Here's why. Not only must each IP plan mesh but you can't share over the internet with windows networking safely. So you invest (many dollars) in VPN capable routers and gear then configure your VPN lan. Sadly this means at each site you need an Internet gateway server as well since we are now on a VPN WAN.

I've only seen one person do this and they were able to answer such questions as what a netmask would be to support 1024 PCs.