What i understand is you want different VPN clients having different IPs to be able to connect to your network and get an IP assigned, DNS etc and also authentication them (which is called xauth Exyended authentication)
If my understanding is correct, then you can do IPSec for your VPN clients to terminate on the Router/PIX authenticat them, then assign IP address etc.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
[toc:faq]Introduction:This document describes details on how NAT-T
works.Background:ESP encrypts all critical information, encapsulating
the entire inner TCP/UDP datagram within an ESP header. ESP is an IP
protocol in the same sense that TCP and UDP are I...