Well-known ports
The TCP/UDP port numbers the CCNA (and the job) expects you to know — searchable, with a built-in quiz to drill them into memory. Look one up, or switch to quiz mode and test yourself. No account, nothing to install.
| Port | Protocol | Service | What it's for |
|---|---|---|---|
20 | TCP | FTP (data) | FTP file-transfer data channel |
21 | TCP | FTP (control) | FTP commands / control channel |
22 | TCP | SSH | Secure remote CLI (also SCP, SFTP) |
23 | TCP | Telnet | Legacy remote CLI — cleartext, insecure |
25 | TCP | SMTP | Sending email between servers |
53 | TCP/UDP | DNS | Name resolution (UDP queries, TCP zone transfers) |
67 | UDP | DHCP (server) | DHCP/BOOTP server listens here |
68 | UDP | DHCP (client) | DHCP client receives here |
69 | UDP | TFTP | Trivial FTP — IOS images, configs |
80 | TCP | HTTP | Web traffic (unencrypted) |
110 | TCP | POP3 | Retrieving email (download) |
123 | UDP | NTP | Network Time Protocol clock sync |
143 | TCP | IMAP | Retrieving email (server-side) |
161 | UDP | SNMP | Network monitoring polling (get/set) |
162 | UDP | SNMP Trap | SNMP notifications to the manager |
179 | TCP | BGP | Border Gateway Protocol peering |
389 | TCP/UDP | LDAP | Directory services (e.g. AD) |
443 | TCP | HTTPS | Secure web traffic (TLS) |
500 | UDP | ISAKMP/IKE | IPsec VPN key exchange |
514 | UDP | Syslog | Logging messages to a server |
520 | UDP | RIP | Routing Information Protocol updates |
587 | TCP | SMTP (submission) | Email submission from clients |
636 | TCP | LDAPS | LDAP over TLS |
3389 | TCP | RDP | Windows Remote Desktop |
Ports FAQ
What are well-known ports?
Well-known ports are the reserved TCP/UDP port numbers from 0 to 1023 that identify common services — like 22 for SSH, 53 for DNS, or 443 for HTTPS. When a client connects, the destination port tells the server which application should handle the traffic. The CCNA expects you to recognize the common ones on sight.
What's the difference between a TCP and a UDP port?
The number can be the same, but TCP and UDP are separate protocols with their own port space. TCP ports carry connection-oriented, reliable traffic (a three-way handshake, acknowledgements); UDP ports carry connectionless, best-effort traffic. DNS famously uses UDP 53 for normal queries but TCP 53 for larger transfers, so it appears under both.
Which ports should I memorize for the CCNA?
Focus on the everyday services: FTP 20/21, SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67/68, TFTP 69, HTTP 80, POP3 110, NTP 123, IMAP 143, SNMP 161/162, HTTPS 443, and Syslog 514. Use the quiz above until you can answer each without hesitating.
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