Virtual Private Dialup Network (VPDN) allows the private networks to span from the client directly to the Home Gateway of choice. For example, mobile users of HP (such as sales personnel) want to be able to always connect to the HP Home Gateway of choice anywhere, anytime. HP would contract for Internet Service Providers (ISPs) that would support VPDN. These ISPs would be configured such that if jsmith@hp.com dials into any of the ISP-provided numbers, the NAS would automatically forward to the HP Home Gateway. The ISP is freed from administering the HP user IP addresses, routing, and other functions tied to the HP user base. The ISP HP administration is reduced to issues of IP connectivity to the HP Home Gateway.
In VPDN terminology, the ISP is the NAS and the HP Gateway is the Home Gateway. The Point to Point (PPP) link is between the client (jsmith) and the Home Gateway. The propagation of the PPP link from the Network Access Server (NAS) and Home Gateway is brought about and maintained by Link-Level Layer 2 Forwarding (L2F).
Understanding Virtual Private Dialup Network
Limiting and Tracking Sessions Per User, Group, or VPDN