nodes (computers, mobile phones,
tablets, etc.) to the Internet. For example, a WiFi router is often used in homes to connect to the Internet. Each
router has a routing table that it consults for forwarding an incoming packet to an outgoing line. It examines the
destination address (IP address) of a received packet and sends the packet to the appropriate network, keeping
in view the current information about the network. Figure 6.22 shows a router that is connected to two switches.
In Figure 6.23, we see several nodes, connected to a router. On receiving a packet from a node, it passes it on to
the Internet.
Router
Switch 1 Switch 2
Hub
Figure 6.22 Router
Node 1
Node 1 Router to the Internet
a
b
c
Node n
Figure 6.23 Use of Router
6.5.9 Gateway
A gateway is a hardware device that operates as a "gate" between networks. A gateway connects the networks that
use different protocols. Thus, a gateway may be thought of as a router with protocol conversion software. The data
from a network that uses a specific protocol is transformed to make it compatible with the outgoing line’s protocol.
Figure 6.24 shows a switch that acts as a gateway for the data flowing from the server to any node (A-E) or vice versa.
Also, data from a node (say D) to a node (say A) will flow via the gateway. Figure 6.25 shows two different networks
connected via a router. Here, the router acts as another gateway. If node B wants to send data to node G, the data will
flow from node B to its server (gateway), passing through the router (gateway). Subsequently, the router will pass the
data to the server of the second network, which will pass it on to node G.
Computer Networks 235

