GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc3531

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

Blanchet Informational [Page 1] RFC 3531 Bits Assignment of an IPv6 Address Block April 2003

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.

Blanchet Informational [Page 2] RFC 3531 Bits Assignment of an IPv6 Address Block April 2003

 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
  ...

Blanchet Informational [Page 3] RFC 3531 Bits Assignment of an IPv6 Address Block April 2003

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.

Blanchet Informational [Page 4] RFC 3531 Bits Assignment of an IPv6 Address Block April 2003

 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.

Blanchet Informational [Page 5] RFC 3531 Bits Assignment of an IPv6 Address Block April 2003

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/

Blanchet Informational [Page 6] RFC 3531 Bits Assignment of an IPv6 Address Block April 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Blanchet Informational [Page 7]

/data/webs/external/dokuwiki/data/pages/rfc/rfc3531.txt · Last modified: 2003/04/24 23:07 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki