There are lot of approaches, how to do this. For me simplest solution, to use computer with opensource unix as gateway, with 3 lan cards. - 1 connect to intern

There are lot of approaches, how to do this. For me simplest solution, to use computer with opensource unix as gateway, with 3 lan cards. - 1 connect to internet, with real ip; - 2 connect to first lan, with grey ip numbers (192.168. , 172. ), which have nat; - 3 connect to second lan, other grey ip network, which NOT have nat, and all packets going from it to 0/0 silently dropped by firewall. All nets use dhcp, and have dns and ntpd for convenience, because it is much easier to change few tables at gateway, than change properties of all devices, when happen some difficulty. Example of difficulty, I few times had troubles with my home lan, because my provider use grey ips in their network and my home net addresses conflicts with providers. So I changed networks of my home lan for some range which is not used by my provider. BTW, this is really big trouble for some cheap hardware routers, because they have limited IP functionality, and for example one of my own such device only work with 192.168. grey ip range, they don't added 172. network range.