Ports are a 16-BIT number which means we can have 16 cells and each cell can have Os And 1s, total numbers that can be generated are approx 65,000. When we request the internet for information and the internet sends back the information, it doesn't know where to send like to your webpage, your email, and other services from where information can be requested.
So the Internet decides which service to send with the help of Port Numbers e.g. Port number of HTTP is 80. Internet Checks this port number and sends the information back to the HTTP site.
Common Port Numbers
Internet Assigned Numbers Authority (IANA) is responsible for the assignment of Port Numbers
From Wikipedia, we get Information about common port numbers
| 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) 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 |
| 546, 547 | DHCPv6 IPv6 version of DHCP |
From 0 to 1023 Port Numbers are reserved ports. These are widely used and reserved for privileged service.
From 1024 through 49151 are registered ports.
49152 through 65535 are dynamic or private ports.
To learn more about Ports Checkout Kunal Kushwaha ' s youtube video on NETWORKING
Checkout My series on Computer Networking