“The digital world is all about IP (internet protocol) addresses. Every device needs an IP in order to communicate on the internet or within a private network. Given there’s not enough public IP addresses out there for every internet-connected device (at least with IPv4), this little thing called NAT becomes extremely important. It stands for network address translation (NAT) and is a function provided by routers to enable multiple devices to access the internet via a single public IP address.
Having more than one NAT device usually happens when you connect your own router to a gateway installed by your internet service provider (ISP) that also includes the NAT and routing functions. Some ISPs install only a simple modem that lacks the NAT and routing, which eliminates the problem altogether. But most ISPs assume you don’t their customers have routers, however, so they’ll provide you with a combo device whether you want it or not.” – Eric Geier / Network World
Find out how to resolve double NAT issues on Network World at link below.