The ports you must know
| Port | Protocol | Service |
|---|---|---|
| 20 / 21 | TCP | FTP — file transfer (20 data, 21 control) |
| 22 | TCP | SSH / SCP / SFTP — secure shell & transfer |
| 23 | TCP | Telnet — unencrypted remote login |
| 25 | TCP | SMTP — sending email |
| 53 | TCP/UDP | DNS — name resolution |
| 67 / 68 | UDP | DHCP — IP assignment (67 server, 68 client) |
| 69 | UDP | TFTP — trivial file transfer |
| 80 | TCP | HTTP — web |
| 110 | TCP | POP3 — receiving email |
| 123 | UDP | NTP — time sync |
| 143 | TCP | IMAP — email with server-side folders |
| 161 / 162 | UDP | SNMP — network management (162 traps) |
| 389 | TCP/UDP | LDAP — directory services |
| 443 | TCP | HTTPS — web over TLS |
| 445 | TCP | SMB — Windows file sharing |
| 636 | TCP | LDAPS — LDAP over TLS |
| 993 / 995 | TCP | IMAPS / POP3S — secure email |
| 3389 | TCP | RDP — Windows remote desktop |
| 3306 / 5432 | TCP | MySQL / PostgreSQL databases |
Plaintext vs secure pairs
Pattern worth memorizing
Many services have a plaintext port and a TLS-secured sibling: HTTP 80 → HTTPS 443, IMAP 143 → IMAPS 993, POP3 110 → POP3S 995, LDAP 389 → LDAPS 636. Prefer the secure port in production.
The three port ranges
- Well-known: 0–1023 (the services above).
- Registered: 1024–49151 (vendor apps, e.g. 3306, 3389).
- Dynamic / ephemeral: 49152–65535 (temporary client-side ports).
Test yourself
4 quick questions — tap an answer to check it instantly. Nothing is sent anywhere.
1. Which port does SSH use?
Answer: B. SSH (and SFTP/SCP) use TCP port 22. Port 23 is Telnet (unencrypted).
2. DNS primarily uses which port?
Answer: A. DNS uses port 53 — UDP for most queries, TCP for zone transfers and large responses.
3. Which is the secure version of IMAP?
Answer: C. IMAPS is port 993. 143 is plain IMAP; 995 is POP3S.
4. RDP (Windows Remote Desktop) uses which port?
Answer: B. RDP uses TCP 3389. 3306 is MySQL, 5432 is PostgreSQL.
Sources
- IANA — Service Name and Transport Protocol Port Number Registry.
- CompTIA Network+ / Security+ exam objectives.
Reference summary only. Authoritative assignments are in the IANA Service Name and Transport Protocol Port Number Registry.