May 20, 2023
In computer networking, a port is a logical construct that allows multiple networked applications or processes to share a single physical communication channel. A port is identified by a 16-bit number, also known as a port number, which is used by the transport layer of the Internet Protocol (IP) suite to identify the specific application or service that is running on a particular device.
Purpose and Usage
The primary purpose of ports is to enable communication between different applications or services that are running on different devices across a network. Without ports, these applications or services would have no way to establish a connection with each other.
Each application or service that needs to communicate over the network must be assigned a unique port number, which is used to identify the application or service to which the communication is intended. Port numbers are divided into three ranges, which are assigned by the Internet Assigned Numbers Authority (IANA):
Well-known ports (0-1023) are reserved for system services and applications that are generally available to all users. Examples include HTTP (port 80), FTP (port 21), and SSH (port 22).
Registered ports (1024-49151) are assigned to applications and services by the IANA, but are available for general use. Examples include BitTorrent (port 6881) and MySQL (port 3306).
Dynamic or private ports (49152-65535) are used by client applications and are assigned dynamically by the operating system. These ports are not reserved and are available for use by any application.
When a client application wants to communicate with a server application, it must specify the IP address and port number of the server. The client then sends a connection request to the server, which responds with an acknowledgement if the port is open and available for communication. If the port is closed or blocked, the server will send a different response, indicating that the connection is not possible.
There are two main types of ports that are used in networking: TCP and UDP.
Transmission Control Protocol (TCP) is a connection-oriented protocol that provides reliable, ordered delivery of data between applications. TCP ports are used by applications that require a reliable, error-free communication channel, such as email, web browsing, and file transfers.
When a TCP connection is established between two devices, a three-way handshake is used to negotiate the connection. The client device sends a SYN (synchronize) packet to the server, which responds with a SYN-ACK (synchronize-acknowledge) packet. The client then sends an ACK (acknowledge) packet to confirm the connection. Once the connection is established, data can be exchanged between the client and server.
TCP ports are assigned to specific applications or services based on their functionality. For example, port 80 is used for HTTP traffic, port 443 is used for HTTPS traffic, and port 25 is used for SMTP traffic.
User Datagram Protocol (UDP) is a connectionless protocol that provides faster, but less reliable, delivery of data between applications. UDP ports are used by applications that need to send data quickly and do not require the reliability or ordered delivery of TCP, such as video streaming, online gaming, and VoIP.
UDP does not establish a connection between devices before transmitting data. Instead, each packet is sent independently and may be lost or duplicated in transit. UDP ports are assigned to specific applications or services based on their functionality, but because UDP does not establish a connection, the port number may not be as critical as it is with TCP.
Ports are a critical component of network security, as they can be used by attackers to gain unauthorized access to a device or network. There are several port-related security measures that can be implemented to help protect against these types of attacks.
Firewalls are used to control access to a device or network by filtering traffic based on a set of predefined rules. A firewall can be configured to block traffic on specific ports to prevent unauthorized access or to only allow traffic on certain ports to specific devices.
Port scanning is a technique used by attackers to identify open ports on a device or network. This information can be used to identify vulnerabilities that can be exploited to gain unauthorized access. Port scanning can be detected and prevented by implementing security measures such as firewalls and intrusion detection systems.
Port knocking is a technique used to hide open ports from attackers by requiring a specific sequence of connection attempts on certain closed ports before the desired port will open. This technique can help prevent unauthorized access, but it can also be difficult to implement and maintain.