Network Working Group                                        M. Blanchet
Request for Comments: 3531                                      Viagenie
Category:Informational                                        April 2003


         A Flexible Method for Managing the Assignment of Bits
                        of an IPv6 Address Block

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document proposes a method to manage the assignment of bits of
   an IPv6 address block or range.  When an organisation needs to make
   an address plan for its subnets or when an ISP needs to make an
   address plan for its customers, this method enables the organisation
   to postpone the final decision on the number of bits to partition in
   the address space they have.  It does it by keeping the bits around
   the borders of the partition to be free as long as possible.  This
   scheme is applicable to any bits addressing scheme using bits with
   partitions in the space, but its first intended use is for IPv6.  It
   is a generalization of RFC 1219 and can be used for IPv6 assignments.

Table of Contents

   1.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.  Description of the Algorithm . . . . . . . . . . . . . . . . .  3
     3.1 Leftmost . . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.2 Rightmost  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     3.3 Centermost . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  5
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  5
       References . . . . . . . . . . . . . . . . . . . . . . . . . .  6
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  6
       Full Copyright Statement . . . . . . . . . . . . . . . . . . .  7

1. Rationale

   IPv6 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.

   RFC 1219 [1] describes an allocation scheme for IPv4 where address
   space is kept unallocated between the leftmost bits of the subnet
   part and the rightmost bits of the host part of the address.  This
   enables the network designer to change the subnet mask without
   renumbering, for the central bits not allocated.

   This work generalizes the previous scheme by extending the algorithm
   so it can be applied on any part of an IP address, which are assigned
   by any assignment authority level (registries, ISPs of any level,
   organizations, ...).  It can be used for both IPv4 and IPv6.

   This document does not provide any recommendation to registries on
   how to assign address ranges to their customers.

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.

3. Description of the Algorithm

   This section describes the assignment of leftmost bits, rightmost
   bits and centermost bits.

3.1 Leftmost

   p1 will be assigned in order as follows:

    Order   Assignment
    1 00000000
    2 10000000
    3 01000000
    4 11000000
    5 00100000
    6 10100000
    7 01100000
    8 11100000
    9 00010000
    ...

   This is actually a mirror of binary counting.

3.2 Rightmost

   pN (the last part) will be assigned in order as follows:

    Order   Assignment
    1 00000000
    2 00000001
    3 00000010
    4 00000011
    5 00000100
    6 00000101
    7 00000110
    8 00000111
    9 00001000
    ...

3.3 Centermost

   pX (where 1 < X < N) will be assigned in order as follows: (for
   example, with a 8 bit predefined length l(pX)=8))

    Order   Assignment
    1 00000000
    2 00001000
    3 00010000
    4 00011000
    5 00000100
    6 00001100
    7 00010100
    8 00011100
    9 00100000
    ...

   The bits are assigned using the following algorithm:

   1.  The first round is to select only the middle bit (and if there is
       an even number of bits  pick the bit following the center)

   2.  Create all combinations using the selected bits that haven't yet
       been created.

   3.  Start a new round by adding one more bit to the set.  In even
       rounds add the preceding bit to the set.  In odd rounds add the
       subsequent bit to the set.

   4.  Repeat 2 and 3 until there are no more bits to consider.

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):

   P1 assigns address space to its customers using leftmost bits:

    10000000  : assigned to C1
    01000000  : assigned to C2
    11000000  : assigned to C3
    00100000  : assigned to C4
    ...

   C2 assigns address space to its customers (C2C1, C2C2, ...) using
   centermost bits:

    0000010000 : assigned to C2C1
    0000100000 : assigned to C2C2
    0000110000 : assigned to C2C3
    ...

   Customers of C2 can use centermost bits for maximum flexibility and
   then the last aggregators (should be a network in a site) will be
   assigned using rightmost 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.

References

   [1]  Tsuchiya, P., "On the assignment of subnet numbers", RFC 1219,
        April 1991.

   [2]  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
        9, RFC 2026, October 1996.

Author's Address

   Marc Blanchet
   Viagenie
   2875 boul. Laurier, bureau 300
   Sainte-Foy, QC  G1V 2M2
   Canada

   Phone: +1 418 656 9254
   EMail: Marc.Blanchet@viagenie.qc.ca
   URI:   http://www.viagenie.qc.ca/

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