How application services use protocols and ports for naming, addressing, remote access, web, mail, and directory operations.
Application protocols define a service’s behavior; transport ports let multiple services share an IP address. Diagnose the service role before focusing on its port number.
| Service | Common port | Primary role |
|---|---|---|
| DNS | 53 | Name resolution |
| DHCP | 67/68 | Automatic IPv4 configuration |
| HTTP / HTTPS | 80/443 | Web and API traffic |
| SSH | 22 | Encrypted remote command access |
| RDP | 3389 | Remote desktop access |
| LDAP / LDAPS | 389/636 | Directory lookup and protected directory access |
| SMTP | 25 or 587 | Mail submission or relay |
| IMAP / POP3 | 143 / 110 | Mail retrieval |
If a service is reachable by IP address but not name, investigate DNS. Use encrypted protocols where the service and environment support them.