Common ports & protocols
Why it matters: Port numbers turn up in firewall-rule, log and scenario questions. Your guide mentions some but has no reference table.
Overview
Supplementary — not from your PDFServices listen on well-known port numbers. Secure versions of old protocols often use a different port, or upgrade the same port to TLS with STARTTLS. Learn them in insecure/secure pairs.
Port reference
| Port | Protocol | Notes |
|---|---|---|
| 20/21 | FTP (TCP) | Insecure file transfer; replace with SFTP or FTPS |
| 22 | SSH / SFTP / SCP (TCP) | Encrypted remote shell and file transfer |
| 23 | Telnet (TCP) | Insecure remote shell; replace with SSH |
| 25 | SMTP (TCP) | Mail transfer between servers |
| 53 | DNS (UDP/TCP) | Name resolution; TCP for zone transfers and large replies |
| 67/68 | DHCP (UDP) | Address assignment |
| 69 | TFTP (UDP) | Trivial file transfer, no authentication |
| 80 | HTTP (TCP) | Unencrypted web |
| 88 | Kerberos (UDP/TCP) | Authentication in Windows domains |
| 110 | POP3 (TCP) | Mail retrieval; secure version 995 |
| 123 | NTP (UDP) | Time sync; accurate time matters for logs and Kerberos |
| 143 | IMAP (TCP) | Mail access; secure version 993 |
| 161/162 | SNMP (UDP) | Monitoring (162 = traps); use SNMPv3 |
| 389 | LDAP (TCP/UDP) | Directory queries; secure version 636 (LDAPS) |
| 443 | HTTPS (TCP) | Web over TLS |
| 445 | SMB (TCP) | Windows file sharing |
| 465 / 587 | SMTPS / submission (TCP) | Mail submission from clients, with TLS |
| 514 | Syslog (UDP) | Log forwarding; 6514 for syslog over TLS |
| 636 | LDAPS (TCP) | LDAP over TLS |
| 989/990 | FTPS (TCP) | FTP over TLS |
| 993 | IMAPS (TCP) | IMAP over TLS |
| 995 | POP3S (TCP) | POP3 over TLS |
| 1433 | Microsoft SQL Server (TCP) | Database; should not face the internet |
| 1812/1813 | RADIUS (UDP) | Authentication and accounting for network access |
| 3306 | MySQL (TCP) | Database |
| 3389 | RDP (TCP) | Remote Desktop; put behind a VPN or gateway |
| 5060/5061 | SIP (TCP/UDP) | VoIP signalling; 5061 with TLS |
Memory tips
- Learn in pairs: HTTP 80 → HTTPS 443, LDAP 389 → LDAPS 636, IMAP 143 → IMAPS 993, POP3 110 → POP3S 995, Telnet 23 → SSH 22.
- SFTP runs over SSH (22). FTPS is FTP over TLS (989/990). They are different protocols.
- In a firewall-rule question, first find which port the service needs, then whether it should be allowed from that source at all.