Ports and Common Services

Share one IP among apps

A port is a 16-bit number from 0 to 65535. The combination of IP address, protocol (TCP or UDP), and port points to a specific service or conversation.

Know well-known ports

Servers usually listen on familiar ports so clients know where to connect.

• 80 HTTP
• 443 HTTPS
• 53 DNS
• 22 SSH
• 25 SMTP email submission paths vary; many clients use 587

Watch client ephemeral ports

When your browser connects to a website on port 443, it picks a temporary high-numbered local port for that conversation. Many connections can run at once, each with its own local port.

Read a socket briefly

People write 203.0.113.10:443 to mean IP 203.0.113.10, port 443. Firewalls often allow or block traffic by port and protocol.