Port

allow data to be sorted effectively
Ports can be opened or closed
Port numbers are included in the data packet’s header
Ports are defined in specification of protocols
TCP has 16-bit (2-bytes) port specification
- it is possible to have 65,535 (2^16-1) possible ports
- other protocols have similar specification hence generally it is the limit
Firewalls protect the computer from harm by blocking specific ports by default
https://en.wikipedia.org/wiki/Port_(computer_networking)

Well known ports

aka system ports
Well known ports are between 0 and 1023 ( $0$ to $2^{10} - 1$ )
On Unix-like operating systems, a process must execute with superuser privileges to be able to bind a network socket to an IP address using one of the well-known ports

Number	Assignment
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) email delivery
53	Domain Name System (DNS) service
67, 68	Dynamic Host Configuration Protocol (DHCP)
80	Hypertext Transfer Protocol (HTTP)
110	Post Office Protocol (POP3)
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
546, 547	DHCPv6, IPv6 version of DHCP

netstat -aon
 
# list of applications involved with the ports
netstat -b