Ports & Protocols Cheat Sheet (Beginner)

> A domain points to an IP, but ports decide *which service* answers. Many “my site/email is down” issues are just the wrong port blocked or misconfigured.

1) What is a port? (simple)

A port is like a door number on a server IP. The IP is the building; the port is the specific door for a service.

2) Web (most common)

• **80 (HTTP)**: website without encryption (often redirects to HTTPS).
• **443 (HTTPS)**: encrypted website (TLS/SSL).

3) Server access

• **22 (SSH)**: secure remote terminal to manage Linux servers.

4) Email basics (inbound vs outbound)

Email has two major directions:

• **Inbound**: other servers delivering email *to you* (uses MX + SMTP).
• **Outbound**: your apps/devices sending email *out to the world* (SMTP submission).

Inbound email (server-to-server)

• **25 (SMTP)**: inbound mail delivery. Many VPS providers restrict port 25 until verified.

Outbound sending (recommended for users/apps)

• **587 (Submission)**: standard for authenticated sending from apps/devices.
• **465 (SMTPS)**: legacy-but-common encrypted submission (some providers use it).

Reading mail on devices

• **IMAP 143 / 993 (IMAPS)**: keep mail on server, sync folders (recommended).
• **POP3 110 / 995 (POP3S)**: downloads mail (often removes from server).

5) Quick troubleshooting questions

• Is the port open in the server firewall/security group?
• Is the service actually listening on that port?
• Are you mixing inbound/outbound ports (25 vs 587)?

6) Beginner-safe defaults

• Website: open **80** and **443**.
• SSH: open **22** but restrict by IP if possible.
• Email: use **587** for sending from devices/apps; use **993** for IMAP.

What to learn next

• How the Web Works
• Email Basics (How Email Works)
• IMAP vs POP3 vs SMTP