termonology

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GATEWAY
Gateway is used to translate one protocol into another protocol. gateway is a component that used in telecommunication. gateway works on multiple point.
In OSI gateway is used at different different level , and depend upon the type of protocol.

in above diagram, we have a 4 office computers and one printer that are connected through a switch and make a LAN . switch is connected with a gateway that help to translate the office computer data and send via. a modem to internet. office data protocol is different and through internal the users uses different different protocol so making a connection between office computer and users computers we used gateways.

as many times we saw "default gateway". what is default gateway???

default gateway is a device that passes the traffic from local subnet to the other subnet. it help to connect local network to internet.

as we saw in the diagram,the office computers are local network and through gateway these connect to the internet.