What are these 100.x.y.z addresses?
Tailscale assigns each node on your network a unique 100.x.y.z address. This address stays stable for each node (a device or a server), which means it should not change, no matter where the device moves to in the physical world.
More specifically, we assign addresses in the 100.64.0.0/10 subnet (from 100.64.0.0 to 100.127.255.255). This is called the "Carrier Grade NAT" (CGNAT) address space, reserved by RFC6598, IANA-Reserved IPv4 Prefix for Shared Address Space.
There are a few reasons to use this address space in particular:
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It doesn't conflict with the commonly-used private addresses your network might already use, such as
10.0.0.0/8and192.168.0.0/16. -
The addresses are intended to be used for intermediate NATted traffic that is neither on your LAN nor on the public Internet. When a device on this network wants to reach the public Internet, they are expected to be NATted once more. This matches how Tailscale uses the addresses.
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The addresses are supposed to be used by Internet Service Providers (ISPs) rather than private networks. Philosophically, Tailscale is a service provider creating a shared network on top of the regular Internet. When packets leave the Tailscale network, different addresses are always used.