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How does a server know which application should handle a request?
A port is a 16-bit number used by transport protocols as part of identifying services and communication sessions. It is not a physical socket or a separate computer. When traffic arrives, the server’s network stack checks the transport protocol and the destination address and port, then matches that endpoint to a socket opened by a listening service.
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- The client identifies the server. It uses the server’s IP address, obtained or already known, to send traffic to the correct host.
- The client selects a transport protocol and destination port. Together, these identify the intended transport endpoint. A familiar example is HTTP traffic sent to TCP port 80, although web services can be configured differently.
- The server matches the local endpoint. The operating system checks the protocol, local address and port against sockets that applications have opened. A matching listener can receive the traffic.
- The service handles the data. The application processes the connection or datagram according to its protocol.
A port number by itself does not name a unique program. It has meaning in the context of a transport protocol and endpoint, and administrators can configure services to use different ports.
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The destination port helps select the service, while the full transport session is distinguished by the protocol, both endpoint IP addresses and both port numbers. In other words, the source address and source port identify the client-side endpoint, and the destination address and destination port identify the server-side endpoint. RFC 6335 describes ports as demultiplexing identifiers used to direct traffic among applications and sessions.
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This is why many clients can communicate with the same server service at once. Their connections can share the same server destination port while having different client-side endpoints. For TCP, a server listens for connection requests and accepted connections have peer endpoints of their own. UDP instead delivers individual datagrams to a matching socket without TCP’s connection setup.
What is the difference between an IP address and a port?
| Identifier | What it helps identify | Role in delivery |
|---|---|---|
| IP address | A network host or interface | Gets traffic to the intended host. |
| Transport port | A service endpoint within a transport protocol and host address | Helps the host deliver traffic to the matching listening socket. |
The mailroom analogy is useful only up to a point: an IP address is like the building address, and a port is like an internal destination. In actual networking, the operating system matches protocol and endpoint information rather than reading a service name from the port number.
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Why do TCP and UDP have separate port contexts?
TCP and UDP are different transport protocols, so a port assignment is interpreted alongside the protocol. A TCP listener and a UDP socket using the same numeric port are not automatically the same endpoint. For example, IANA’s registry lists HTTP on port 80 for TCP and UDP; that registry entry does not mean every web deployment uses either one. For the familiar ordinary web-request example, TCP port 80 is the relevant convention.
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What do port numbers 0–65535 mean?
IANA groups port numbers into registry categories. These categories describe registration ranges; they are not universal rules for how every operating system selects its temporary local ports.
| IANA category | Port range |
|---|---|
| System | 0–1023 |
| User | 1024–49151 |
| Dynamic/Private | 49152–65535 |
For current service and transport assignments, consult the IANA Service Name and Transport Protocol Port Number Registry. An assignment is a convention recorded in the registry, not a guarantee about what a particular server is running.
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Does an open or registered port prove which service is running?
No. A registered port can be used for something other than its assigned service, and a familiar port number does not establish that traffic is legitimate. IANA explicitly warns: “ASSIGNMENT OF A PORT NUMBER DOES NOT IN ANY WAY IMPLY AN ENDORSEMENT OF AN APPLICATION OR PRODUCT, AND THE FACT THAT NETWORK TRAFFIC IS FLOWING TO OR FROM A REGISTERED PORT DOES NOT MEAN THAT IT IS ‘GOOD’ TRAFFIC, NOR THAT IT NECESSARILY CORRESPONDS TO THE ASSIGNED SERVICE.” Administrators should configure firewalls based on what the traffic is, not on the port number alone.
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Why might the public port not match the service’s local port?
Port forwarding, network address translation (NAT), reverse proxies, containers and firewall rules can affect how a public endpoint maps to an internal service. A request that reaches a particular public IP address and port may be forwarded or proxied to a different address or port inside the network. The simple direct-match explanation applies when no such mapping changes the destination; RFC 6335 also notes constraints on multiple instances sharing a port and on mappings through a NAT gateway.
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For the standards-level definitions and port-management details, see RFC 6335. IANA’s IANA 101 briefing paper also uses HTTP port 80 as a basic illustration.
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