Is this a request for help? (If yes, you should use our troubleshooting guide and community support channels, see http://kubernetes.io/docs/troubleshooting/.):

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Is this a BUG REPORT or FEATURE REQUEST? (choose one):
BUG REPORT

Kubernetes version (use kubectl version):
latest

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What happened:
cidr_set.go in pkg/controller/node currently uses a bitmap to track the subnets allocated.
The current allocation logic assumes a small allocation range. This bitmap could grow very large with the introduction of IPv6.
The v6 code in cidr_set.go currently limits the difference between the cluster mask size and subnet mask size to 16. This restriction could be removed if the bitmap was compressed.

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