How services communicate.
WHAT A PROTOCOL IS
An agreed set of rules for exchanging data.
WHAT A PORT IS
A number identifying a particular service on a device.
WHY THEY EXIST
One address, many services.
WHAT THE COMMON PROTOCOLS ARE
Web traffic, encrypted and unencrypted Mail sending and retrieval File transfer, plain and encrypted Secure remote access Name resolution Automatic address assignment Time synchronisation
WHAT TO ALWAYS PREFER
The encrypted variant, where both exist.
WHY
Plain variants transmit credentials readable.
WHAT THE TWO TRANSPORT APPROACHES ARE
Connection-oriented, confirming delivery and resending what is lost Connectionless, faster with no guarantee
WHAT EACH SUITS
The first: web, mail, file transfer
The second: name resolution, voice and video
WHY THAT MATTERS IN SUPPORT
A firewall rule for one does not cover the other.
WHAT TO TEST WHEN A SERVICE IS UNREACHABLE
Whether the port is reachable, which distinguishes a blocked port from a failed service.
