Whenever I help a company deploy IPv6, the first question I'm asked is how to get provider independent IPv6 address space from ARIN. ARIN has new policy guidelines that affect how a company approaches its allocation justification request for IPv6 vs. what was required for IPv4.
First, I can't emphasis enough how important it is for you to change your mindset when talking about IPv6 address requests. Remember, for the regional registry, IPv4 addresses are now a scarce resource and it has had policies in place for a while to conserve them. For IPv6 the whole process is turned on its head. The goal with IPv6 allocations is to give more than enough IPv6 address space. They want to prevent an organization from needing to come back to request more and also to avoid fragmented route prefix advertising in the core Internet routing tables.
The quick and dirty for those that have an existing ASN and are BGP multi-homed network is that you automatically qualify for a /48 delegation from ARIN where a /48 is considered a single "site." Translating that into number of subnets you have to build out as /64 networks is 64-48=16 which would be 2^16 or 65,536 subnets. Each subnet is a /64 which is 2^64 or 18,446,744,073,709,551,616 hosts.
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