What is the difference between Well-Known, Registered, and Ephemeral ports?

Ports 0–1023 are Well-Known (reserved for privileged system services like HTTP, SSH). Ports 1024–49151 are Registered (assigned to user applications like MySQL, Redis). Ports 49152–65535 are Ephemeral (allocated dynamically by operating systems for client sockets).

Why must certain ports like 23 (Telnet) and 21 (FTP) be blocked on corporate firewalls?

Legacy protocols like Telnet and unencrypted FTP transmit authentication credentials and session payloads in plaintext, exposing networks to eavesdropping and credential harvesting.

Network Port Reference & Protocol Directory

Search standard TCP and UDP port numbers, protocol specifications, and security implications for firewall administrators.

Search Port Number, Service, or Keyword

Protocol Category

Port Specification & Security Profile Standard / Secure

Port Number

443

Service / Protocol

HTTPS (HTTP over TLS)

Transport Layer

TCP (and UDP in HTTP/3)

IANA Allocation Range

Well-Known (0–1023)

Architectural Description & Security Guidance

Encrypted World Wide Web traffic over TLS. Ubiquitous modern internet communications standard. Mandatory on production firewalls.

🛡️ Firewall Rule of Thumb: Follow the principle of least privilege: default-deny all inbound traffic and only permit explicitly required ports (e.g. 443 for web ingress, 22 restricted strictly to VPN/bastion IPs).

IANA Port Numbering Architecture (RFC 6335)

The Internet Assigned Numbers Authority (IANA) manages the 16-bit port number space (0 to 65535) used by transport protocols (TCP, UDP, SCTP, DCCP) to multiplex communication endpoints across an operating system.

Port Ranges & Allocation Blocks

Well-Known Ports (0–1023): Managed strictly by IANA. Historically required administrative (root/Administrator) privileges to bind. Includes DNS (53), DHCP (67/68), NTP (123), and SSH (22).
Registered Ports (1024–49151): Listed by IANA upon request for specific commercial and open-source applications, including MS SQL (1433), Oracle (1521), MySQL (3306), and Kubernetes API (6443).
Dynamic / Private / Ephemeral Ports (49152–65535): Operating systems allocate these ports transiently for outbound socket connections.