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How do two computers communicate with each other?

For two computers to communicate, they must both use the same protocol and same connection (for example, a leased line). For an application to send or receive data, it must use a particular protocol designed for that application and open up a port on the network adapter to make a connection to another computer.

 

For example, let’s say you want to visit www.google.com. You would open a browser and type http://www.bing.com. The protocol being used is HTTP, short for Hypertext Transfer Protocol, which makes the connection to the web server: google.com. The HTTP protocol would select an unused port on your computer (known as an outbound port) to send and receive data to and from google.com. On the other end, bing.com’s web server will have a specific port open at all times ready to accept sessions.

In most cases the web server’s port is 80, which corresponds to the HTTP protocol. This is known as an inbound port.

Transmission Control Protocol (TCP) sessions are known as connection-oriented sessions. This means that every packet that is sent is checked for delivery. If the receiving computer doesn’t receive a packet, it cannot assemble the message and will ask the sending computer to transmit the packet again. No one packet is left behind.

User Datagram Protocol (UDP) sessions are known as connectionless sessions. UDP is used in streaming media sessions, Voice over IP (VoIP), for protocols that use a simple query and response such as DNS, and gaming. In these cases if a packet is dropped, it is not asked for again. Let’s say you were listening to some streaming music and you heard a break in the song or a blip of some kind. That indicates some missing packets, but you wouldn’t want those packets back because by the time you get them you would be listening to a totally different part of the music stream.
 

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