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Ports in Linux

The Internet Assigned Numbers Authority (IANA) is responsible for maintaining the official list of well-known ports (or registerred ports) and their associated applications/services. The IANA assigns port numbers in the range of 0 to 1023 for well-known ports.

The registry is called 'Service Name and Transport Protocol Port Number Registry' and it is available here.

Most Common Ports:

Port Service
Port 21: FTP (File Transfer Protocol) - used for file transfers.
Port 22: SSH (Secure Shell) - used for secure remote access to servers.
Port 25: SMTP (Simple Mail Transfer Protocol) - used for email transmission.
Port 53: DNS (Domain Name System) - used for domain name resolution.
Port 80: HTTP (Hypertext Transfer Protocol) - used for web browsing.
Port 443: HTTPS (Hypertext Transfer Protocol Secure) - used for secure web browsing (encrypted)
Port 110: POP3 (Post Office Protocol version 3) - used for receiving email from a server.

Most popular applications uses a specific port as their default, as a convention. When installing the applications, these allow the user to override, in case of conflict.

This is not a exhaustive list, but the applications used in my products.

Port Application
1313 Hugo server
3306 MySQL
5000 Flask
5432 PostgreSQL
6379 Redis

When troubleshooting network connectivity or security issues, knowing the well-known ports associated with specific applications can be helpful. However, keep in mind that applications and services may use different port numbers, especially custom ones. To verify the exact port usage on a system, you can use the following utilities.

List Open Files

# to install
sudo apt install lsof 
# to check ports
sudo lsof -i -P -n | grep LISTEN

Network Statistics utilty. 

# to install
sudo apt install net-tools 
# to check ports
netstat -tulpn | grep LISTEN

Socket-related Statistics utility 

# to install
sudo apt install iproute2
# to check ports
sudo ss -tulwn | grep LISTEN