>If I keep 3 public IP address for this IP pool,is it means that only 3 users can concurrent at the same time?
If you do NAT, the answer is yes.
I'd do PAT w/ a few inside hosts. If you use PAT you can have "any" inside host, that can communicate to the oustide using the PAT address. This means that outgoing conenctions using the PAT IP address as the source IP, but they are differenciated by port numbers.
From versions pix 5.x PAT address can be the same as OUTSIDE IP address. From 6.1.1 you can use the OUSTIDE address for port-mapping too. - So finally we have reached the capability of a Cisco router IOS 11.2 :D In this way you only have to have 1 public IP address on the OUTSIDE - under some simple circumstances, of course- and can have public servers line www, mail etc. on this public address.
So use PAT, and conserve your public addresses for the future. Maybe later you'll need them.
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...
Table of Contents 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 an...