IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes
RFC 5942

Internet Engineering Task Force (IETF)                          H. Singh
Request for Comments: 5942                                     W. Beebee
Updates: 4861                                        Cisco Systems, Inc.
Category: Standards Track                                    E. Nordmark
ISSN: 2070-1721                                             Oracle, Inc.
                                                               July 2010

 IPv6 Subnet Model: The Relationship between Links and Subnet Prefixes

Abstract

   IPv6 specifies a model of a subnet that is different than the IPv4
   subnet model.  The subtlety of the differences has resulted in
   incorrect implementations that do not interoperate.  This document
   spells out the most important difference: that an IPv6 address isn't
   automatically associated with an IPv6 on-link prefix.  This document
   also updates (partially due to security concerns caused by incorrect
   implementations) a part of the definition of "on-link" from RFC 4861.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5942.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Singh, et al.                Standards Track                    [Page 1]
RFC 5942                    IPv6 Subnet Model                  July 2010

   This document may contain material from IETF Documents or IETF
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Table of Contents

   1. Introduction ....................................................2
   2. Requirements Language ...........................................4
   3. Host Behavior ...................................................4
   4. Host Rules ......................................................7
   5. Observed Incorrect Implementation Behavior ......................8
   6. Updates to RFC 4861 .............................................9
   7. Conclusion ......................................................9
   8. Security Considerations .........................................9
   9. Contributors ....................................................9
   10. Acknowledgements ...............................................9
   11. References ....................................................10
      11.1. Normative References .....................................10
      11.2. Informative References ...................................10

1.  Introduction

   IPv4 implementations typically associate a netmask with an address
   when an IPv4 address is assigned to an interface.  That netmask
   together with the IPv4 address designates an on-link prefix.  Nodes
   consider addresses covered by an on-link prefix to be directly
   attached to the same link as the sending node, i.e., they send
   traffic for such addresses directly rather than to a router.  See
   Section 3.3.1 of [RFC1122].  Prior to the development of subnetting
   [RFC0950] and Classless Inter-Domain Routing (CIDR) [RFC4632], an
   address's netmask could be derived directly from the address simply
   by determining whether it was a Class A, B, or C address.  Today,