In computer networking, a port is a logical endpoint that identifies a specific process or service running on a host. Ports are numbered from 0 to 65535 and work in conjunction with IP addresses to direct network traffic to the correct application. When a client connects to a server, it specifies both the IP address and the port number -- for example, connecting to port 443 on a web server to access HTTPS content. Ports are divided into three ranges: well-known ports (0-1023) assigned to standard services by IANA, registered ports (1024-49151) used by software applications, and dynamic or ephemeral ports (49152-65535) used temporarily by client-side connections.
Network communication primarily uses two transport layer protocols: TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP is connection-oriented, providing reliable, ordered delivery of data through a three-way handshake and acknowledgment mechanism. UDP is connectionless, offering faster but unreliable delivery without the overhead of establishing a connection. Most port scanning focuses on TCP ports, since the majority of internet services use TCP for reliable communication.
Some of the most commonly scanned ports and their associated services include:
Port scanning is the process of probing a host to determine which ports are open and listening for connections. It is one of the most fundamental techniques in network security assessment. System administrators use port scans to audit their own infrastructure, verify that only intended services are publicly accessible, and detect unauthorized services that may have been started by malware or misconfiguration. Open ports that are not needed represent an increased attack surface -- each open port is a potential entry point for attackers to exploit vulnerabilities in the listening service.
Firewalls are the primary defense mechanism for controlling port access. They filter incoming and outgoing traffic based on rules that specify which ports and IP addresses are allowed or blocked. A well-configured firewall closes all unnecessary ports and restricts access to critical services like SSH and database ports to trusted IP addresses only. Regular port scanning helps ensure that firewall rules are working as expected and that no unintended services are exposed. Our free port scanner tool lets you quickly check which ports are open on any host, helping you verify your server's security posture and troubleshoot connectivity issues with specific services.