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network-programmingipipv6subnet

Creating an IPv6 subnet with 64 bits prefix

I'm having a look at this IPv6 calculator to learn how IPv6 subnets work, however there's something I don't understand.

I'm trying to create one subnet with address:

2001:db8:8714:3a90:/64

When I do so it tells me that 1 bit is required and that 63 bits are remaining, which makes sense. However, when I click on "Calculate subnet", it tells me:

Error: You do not have enough room for this many subnet levels with this prefix. Remove some subnet levels or increase the prefix and try again.

This is the part I don't get. With 64 bits available, there should be more than enough for a subnet, or am I missing something about the way IPv6 subnetting work?

Solution

  • No, /64 is the largest prefix you can use. In there you do have 64 bits for nodes but you can't create any further subnets.

    This is to do with the fact that IPv6 mandates that addresses for stateless autoconfiguration (as per RFC 4862) must fit into the host part. And since these addresses take up 64bits in the host part, there would be no room if you split a /64 further.