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Network Working Group                                        M. BlanchetRequest for Comments: 3531                                      ViagenieCategory:Informational                                        April 2003A Flexible Method for Managing the Assignment of Bitsof an IPv6 Address BlockStatus 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 ofRFC 1219 and can be used for IPv6 assignments.Table of Contents1.  Rationale  . . . . . . . . . . . . . . . . . . . . . . . . . .22.  Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . .23.  Description of the Algorithm . . . . . . . . . . . . . . . . .33.1 Leftmost . . . . . . . . . . . . . . . . . . . . . . . . . .33.2 Rightmost  . . . . . . . . . . . . . . . . . . . . . . . . .33.3 Centermost . . . . . . . . . . . . . . . . . . . . . . . . .44.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .45.  Security Considerations  . . . . . . . . . . . . . . . . . . .56.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .5       References . . . . . . . . . . . . . . . . . . . . . . . . . .6       Author's Address . . . . . . . . . . . . . . . . . . . . . . .6       Full Copyright Statement . . . . . . . . . . . . . . . . . . .7Blanchet                     Informational                      [Page 1]

RFC 3531        Bits Assignment of an IPv6 Address Block      April 20031. 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 20033.3 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 2003References   [1]  Tsuchiya, P., "On the assignment of subnet numbers",RFC 1219,        April 1991.   [2]  Bradner, S., "The Internet Standards Process -- Revision 3",BCP9,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 2003Full 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]