• It is represented by a 16-bit Number
• Each IP address can have port numbers ranging from 0 to 65535
• Although a process can use many ports at a time, a single port can exclusively used by only one process
• The destination port number must be known before sending messages
• the port number space is managed by IANA, along with the IP address space
• A service is a class of application, for example, a web servre, DNS server, which is usually tied to a specific applicaton protocol
• Ports are assigned to specific services, not to specific user-level application
• TCP, UDP packet headers both include source and destination port numbers.

TCP and UDP 헤더 모두 Source Port와 Destination Port 필드를 필수로 포함한다. 대신 UDP 에서는 체크섬(Checksum) 필드에서 포트 번호를 선택적으로 포함한다.

IP address + PortNumber = Socket Address