Please read doc/config.txt before using 3proxy.

In all examples next scenario is used:

3proxy is installed on multihomed host. IP address of internal interface
is 192.168.1.1. IP address of external interface is 10.1.1.1.
Internal network has few subnetworks for 192.168.0.0/16.
Users are named user1, user2, etc.

You use external DNS server 10.1.2.1 and 10.1.2.2

Provider has networks with free access. A list of networks is published
on the provider's Web server.

Provider has proxy server 10.1.2.5. Traffic from proxy server is not free, but
is cheaper than traffic from non-free networks.