What are port numbers and what are some common ports?

Port numbers identify specific processes/services on a host, enabling multiple services on one IP. 16-bit numbers (0-65535). Ranges: 0-1023 well-known (privileged), 1024-49151 registered, 49152-65535 dynamic/private. Common ports: HTTP (80), HTTPS (443), SSH (22), FTP (21), SMTP (25), DNS (53), MySQL (3306), PostgreSQL (5432), Redis (6379). Socket = IP + port combination for unique connection endpoint.