More Application Layer Protocols
Beyond HTTP, DNS, DHCP, and email, there are many other Application Layer protocols that serve specific purposes. Let's look at the most important ones.
FTP (File Transfer Protocol)
FTP is one of the oldest internet protocols, designed for transferring files between a client and a server.
Client ──FTP──→ Server
Port 21 (control) + Port 20 (data)
- How it works: The client connects to the server on port 21 for commands (login, list files, navigate directories). Data transfers happen on port 20.
- Authentication: Usually requires a username and password, though anonymous FTP is common for public file servers.
- Security concern: FTP transmits credentials in plaintext. Use FTPS (FTP over TLS) or SFTP (SSH File Transfer Protocol) for secure transfers.
SSH (Secure Shell)
SSH provides encrypted remote access to a computer. Instead of Telnet (which sends everything in plaintext), SSH encrypts the entire session.
Your Computer ──SSH──→ Remote Server
(Port 22)
- Remote login — Access a server's command line from anywhere.
- Remote file transfer — SCP (Secure Copy Protocol) uses SSH to transfer files.
- Port forwarding — Tunnel other protocols through SSH for encryption.
- Key-based authentication — Use cryptographic keys instead of passwords for better security.
Telnet
Telnet is the predecessor to SSH — it provides remote terminal access but without encryption. Everything you type (including passwords) is transmitted in plaintext.
Your Computer ──Telnet──→ Remote Server
(Port 23)
Telnet is rarely used today because of its security vulnerabilities. SSH has replaced it almost everywhere. But you might encounter it in legacy systems or network device configuration.
SNMP (Simple Network Management Protocol)
SNMP is used by network administrators to monitor and manage network devices — routers, switches, servers, printers. It allows administrators to check device status, configure settings, and receive alerts about problems.
Management Station ←──SNMP──→ Network Devices
(Port 161)
SNMP agents run on each managed device, collecting information about CPU usage, network traffic, errors, and other metrics. The management station polls these agents or receives traps (unsolicited alerts) about problems.
Protocol Summary
Protocol │ Port │ Purpose │ Transport
─────────┼──────┼────────────────────────┼──────────
HTTP │ 80 │ Web browsing │ TCP
HTTPS │ 443 │ Secure web browsing │ TCP
DNS │ 53 │ Name resolution │ UDP/TCP
DHCP │ 67/68│ Auto IP config │ UDP
SMTP │ 25 │ Sending email │ TCP
POP3 │ 110 │ Retrieving email │ TCP
IMAP │ 143 │ Managing email │ TCP
FTP │ 20/21│ File transfer │ TCP
SSH │ 22 │ Secure remote access │ TCP
Telnet │ 23 │ Remote access (insecure)│ TCP
SNMP │ 161 │ Network management │ UDP