FREE QUICK REFERENCE

Common ports & protocols

Memorize the service, then understand why it matters in troubleshooting and security.

Secure remote administration.

Legacy plaintext remote terminal; prefer secure alternatives.

Email transfer between mail systems.

Name resolution; commonly UDP and TCP depending on operation.

67/68

IPv4 address configuration between server and client.

Unencrypted web traffic.

110

Legacy email retrieval.

123

Network time synchronization.

143

Email access and mailbox synchronization.

161/162

Network management queries and traps.

389

Directory services.

443

HTTP protected with TLS.

445

Windows file and resource sharing.

3389

Microsoft Remote Desktop.

Port numbers are useful clues, not proof of an application. Services can run on non-default ports, so validate actual traffic and configuration.