How a machine runs several services.
WHAT A PORT IS
A number identifying a particular service on a device.
WHY THEY EXIST
One address, many services.
The port says which one a connection is intended for.
HOW IT WORKS
A server listens on a port A client connects to that address and port The correct service receives the connection
WHAT WELL-KNOWN PORTS ARE
Conventional numbers for common services, so clients know where to connect.
WHAT THAT MEANS PRACTICALLY
Web, mail and file services each have conventional ports.
Your client uses them without you specifying.
WHEN YOU SPECIFY ONE MANUALLY
Mail client configuration Non-standard services Anything on an unusual port
WHAT BLOCKS PORTS
Firewalls Networks that restrict outbound traffic Providers blocking particular ports
WHAT THAT EXPLAINS
A service working on one network and not another.
WHAT TO TEST
Whether the port is reachable, rather than assuming the service is down.
