How common network services use ports and protocols for web access, name resolution, addressing, remote access, mail, and directory lookup.
A port is a logical endpoint that lets multiple network services share an IP address. Troubleshooting begins by identifying the service that matches the failed task.
| Service | Common port | Purpose |
|---|---|---|
| SSH | 22 | Encrypted remote terminal access |
| HTTP / HTTPS | 80 / 443 | Web traffic; HTTPS protects the application session with TLS |
| DNS | 53 | Hostname resolution |
| DHCP | 67 / 68 | Automatic IPv4 configuration |
| RDP | 3389 | Remote desktop access |
| SMTP | 25 or 587 | Mail submission or relay |
| IMAP / POP3 | 143 / 110 | Mail retrieval |
| LDAP / LDAPS | 389 / 636 | Directory lookup; LDAPS uses TLS |
If a host can reach a service by IP address but not by name, investigate DNS before changing a cable or remote-desktop configuration. Likewise, a client without automatic addressing points toward DHCP rather than DNS.