There’s a finite number of public IPv4 addresses and the IPv6 address space was specified to solve this problem some 20 years ago, long before Kubernetes was conceived of. But because it was originally developed inside Google and it’s only relatively recently that cloud services like Google and AWS have started to support IPv6 at all, Kubernetes started out with only IPv4 support.
As of 2017, three were only 38 million new IPv4 addresses available to be allocated by registrars worldwide (none of those are in the US, so anyone needing more IPv4 addresses has to find someone willing to sell ones they’re not using).
That means even enterprises who are slower to move off IPv4 because they can deal with the address shortage using technologies like NAT can will run into problems, Tim Hockin, principal software engineer at Google Cloud told the New Stack. “Kubernetes makes very liberal use of IP addresses (one IP per Pod), which simplifies the system and makes it easier to use and comprehend. For very large installations this can be difficult with IPv4.
Read more at The New Stack