Commonly used ports are typically highly secure, while other ports may be overlooked and vulnerable to hackers. Commonly hacked TCP port numbers include port 21 (FTP), port 22 (SSH), port 23 (Telnet), port 25 (Simple Mail Transfer Protocol or SMTP), port 110 (POP3), and port 443 (HTTP and Hypertext Transfer Protocol Secure or HTTPS). Commonly targeted TCP and UDP ports include port 53 (DNS), ports 137 to 139 (Windows NetBIOS over TCP/IP), and 1433 and 1434 (Microsoft SQL Server).