Network Working Group M. Blanchet Request for Comments: 3531 Viagenie Category:Informational April 2003A Flexible Method for Managing the Assignment of Bits of an IPv6 Address Block
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RFC 3531 Bits Assignment of an IPv6 Address Block April 2003 1. RationaleIPv6 addresses have a flexible structure for address assignments. This enables registries, internet service providers, network designers and others to assign address ranges to organizations and networks based on different criteria, like size of networks, estimated growth rate, etc. Often, the initial assignment doesn't scale well because a small network becomes larger than expected, needing more addresses. But then, the assignment authority cannot allocate contiguous addresses because they were already assigned to another network.
2. Scheme
We define parts of the IP address as p1, p2 , p3, ... pN in order, so that an IP address is composed of these parts contiguously. Boundaries between each part are based on the prefix assigned by the next level authority. Part p1 is the leftmost part probably assigned to a registry, Part p2 can be allocated to a large internet service provider or to a national registry. Part p3 can be allocated to a large customer or a smaller provider, etc. Each part can be of different length. We define l(pX) the length of part X.+------+------+------+------+------+------+ | p1 | p2 | p3 | p4 | ... | pN | +------+------+------+------+------+------+ <------- ipv6 or ipv4 address ------------>The algorithm for allocating addresses is as follows: a) for the leftmost part (p1), assign addresses using the leftmost bits first b) for the rightmost part (pN), assign addresses using the rightmost bits first c) for all other parts (center parts), predefine an arbitrary boundary (prefix) and then assign addresses using the center bits first of the part being assigned. This algorithm grows assigned bits in such way that it keeps unassigned bits near the boundary of the parts. This means that the prefix between any two parts can be changed forward or backward, later on, up to the assigned bits.
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RFC 3531 Bits Assignment of an IPv6 Address Block April 2003 3. Description of the AlgorithmThis section describes the assignment of leftmost bits, rightmost bits and centermost bits.
3.1 Leftmost
p1 will be assigned in order as follows:
3.2 Rightmost
pN (the last part) will be assigned in order as follows:
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RFC 3531 Bits Assignment of an IPv6 Address Block April 2003 3.3 CentermostpX (where 1 < X < N) will be assigned in order as follows: (for example, with a 8 bit predefined length l(pX)=8))
4. Example
As an example, a provider P1 has been assigned the 3ffe:0b00/24 prefix and wants to assign prefixes to its connected networks. It anticipates in the foreseeable future a maximum of 256 customers consuming 8 bits. One of these customers, named C2, anticipates a maximum of 1024 customer's assignments under it, consuming 10 other bits. The assignment will be as follows, not showing the first 24 leftmost bits (3ffe:0b00/24: 00111111 11111110 00001011):
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RFC 3531 Bits Assignment of an IPv6 Address Block April 2003
P1 assigns address space to its customers using leftmost bits:
Putting all bits together for C2C3:
P1 |C2 |C2C3
00111111 11111110 00001011 01000000 00001100 00
<-------> <------>
growing bits
By using this method, P1 will be able to expand the number of customers and the customers will be able to modify their first assumptions about the size of their own customers, until the "reserved" bits are assigned.
5. Security Considerations
Address assignment doesn't seem to have any specific security consideration.
6. Acknowledgements
Thanks to Steve Deering, Bob Hinden, Thomas Narten, Erik Nordmark, Florent Parent and Jocelyn Picard for their very useful comments on this work.
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RFC 3531 Bits Assignment of an IPv6 Address Block April 2003
References
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RFC 3531 Bits Assignment of an IPv6 Address Block April 2003
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