If you're looking at networks on the control panel, you might have noticed a new feature: below IPv4 address management configuration there is now an IPv6 option.
IPv6 has always worked over ZeroTier, both link-local and any other addressing schemes assigned to ZeroTier devices. But so far the ZeroTier network configuration UI hasn't contained any IPv6-related options for address management. Now we have one: ZeroTier-Mapped RFC4193 addressing.
The IPv6 address space is large: 128 bits per IP. ZeroTier network IDs are 64 bits, and device addresses are 40. 64 + 40 is 104 bits, which is less than 128. This allows us to use network IDs and ZeroTier device IDs to create static globally unique private IPv6 addresses. If you enable this option, within a few minutes the devices on your networks will be assigned IPv6 addresses like:
We've enabled this addressing on Earth, our test public network. Earth's ZeroTier network ID is 8056c2e21c000001, and if you look closely you'll see it there inside the IP after the IPv6 private prefix of 0xfd. After that is 0x99 and 0x93, two arbitrary bytes of padding, and then there's a device ID of 89e92ceee5.
This provides a very nice, semantically meaningful scheme for static IPv6 addressing that guarantees unique addressing across all networks. It also opens the doorway to a mode of operation that could be very good for mobile and Internet-of-things applications.
For very low power operation as well as for very very large networks, it would be beneficial to do away with multicast and broadcast. While multicast is useful on normal LANs, in these areas of application it imposes additional power consumption, memory, and bandwidth requirements that aren't really needed. By using an IPv6 scheme that embeds both network and device ID semantically into the address, it becomes possible to emulate IPv6 NDP (IPv6's equivalent of ARP) and instantly resolve IPv6 addresses to MAC addresses without multicast queries.
So if you've wanted to try IPv6 addressing, give it a shot. It won't interfere with IPv4.
Edit: It appears that adding IPv6 addressing does cause issues with the Android client. Since Android VPN endpoints are limited to a single IP, we will likely force Android to ignore IPv6 for now unless we can find a better solution. So for now if you are using Android devices on your network, you should probably wait to try this addressing mode. Fixed!