Common Network Protocols
A comprehensive guide to the application layer protocols that make everyday internet services work, including SSH, FTP, SMTP, and DHCP.
What is a Protocol?
A protocol is a set of formal rules that govern how data is transmitted and received over a network. Without protocols, devices would send unstructured bits that other machines wouldn't know how to interpret.
Essential Application Layer Protocols
1. DHCP (Dynamic Host Configuration Protocol)
Automatically assigns IP addresses, subnet masks, default gateways, and DNS servers to devices when they connect to a network. Without DHCP, you would have to manually configure every device's network settings.
- Port: UDP 67 (server), UDP 68 (client)
2. DNS (Domain Name System)
Translates human-readable domain names (like google.com) into computer-readable IP addresses (like 142.250.190.46). See DNS Explained for more details.
- Port: UDP/TCP 53
3. SSH (Secure Shell)
Allows secure remote command-line access to servers and network devices. All traffic is encrypted to prevent password sniffing.
- Port: TCP 22
4. FTP & SFTP (File Transfer Protocol)
- FTP — Used to transfer files between client and server. Plaintext, insecure. (Ports TCP 20, 21).
- SFTP (SSH File Transfer Protocol) — Moves files over an encrypted SSH connection. Secure. (Port TCP 22).
5. Email Protocols
Three protocols work together to send and receive emails:
- SMTP (Simple Mail Transfer Protocol) — Used for sending emails from client to server or between servers. (Ports: 25, 587, or 465).
- IMAP (Internet Message Access Protocol) — Used for retrieving emails. Messages stay on the server, allowing sync across multiple devices. (Ports: 143 or 993 secure).
- POP3 (Post Office Protocol v3) — Downloads emails to a single device and deletes them from the server. Legacy protocol. (Ports: 110 or 995 secure).
6. NTP (Network Time Protocol)
Synchronizes the clocks of computers and servers on a network. Critical for security logs, authentication systems, and database consistency.
- Port: UDP 123
Protocol Port Summary Table
| Protocol | Description | Transport | Default Port |
|---|---|---|---|
| SSH | Secure Remote Login | TCP | 22 |
| DNS | Domain Name Resolution | UDP/TCP | 53 |
| DHCP | IP Auto-Configuration | UDP | 67/68 |
| HTTP | Unencrypted Web Traffic | TCP | 80 |
| HTTPS | Encrypted Web Traffic | TCP | 443 |
| SMTP | Sending Email | TCP | 25/587 |
| IMAP | Receiving Email (Sync) | TCP | 993 |
What's Next?
Learn how the web protocol works in HTTP & HTTPS, or explore the underlying transport protocols in TCP/IP.