Ports

Ports are numeric identifiers at the transport layer used to distinguish individual services and applications on a system. Together with the IP address, they form a unique connection endpoint. TCP and UDP use ports in the range from 0 to 65535, divided into well-known ports up to 1023, registered ports up to 49151 and dynamic ports above that. Well-known assignments include ports 80 and 443 for HTTP and HTTPS, 22 for SSH, 25 for SMTP, 53 for DNS and 3389 for RDP. In firewalls, access lists and NAT rules, ports are a central criterion for permitting or blocking connections. In a hardware context, the term also refers to the physical interfaces on switches, routers and servers.