What are well-known ports?

Well-known ports are the range 0 to 1023, assigned by IANA to core internet services such as HTTP, SSH, and DNS. On most operating systems, binding to a port in this range requires administrative privileges.

What is the difference between TCP and UDP for a port?

TCP is connection-oriented and reliable, used by services like HTTP and SSH. UDP is connectionless and lightweight, used by DNS queries, DHCP, and NTP. Some services, like DNS, use both depending on the message size.

What service runs on port 443?

Port 443 is HTTPS, which is HTTP over TLS. Modern browsers also use it for HTTP/3, which runs over UDP using QUIC, so 443 carries both encrypted web traffic over TCP and over UDP.

Why are ports 1024 and above not listed here?

Ports 1024 to 49151 are registered ports assigned to specific applications, and 49152 to 65535 are dynamic or ephemeral ports used for outgoing connections. This reference focuses on the well-known range 0 to 1023.

Should I open all these ports on my firewall?

No. Open only the ports for services you actually run, and prefer the encrypted variants such as 22, 443, 465, 993, and 995. Leaving unencrypted legacy services like Telnet on port 23 exposed is a security risk.

What is the TCP/UDP Well-Known Port Reference?

Search the IANA well-known ports 0 to 1023 by number, service name, or keyword and see the assigned service, transport protocol, and what it is used for. A fast reference for sysadmins and developers. It runs free in your browser on Gera Tools, with nothing uploaded.

TCP/UDP Well-Known Port Reference

Name: TCP/UDP Well-Known Port Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Look up any well-known port

Network services listen on numbered ports, and the lowest range — 0 to 1023 — is reserved for the core protocols of the internet. IANA assigns these well-known ports so that, for example, web traffic always finds a server on port 80 or 443. This reference lets you search by port number, service name, or keyword and shows the assigned service and transport protocol.

How it works

Every entry follows the IANA Service Name and Transport Protocol Port Number Registry. A port assignment specifies a number, a service name, and whether the service uses TCP, UDP, or both. TCP is reliable and connection-oriented; UDP is fast and connectionless. Some services appear under both — DNS on port 53 uses UDP for small queries and TCP for larger transfers and zone moves. The search matches your query against the port number, the service name, and the description.

Commonly referenced well-known ports

Port	Service	Protocol	Common use
20, 21	FTP	TCP	File transfer (data and control)
22	SSH	TCP	Secure remote shell
25	SMTP	TCP	Email delivery (server-to-server)
53	DNS	UDP/TCP	Domain name resolution
80	HTTP	TCP	Unencrypted web traffic
110	POP3	TCP	Email retrieval (legacy)
143	IMAP	TCP	Email retrieval (sync)
443	HTTPS	TCP/UDP	Encrypted web (TLS + QUIC)
465	SMTPS	TCP	Email submission over TLS
587	Submission	TCP	Email submission (STARTTLS)
993	IMAPS	TCP	IMAP over TLS
995	POP3S	TCP	POP3 over TLS

This is a selection of the most commonly referenced ports. Use the search above to look up any port 0–1023.

TCP vs. UDP — when each is used

TCP (Transmission Control Protocol) establishes a connection before sending data and guarantees delivery and ordering. Every packet is acknowledged; lost packets are retransmitted. This reliability comes at the cost of latency from the handshake and acknowledgement overhead. TCP is used where data integrity matters: web pages (HTTP/HTTPS), email, SSH, file transfers.

UDP (User Datagram Protocol) fires packets without connection setup or delivery guarantees. This makes it faster and lower-latency, but the application must handle lost packets itself if it cares about them. UDP is used where speed or real-time delivery matters more than guaranteed order: DNS queries (where a lost query just means the client sends another one), streaming video, online gaming, and VoIP.

Some protocols use both: DNS uses UDP for standard queries (fast, small) and TCP for large responses and zone transfers. HTTPS on HTTP/3 uses QUIC, which runs over UDP but reimplements reliability at the application layer.

Security guidance for firewall configuration

When configuring a firewall, the key principle is allow only what you actively run and block everything else. Some guidance by category:

Open on internet-facing servers: 22 (SSH, key-auth only), 80 (redirect to 443), 443 (HTTPS).
Open only from trusted sources: 3306 (MySQL), 5432 (PostgreSQL), 6379 (Redis), 27017 (MongoDB). These are registered ports, not in this reference, but they should never be exposed to the internet.
Block or disable: 23 (Telnet), 21 (FTP), 110 (POP3), 143 (IMAP) — unencrypted legacy services. Use SSH, SFTP, IMAPS, and POP3S instead.

Binding to a well-known port usually requires root or administrator rights, which is why application servers often run on higher ports behind a reverse proxy. All lookups run locally in your browser.