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Network World - 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.
Not bad but believe it or not there are a lot of use cases where even 18 quintillion IP addresses will not be enough depending on what your organization is providing in terms of services. To reduce the amount of work involved in justifying an allocation request, ARIN has made some simple breakdowns based on the number of sites an organization has or will have within the next 12 months. An initial size allocation will be based off the largest site you operate and the following:
If you have more than 49,152 sites you should look at ARIN’s ISP Address Space Guidelines, that will cover the allocation requirements for much larger organizations.
As you can tell, it is pretty simple. You take the largest site you have and use that as the allocation basis. More than likely it fits within the /48 definitions. If so, then the allocation rules above (which allocate on natural nibble boundaries) are very generous. A nibble is four bits (i.e. one hexadecimal character) and in this case a natural subnet boundary is 4 bits, aka “nibble”, resulting in natural IPv6 subnets of /48, /52, /56, /60 and /64, for example. Keep in mind, the largest site you have dictates the use case so the reality is even if you have a smaller remote office with 12 folks they will get a /48 in this design. It allows you to grow that site to be identical to your largest current site topology.