>Whether I must design a public IP pool for NAT?
Yes, if you want to connect to the Internet.
>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.
Bests,
Attila
