The document describes and compares private IP-based and real IP-based VPN solutions. Private IP VPNs use fixed private IPs without internet connectivity, requiring only one NIC for the VPN server. Real IP VPNs use public IPs with mandatory internet access, exposing the VPN server to the internet and requiring two NICs. Both solutions establish connections between remote users and the VPN server by tunneling through either a PDSN or the internet.