This simple subnet calculator enumerates the subnets resulting from carving up an IPv6 block. You'll need to do this when you get IPv6 space from your ISP. Enter the IPv6 block address you received from your ISP (or whatever block address you'd like for that matter!). Then enter the prefix length of the block in the prefix length field or the left slider. The tool will automatically calculate the minimum length of the prefix by default when you enter an IPv6 block address, but you can override this. So if your ISP gives you the 2001:db8:8f00::/48 block, enter 2001:db8:8f00:: in the IPv6 block field, then enter 48 in the prefix length field.
Now let's say you need to carve this block into 16 subnets in order to allocate address space for each of your 16 core routers (or whatever your top level allocation is designed for). Allocating 16 subnets requires an increment of 4 bits over the prefix length you entered. So given you've entered 48 in the prefix length field, enter (or use the right slider control) 52 in the CIDR Length field. Then click the Calculate button and you will see a listing of the 16 resulting subnet addresses.
The "Enforce nibble allocation" checkbox assures your subnets will be allocated on nibble (4-bit mulitple) boundaries for easier management and DNS administration. But you can uncheck this if you'd like to experiment with non-nibble allocations.
The goal of this simple tool is to provide a practical introduction to working with IPv6 addresses. Enjoy!
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