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.
The obvious benefit to this type of solution is the fact that a single IPv4 address can support thousands of customer subscribers, drastically increasing the usable life of IPv4. So what's the problem? If ISPs are ensuring their IPv6 subscribers will still have IPv4 connectivity for the foreseeable future through this dual-stack scheme using shared IPv4 addresses, why do you need to get your organization's content on the IPv6 Internet anytime soon?
To answer that question, we have to look more closely at this LSN technology and ask ourselves if it will introduce problems as clients try to connect to your systems. And, indeed, in many circumstances we see this is the case. These problems can be broken down into three categories:
1. Functional: there is no guarantee that ISPs LSN solutions will work with your application. And the fact that each ISP may deploy different LSN solutions from different vendors means that you have no reasonable way of testing every possible technology. The result could be your application not working properly with potentially large swaths of the world population.
2. Performance: LSN solutions, just like traditional NAT solutions, maintain a state table for flows that traverse the device. Every packet that flows through the device triggers a lookup in the state table. Do you see a problem here? In fact there are several.
First, this has the potential of introducing a bottleneck, causing your website to load more slowly in the user's browser or worse, causing connections to be dropped by the LSN device. As the load on LSN devices increases, the problem will be compounded. And second, since all traffic in both directions for a given session must flow through the LSN device, your traffic has only one way into and out of the ISPs network. Think of the Golden Gate Bridge...all traffic must take a single path to the other side.