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A network port is a number in a TCP or UDP header that helps a computer deliver incoming traffic to the right service or flow. It is not a physical socket, and it does not prove which application is using it. TCP and UDP both use port numbers, but they handle communication differently: TCP provides a reliable, in-order byte stream, while UDP sends datagrams without guaranteeing delivery or duplicate protection.
What a port number identifies
A port number is a transport-layer value carried in a TCP or UDP header. Combined with network endpoint information, it helps the receiving system direct traffic to the appropriate service or flow. TCP uses ports to identify application services and distinguish multiple flows.
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Keep these terms separate:
- IP address: identifies a network endpoint so traffic can be routed to the right host or interface.
- Transport protocol: TCP or UDP, which defines how transport-layer communication is handled.
- Port number: a numeric field used at an endpoint to direct traffic to a service or flow.
- Service name: a human-readable label, such as SSH or HTTP, associated with a registered port and transport.
A port is not a physical connector on a computer or router. A service name is also not the number itself: software can use a different port from the one commonly associated with its service.
How TCP and UDP ports differ
| Transport | Communication model | Delivery and ordering | What the port helps identify |
|---|---|---|---|
| TCP | Connection-oriented | Provides a reliable, in-order byte stream. | A service and distinct flows at an endpoint. |
| UDP | Datagram-based and minimal | UDP itself does not guarantee delivery or protect against duplicates. | The service or flow intended to receive each datagram. |
These are differences in protocol behavior, not a universal speed ranking. The standards define TCP’s and UDP’s services; they do not establish that UDP is always faster or TCP is inherently slow.
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What IANA’s port ranges mean
The IANA Service Name and Transport Protocol Port Number Registry divides the 16-bit port-number space into three ranges. The registry reported these labels and values as of its 2026-09-11 update:
| IANA range name | Port numbers | Meaning |
|---|---|---|
| System Ports | 0–1023 | Ports assigned through IANA procedures. |
| User Ports | 1024–49151 | Ports assigned through IANA procedures. |
| Dynamic and/or Private Ports | 49152–65535 | Dynamic ports are not assigned. |
The IANA registry explains that System and User port assignments follow the procedures in RFC 6335. These categories describe registry ranges; they do not mean every operating system applies identical permissions or rules to every port.
Registered ports are conventions, not guarantees
The registry lists SSH on TCP port 22 and HTTP on TCP port 80. These are registered/default examples, not a promise that every SSH or HTTP service uses those ports. Services can run on other ports, and traffic observed on a registered port may not be the service named in the registry.
IANA registration coordinates port use; it does not endorse a product or application, authenticate traffic, or establish that traffic is safe. Joe Touch’s IETF Best Current Practice, RFC 7605 (August 2015), puts the security implication plainly: “Security SHOULD NOT rely on assigned port number distinctions alone; every service, whether secure or not, is likely to be attacked.”
How to use ports when troubleshooting or configuring a firewall
A port number is a useful clue when diagnosing connectivity, but it is only one part of the picture. Check the transport protocol as well as the number: a TCP rule for port 80 is not the same as a UDP rule for port 80. Also verify the destination address, the service’s actual configuration, and the rule’s direction and scope.
- If a connection fails, confirm which protocol and port the service is configured to use rather than relying only on its familiar default.
- When reviewing a firewall rule or port scan, treat the port as an indication of where traffic is directed—not proof of the application’s identity or trustworthiness.
- To identify a service, use additional evidence, such as host configuration and application-level behavior; a port number alone cannot verify it.
For the protocol definitions, see RFC 9293 for TCP and RFC 768 for UDP. For current registered names and port assignments, consult the IANA Service Name and Transport Protocol Port Number Registry.
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