Is it possible to configure an internet gateway so that its outer interface (oif - internet side) has 255 (virtual, public) IP addresses, and when an internal client sends a package to a specific port of the internal interface (iif - internal side), it is passed through to the internet via a specific IP of the outer interface?

With other words:
any traffic to iif : port1 should passed through to oip1, and to the internet from there,
iif : port2 to oip2,
iif : port3 to oip3,
etc...

The destinations of the packages should see as if the source of these packages were oip1, oip2, oip3, respectively.

I wonder if this is the so-called reverse NAT operation?

Can it be configured with iptables (or is it possible at all)?

It is called one-to-one or static NATing. More info can be found at http://www.linuxhomenetworking.com/w...Using_iptables.