Who asks and who answers.
WHAT A CLIENT IS
A device or program making a request.
Your browser, your mail application, your phone.
WHAT A SERVER IS
A device or program answering requests.
A web server, a mail server, a database server.
WHAT THAT MEANS
Server is a role, not necessarily a machine.
One machine can run several servers, and a machine can be both client and server.
HOW A CONNECTION IS MADE
The client contacts the server at its address, on a specific port.
The server accepts and they exchange data.
WHY THE DIRECTION MATTERS
Servers listen. Clients initiate.
A client behind a router can reach servers freely; a server needs to be reachable.
WHAT THAT EXPLAINS
Why hosting a service at home is harder than browsing from home.
WHAT LISTENING MEANS
A program waiting for connections on a particular port.
WHAT TO UNDERSTAND
Anything listening and reachable from the internet will be found by automated scanning.
