The protocols of this layer provide host-to-host communication services for applications. It provides services such as connection-oriented communication, reliability, flow control, and multiplexing.
 RFC 1122
The port numbers are divided into three ranges: the well-known ports, the registered ports, and the dynamic or private ports.
The well-known ports (also known as system ports) are those from 0 through 1023. The requirements for new assignments in this range are stricter than for other registrations, examples include:
- 20: File Transfer Protocol (FTP) Data Transfer
- 21: File Transfer Protocol (FTP) Command Control
- 22: Secure Shell (SSH) Secure Login
- 23: Telnet remote login service, unencrypted text messages
- 25: Simple Mail Transfer Protocol (SMTP) E-mail routing
- 53: Domain Name System (DNS) service
- 80: Hypertext Transfer Protocol (HTTP) used in the World Wide Web
- 110: Post Office Protocol (POP3)
- 119: Network News Transfer Protocol (NNTP)
- 123: Network Time Protocol (NTP)
- 143: Internet Message Access Protocol (IMAP) Management of digital mail
- 161: Simple Network Management Protocol (SNMP)
- 194: Internet Relay Chat (IRC)
- 443: HTTP Secure (HTTPS) HTTP over TLS/SSL
The registered ports are those from 1024 through 49151. IANA maintains the official list of well-known and registered ranges. The dynamic or private ports are those from 49152 through 65535. One common use for this range is for ephemeral ports.