Network World - The author is a Senior Network Engineer specializing in large-scale enterprise and data center network design for the Department of Defense
Will the world end? Will the Internet grind to a screeching halt? Will your computer systems disintegrate into a pile of bits and bytes? In short, no. At least not yet. But you may want to consider a few things.
ISPs aren't stupid enough to cut off IPv4 access as they begin rolling out IPv6. If they did, only a tiny fraction of websites on the Internet would be accessible at this time because most content providers haven't yet connected their Internet-accessible systems to the IPv6 Internet. The ISP's subscribers would revolt, flood the ISP with service calls, and take their business elsewhere.
But this presents an interesting dilemma for ISPs. If the reports of IPv4 shortages are true (and they are), how does a service provider continue to expand its subscriber base? This problem is most acute in Asia where the growing middle class is coming online and ISPs are starting to run out of IPv4 space. America and Europe aren't far behind.
ISPs in this situation are starting to deploy IPv4 and IPv6 in a dual-stack configuration for their customers. The IPv6 addresses are globally unique, but the IPv4 address is shared by multiple customers. This sharing of IPv4 addresses is a band-aid for IPv4 address depletion. How does it work? By adding another layer of Network Address Translation (NAT).
Take a look at Figure 1 below. Think of how your broadband connection at home works. You have a cable or DSL modem that connects to your service provider and probably acts as a wireless access point, enabling your laptop, iPad, or PlayStation to connect simultaneously to the Internet. Your wireless router is assigned a single publicly routable, globally-unique IPv4 address (D in the diagram) by your ISP, and all devices inside your house use private addresses (A, B, and C) to communicate locally.
The router translates A, B, and C to D when your devices are communicating with other computers on the Internet using NAT.
The problem for ISPs is the fact that there aren't enough globally-unique IPv4 addresses (D in the diagram) to assign to every new customer, so they are adding another layer of NAT (see Figure 2).
As you can see in the diagram, two layers of NAT are taking place for IPv4. In the first layer, the home router translates the private IPv4 addresses (A, B, and C) to an IPv4 address assigned by the ISP (D for customer 1, F for customer 2, and G for customer 3), just like in Figure 1. However, instead of D, F, and G being globally-unique, they are private addresses, and are themselves translated to E. This technology is known by multiple names, such as carrier-grade NAT (CG-NAT), large-scale NAT (LSN), or NAT444.
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