Ports

See book 6.2.1 and 6.5.2

Port - Logical (not physical) connection to computer (server).
One hardware link: Many ports.
One host (physical server) can run many services (listening processes) at different addresses.

IP address = Address of a host.
IP address + port = Address of a process (service) on a host.

List of ports.
1 to 65535 (16 bit no).

List of ports

Ports set aside for some "well-known" services:

20 - ftp data
21 - ftp control (commands)
22 - ssh / sftp / scp
23 - telnet
25 - SMTP - standard for email transmission from node to node
53 - DNS
80 - http
110 - POP3 - retrieve email to local client
143 - IMAP - more advanced email client
443 - https (http over SSL/TLS)
554 - rtsp
989 - ftps (ftp over SSL/TLS) (data)
990 - ftps (control)
1935 - rtmp
8080 - http alternate

Full list:

IANA
- port numbers

"Syntax" of these protocols (data to be sent/received) is different to each other.
Port number gives the server an idea of what to expect / how to interpret the data sent.
Multiple ports on 1 server:
- Server machine may run multiple server processes, each contactable on different port.
Multiple clients connect to 1 port:
- Conversely, multiple clients may want to contact same port (e.g. Web server).
  Client creates socket at its end. Sends request to server (at port no). Server creates socket at its end dedicated to that client.
  One port: Many sockets to that port.