[squid-users] users authorisation-> ip

Hello!

Sorry, I know almost nothing about squid, so my question can be not
asked correctly.

We need to use one proxy server for several firms.
Each firm will pay to ISP, every firm have block of ip addresses, but
one connection to ISP.

What we want-
We need to assign several ip addresses from each firm block to proxy server.
User authorises on proxy, now proxy knows from which fir user is.
And proxy need to send request from ip, belonged to firm's block.

Is it possible with squid?
Or, may be, with another proxy?