generate_unique_local.rb

require 'ipaddr_extensions'

    def generate_unique_local(mac, subnet_id = 0, locally_assigned=true)
      now = Time.now
      ntp_time = (now.to_i << 32) + now.nsec
      system_id = '::/64'.to_ip.eui_64(mac).to_i
      integer_key = (ntp_time << 64) + system_id
      key = []
      16.times do |i|
        key << ((integer_key >> (i*8)) & 0xff)
      end

      global_id = Digest::SHA1.digest(key.pack('C' * 16))[-8..-1].unpack('Q').first & 0xffffffffff

      prefix = (126 << 121) + # 0xfc00
               ((locally_assigned ? 1 : 0) << 120) + # L = 1 || 0
               (global_id << 80) +
               ((subnet_id & 0xffff) << 64) # 16 bit subnet_id

      prefix.to_ip(Socket::AF_INET6).tap { |p| p.length = 64 }
    end