Common Port Reference

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TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) ports are 16-bit numbers (0–65535) that identify specific applications or services on a networked device. When data arrives at a host, the port number tells the operating system which application should receive it. TCP provides reliable, ordered delivery with error checking, making it suitable for web, email, and file transfers. UDP provides faster, connectionless delivery without guaranteed delivery, making it suitable for DNS queries, video streaming, and gaming.

Well-known ports are port numbers 0–1023 assigned by IANA (Internet Assigned Numbers Authority) to standard network services. They require elevated (root/administrator) privileges to bind on most operating systems. Examples include port 80 for HTTP, port 443 for HTTPS, port 22 for SSH, port 25 for SMTP, and port 53 for DNS. These assignments are defined in RFC 1340 and are globally recognized.

Registered ports are port numbers 1024–49151 that are registered with IANA for specific services but do not require elevated privileges to bind. They are used by well-known applications and protocols that emerged after the well-known range was filled. Examples include port 3306 for MySQL, port 5432 for PostgreSQL, port 3389 for RDP, port 6379 for Redis, and port 8080 as an HTTP alternative.

Dynamic (ephemeral) ports are port numbers 49152–65535 that are not assigned to specific services. The operating system assigns them temporarily to client-side connections — when your browser connects to a web server, the server uses port 80 or 443, but your browser is assigned a random ephemeral port for the return traffic. These ports are also called private ports and are never registered with IANA.

Some ports are associated with protocols that transmit data in cleartext without encryption, making them vulnerable to eavesdropping and man-in-the-middle attacks. Examples include port 21 (FTP), port 23 (Telnet), port 80 (HTTP), and port 1723 (PPTP VPN). For Telnet, SSH (port 22) is the secure replacement. For FTP, SFTP (over SSH) or FTPS (port 990) are the secure alternatives. For HTTP, HTTPS (port 443) should be used instead.

Firewalls use port numbers as part of their rules to allow or block network traffic. A rule might allow inbound TCP traffic on port 443 (HTTPS) while blocking port 23 (Telnet). Stateful firewalls track connections and automatically allow return traffic for established connections. Understanding which services use which ports is essential for writing effective firewall rules that allow necessary services while minimizing the attack surface.