Network+ (N10-009) · ELI Explains Port Numbers (book)
Commonly Confused Ports
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and 21 — FTP Data vs. Control
Port 21 handles FTP commands and authentication. Port 20 handles the actual file data in active mode. Think of 21 as the front desk where you place your order, and 20 as the back door where the files are delivered.
and 23 — SSH vs. Telnet
Both provide remote command-line access, but SSH on port 22 encrypts everything while Telnet on port 23 sends all data — including passwords — in plaintext. Always prefer SSH.
, 465, and 587 — SMTP Relay and Submission
Port 25 routes email between mail servers. Port 587 is the message-submission service commonly used with STARTTLS. Port 465 is standardized for submission over implicit TLS.
DNS over TCP vs. UDP
Most DNS queries use UDP on port 53 for speed. TCP on the same port handles zone transfers between DNS servers and responses too large for a single UDP packet.
and 68 — DHCP Server vs. Client
DHCP servers listen on port 67. DHCP clients send requests from port 68. When joining a network, your device broadcasts from 68 to reach a server on 67.
and 443 — HTTP vs. HTTPS
Both deliver web pages. Port 80 sends everything in plaintext. Port 443 wraps the same content in TLS encryption, protecting passwords, personal data, and browsing privacy.
and 143 — POP3 vs. IMAP
POP3 retrieves mail and may delete server copies according to client settings and server policy. IMAP works with server-hosted mailboxes so multiple devices can stay synchronized.
, 138, 139, and 445 — NetBIOS Suite vs. SMB Direct
Ports 137–139 are legacy NetBIOS services. Port 445 is modern SMB directly over TCP without NetBIOS. Modern Windows file sharing uses 445.
and 162 — SNMP Query vs. Trap
Port 161 receives polling requests from management systems. Port 162 receives unsolicited alerts from devices. Think: 161 asks "how are you?", 162 shouts "something broke!"
and 636 — LDAP vs. LDAPS
Both are used with directory services. Port 636 starts with implicit TLS. Port 389 may be plaintext or may be upgraded with StartTLS; verify that TLS is required and certificates are validated.
and 990 — FTPS Data vs. Control
Ports 990 and 989 are assigned to legacy implicit-FTPS control and data channels. Modern explicit FTPS normally upgrades a port 21 control session and negotiates data ports. SFTP is a different protocol over SSH on port 22.
and 995 — IMAPS vs. POP3S
Both provide encrypted email retrieval. IMAPS on port 993 works with a server-hosted mailbox for multi-device synchronization. POP3S on port 995 retrieves mail and may delete server copies according to client settings and server policy.
and 5900 — RDP vs. VNC
Both provide graphical remote access. RDP is native to Windows and supports encrypted transport. VNC security capabilities vary by implementation; use a secure implementation or protect it with a VPN or SSH tunnel.
and 5986 — WinRM HTTP vs. HTTPS
Both carry WinRM and PowerShell remoting. Port 5985 uses HTTP and can still use message-level encryption with appropriate authentication. Port 5986 adds HTTPS transport encryption and certificate-based server identity.
, 8080, and 8443 — Alternate Web Ports
Ports 8000 and 8080 are alternate unencrypted HTTP ports for development and proxies. Port 8443 is the encrypted HTTPS alternate. These are side doors to the same web services on 80 and 443.
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