A Flexible Method for Managing the Assignment of Bits of an IPv6 Address Block
A method for managing IPv6 address space without committing all bits upfront.
Blanchet, M., "A Flexible Method for Managing the Assignment of Bits of an IPv6 Address Block", RFC 3531, DOI 10.17487/RFC3531, April 2003, <https://www.rfc-editor.org/info/rfc3531>.
@misc{rfc3531,
series = {Request for Comments},
number = 3531,
howpublished = {RFC 3531},
publisher = {RFC Editor},
doi = {10.17487/RFC3531},
url = {https://www.rfc-editor.org/info/rfc3531},
author = {M. Blanchet},
title = {{A Flexible Method for Managing the Assignment of Bits of an IPv6 Address Block}},
pagetotal = 7,
year = 2003,
month = apr,
} RFC text
Contents
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/
Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
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included on all such copies and derivative works. However, this
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§1The Problem with Fixed Allocations
IPv6 allows flexible address assignment, but initial allocations often fail to scale. A small network may grow larger than expected, needing more addresses. When this happens, the assignment authority cannot allocate contiguous addresses because other networks got them.
RFC 1219 described an IPv4 allocation scheme that keeps unallocated space between the subnet part (on the left) and the host part (on the right) of an address. This lets network designers change the subnet mask later without renumbering the central bits.
RFC 3531 generalizes this approach to work on any parts of an IP address at any allocation authority level—registries, ISPs, organizations, and others. It applies to both IPv4 and IPv6.
If you run out of room in your address plan, you may not be able to fix it without renumbering everything. This memo shows how to leave space in the middle so you can change your plan later.
§2The Allocation Algorithm
Define an IP address as parts p1, p2, p3, ... pN in order, where boundaries between parts are based on what each assignment authority assigns. Part p1 is typically the leftmost part assigned by a registry. Part p2 might go to a large ISP or national registry. Part p3 to a customer or smaller provider. Each part has a length l(pX).
The algorithm allocates addresses differently in each part:
- For p1 (leftmost): assign using leftmost bits first
- For pN (rightmost): assign using rightmost bits first
- For center parts: assign using center bits first
This keeps unassigned bits near the boundaries between parts. The prefix boundary between any two parts can shift forward or backward later, up to the assigned bits.
From the left side, hand out numbers starting with the left edge. From the right side, hand out from the right edge. In the middle, hand out from the center. This leaves gaps at the edges so you can move the boundaries.
- IPv6 Address
- p1 (registry) assigned leftmost
- p2 (ISP) assigned centermost
- p3 (customer) assigned centermost
- pN (host) assigned rightmost
§3Detailed Assignment Methods
Leftmost assignment uses a mirror of binary counting: 00000000, 10000000, 01000000, 11000000, 00100000, and so on. This assigns bits one at a time from the left edge, creating new combinations as each round adds bits.
Rightmost assignment uses standard binary counting: 00000000, 00000001, 00000010, 00000011, 00000100, and so on. Bits fill from the right edge.
Centermost assignment for middle parts with a predefined length works by selecting the middle bit first (or the bit following center if even), creating all combinations of selected bits, then expanding by adding one more bit per round. Even rounds add the preceding bit; odd rounds add the subsequent bit. For an 8-bit part, the order is 00000000, 00001000, 00010000, 00011000, 00000100, 00001100, 00010100, 00011100, 00100000, and so on.
Three different counting tricks: left edge counts from left, right edge counts normally, and middle parts count from the center outward to both sides.
§4Example: Multi-Level Address Delegation
Provider P1 holds 3ffe:0b00/24 and allocates to customers. It expects 256 customers (8 bits) and one customer C2 expects 1024 sub-customers (10 more bits).
P1 assigns to its customers using leftmost bits: 10000000 to C1, 01000000 to C2, 11000000 to C3, 00100000 to C4. C2 assigns to its customers using centermost bits: 0000010000 to C2C1, 0000100000 to C2C2, 0000110000 to C2C3. C2C3's full address becomes the P1 prefix (3ffe:0b00:0b00), P1's allocation to C2 (01000000), and C2's allocation to C2C3 (00001100 00).
By this method, P1 can add more customers and C2 can adjust assumptions about sub-customer size until the reserved bits are assigned.
A provider gives addresses to customers on the left, customers give addresses to sub-customers in the middle, and they both have room to grow by using the unassigned bits at the edges.