Chapter 1: Networking Fundamentals
1.3 Ports
In short: A port is a 16-bit number (0–65535) that identifies a specific process/service on a host.
Now, once the packet reaches the right machine, how does it reach the right program? Bagha, that is the job of the port.
A port is a 16-bit number (0–65535) that identifies a specific process/service on a host. One server with one IP can run a web server (port 80), SSH (port 22) and MySQL (port 3306) at the same time because each listens on a different port. The combination IP:port (e.g. 13.233.10.25:443) is called a socket.
| Range | Name | Meaning |
|---|---|---|
| 0 – 1023 | Well-known ports | Reserved for standard services; need root to bind on Linux |
| 1024 – 49151 | Registered ports | Used by applications (MySQL, MongoDB, dev servers) |
| 49152 – 65535 | Dynamic / ephemeral | Temporary client-side ports chosen by the OS |
Common ports you must remember
| Port | Protocol | Service | Where you will see it |
|---|---|---|---|
| 20, 21 | TCP | FTP (data, control) | Legacy file transfer (avoid; insecure) |
| 22 | TCP | SSH / SCP / SFTP | Logging in to EC2, copying files |
| 23 | TCP | Telnet | Legacy, insecure — never open |
| 25 | TCP | SMTP | Sending e-mail (AWS throttles port 25 by default) |
| 53 | UDP/TCP | DNS | Domain name lookups |
| 67, 68 | UDP | DHCP | Automatic IP assignment |
| 80 | TCP | HTTP | Websites (unencrypted) |
| 110 / 143 | TCP | POP3 / IMAP | Receiving e-mail |
| 443 | TCP | HTTPS | Websites with TLS/SSL |
| 3000 | TCP | Node.js / React dev server | Express apps, npm start |
| 3306 | TCP | MySQL / MariaDB | Database |
| 3389 | TCP | RDP | Windows Remote Desktop |
| 4200 | TCP | Angular dev server | ng serve |
| 5000 | TCP | Flask dev server | flask run |
| 5432 | TCP | PostgreSQL | Database |
| 6379 | TCP | Redis | Cache |
| 8000 | TCP | Gunicorn / Django dev server | Python apps |
| 8080 | TCP | Alternate HTTP / Tomcat / Jenkins | Java apps, proxies |
| 27017 | TCP | MongoDB | NoSQL database |
Security rule of thumb
Only ports 22, 80 and 443 normally need to be open to the internet. Application ports (3000, 5000, 8080) should sit behind a reverse proxy (Nginx/Apache), and database ports (3306, 27017) should never be open to 0.0.0.0/0.