How many bits are left for subnets?

After calculating your /48 prefix, you have 16 bits left for creating subnets.

Here is the breakdown of the 128-bit IPv6 address structure for 6to4:

1. 6to4 Prefix (16 bits): 2002
2. Public IPv4 (32 bits): Converted to hex (e.g., c000:0201)
   • Total so far: 48 bits.
3. Subnet ID (16 bits): This is the space you own to create internal networks.
   • Total so far: 64 bits.
4. Interface ID (64 bits): The "Host" part of the address (e.g., ::1).

Why 16 bits?

In IPv6, the standard size for a single network segment (like a LAN or a VLAN) is a /64. Since the 6to4 transition mechanism gives you a /48:

64 \text{ (Standard Subnet)} - 48 \text{ (Your 6to4 Prefix)} = \mathbf{16 \text{ bits}}

What does this mean in practice?

With 16 bits for subnets, you can create 65,536 individual subnets (2^{16}), and each of those subnets can hold billion of billions of devices!

In the command you just ran:
sudo ip -6 addr add 2002:c000:0201::1/64 dev tun6to4

• The 2002:c000:0201 is your /48 prefix.
• The 0000 (which is hidden between the colons 0201::1) is the first of your 65,536 available subnets.

You are now ready to bring the interface up if you haven't already