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As IPv4 addresses are running out rapidly, a time will come when there will not be even single IPv4 address available for allocation. But, huge number of devices will still be running on IPv4.

However, if someone wants to launch server in that time, they will not have IPv4 address available. They will have to use IPv6 only. (Even if they want to listen using Dual Stack, they still need to have IPv4 address, along with IPv6, for their server. Please correct me if my understanding is wrong here)

Now, what would be it meant for them, as existing huge base of IPv4 customers will not be able to connect to their servers ? How this situation can be addressed ?

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You will be able to obtain IPv4 addresses, but you will have to pay a lot of money for them. –  Michael Hampton May 15 at 17:24

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I think you have some misconceptions about IPv4 exhaustion. Just because there are not more IPv4 addresses to be allocated does not mean there are no IPv4 addresses available. ISPs (generally Tier-1 providers) and many older established companies still have lots of IPv4 space left they can assign to customers. What it does mean is that RIRs have no more addressing space left to assign. This means ISPs and companies will have to make do with what they already have. The United States and Europe have managed to snarf up the lions share of the IPv4 space so the shortage is going to be felt much sooner and harder in Southeast Asia. On top of this, there is always NAT which lets your stretch your existing IPv4 space even further.

When there are no more IPv4 address to be allocated (already happened!) it is not like IPv4 immediately stops working. The existing IPv4 customers will still be able to use their existing IPv4 addresses and traverse the existing IPv4 internet. It's not like a magical switch gets thrown and IPv4 becomes broken. This is whole point of running dual stack.

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Certain areas in Southeast Asia have no IPv4 addresses left for allocation at all, and unless you can find someone willing to give up their IPv4 address block, all you can get is IPv6. It is already happening. –  Michael Hampton May 15 at 17:29
    
Thanks! Your post helped me to some extent. Some ISPs can still go long way with existing IPv4. "The existing IPv4 customers will still be able to use their existing IPv4 addresses and traverse the existing IPv4 internet" that was my key point. In that situaion only existing customers will have IPv4 addresses. For server being newly launched (and if ISP too won't have any IPv4) dual stack also won't be of use? –  Andrew May 15 at 18:15
    
@Andrew - As MichaelHampton points out it depends. If your ISP is retaining some IPv4 space as reserve you might be able to get some at a hefty price. If they have no reserves and/or you do not want to pay for it IPv6 is your only option for a new deployment. –  kce May 15 at 19:14
    
@kce: I agree. But in that case (when am only on IPv6) there is technically no way for IPv4 customers to reach my server. Is my understanding correct? –  Andrew May 16 at 5:09
    
@Andrew Yes, you need to run your server dual-stack for at least the next few years. –  Michael Hampton May 17 at 18:00

Most likely it will be a matter of who will pay the most for the IPv4 addresses. Before the scenario you describe occurs, something else will happen.

ISPs will start using Carrier Grade NAT for all their new customers. Anybody who previously had a public IP from their ISP will lose it, if they ever need to switch to another ISP. This is going to result in a slow release of a small number of IPv4 addresses, and simultaneously reduce the quality of the service delivered to customers as a consequence.

ISPs who have been allowing customers to use multiple IPv4 addresses will start taking some of those away from the customers. That already happened to me once.

Should it happen that there are people who would like to deploy a server, but cannot get an IPv4 address due to cost or otherwise, there are still technical solutions which can be applied.

Frontends can be used to share a single IPv4 address between a large number of servers. I have developed one such frontend myself. I see no technical reason such a solution couldn't be scaled up to handle arbitrary number of sites with the number of IPv4 addresses being used remaining constant.

Others have suggested that hosting providers deploy reverse proxies on a few of the IPv4 addresses they have and let those provide connectivity to a large IPv6 only data center.

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