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A port in computer hardware is a jack or socket that peripheral hardware plugs into.
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A port in networking is a virtual point where network connections start and end. Ports are software-based and managed by a computer's operating system. Each port is associated with a specific process (network protocol). Ports allow computers to easily differentiate between different kinds of traffic: emails go to a different port than webpages, for instance, even though both reach a computer over the same physical network connection.
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A port number is a 16-bit unsigned integer, thus ranging from 0 to 65,535 (2^16 - 1), although not all are in common use.
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The port numbers are divided into three ranges :
- System or well-known ports : 0 – 1023
- User or registered ports : 1024 – 49151
- Dynamic or private ports : 49152 – 65535
There's no technical difference between a "well-known" port, or a "registered port", or a "dynamic" port. Any of them can be used for a service/daemon to listen on, any of them can be port-forwarded in a NAT gateway, and any of them can be used for a client to initiate a connection from.
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The client and server don't need to use the same port. Client generally uses random port (e.g,2000) to connect to a server with a specific port (e.g, 80). The server responds back to that machine using the random port.
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Ports are a transport layer (layer 4) concept. Only a transport protocol such as the TCP or UDP can indicate which port a packet should go to. TCP and UDP headers have a section for indicating port numbers. Network layer protocols — for instance, the Internet Protocol (IP) — are unaware of what port is in use in a given network connection. In a standard IP header, there is no place to indicate which port the data packet should go to. IP headers only indicate the destination IP address, not the port number at that IP address.
Some of the most commonly used ports :
| Port Number | Transport Protocol | Service |
|---|---|---|
| 20,21 | TCP | FTP |
| 22 | TCP | SSH |
| 23 | TCP | Telnet |
| 25 | TCP | SMTP |
| 53 | TCP & UDP | DNS |
| 80 | TCP | HTTP |
| 110 | TCP | POP3 |
| 123 | UDP | NTP |
| 179 | TCP | BGP |
| 443 | Transport Layer Security(TLS) | HTTPS |