The technology necessary for building virtual private networks (VPNs) includes strong encryption algorithms and secure tunneling protocols like OpenVPN, L2TP, and IPsec. These technologies facilitate secure data transmission over public networks by encrypting traffic and creating private tunnels. Implementation of authentication measures, such as username/password combinations or digital certificates, further strengthens VPN security. This combination of technologies ensures privacy and security for users accessing the internet remotely or on unsecured networks.