Issues with Private IP Addressing in the Internet
RFC 6752

Internet Engineering Task Force (IETF)                        A. Kirkham
Request for Comments: 6752                            Palo Alto Networks
Category: Informational                                   September 2012
ISSN: 2070-1721

           Issues with Private IP Addressing in the Internet

Abstract

   The purpose of this document is to provide a discussion of the
   potential problems of using private, RFC 1918, or non-globally
   routable addressing within the core of a Service Provider (SP)
   network.  The discussion focuses on link addresses and, to a small
   extent, loopback addresses.  While many of the issues are well
   recognised within the ISP community, there appears to be no document
   that collectively describes the issues.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc6752.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Kirkham                       Informational                     [Page 1]
RFC 6752          Private IP Addressing in the Internet   September 2012

Table of Contents

   1. Introduction ....................................................2
   2. Conservation of Address Space ...................................3
   3. Effects on Traceroute ...........................................3
   4. Effects on Path MTU Discovery ...................................6
   5. Unexpected Interactions with Some NAT Implementations ...........7
   6. Interactions with Edge Anti-Spoofing Techniques .................9
   7. Peering Using Loopbacks .........................................9
   8. DNS Interaction .................................................9
   9. Operational and Troubleshooting Issues .........................10
   10. Security Considerations .......................................10
   11. Alternate Approaches to Core Network Security .................12
   12. References ....................................................13
      12.1. Normative References .....................................13
      12.2. Informative References ...................................13
   Appendix A.  Acknowledgments ......................................14

1.  Introduction

   In the mid to late 1990s, some Internet Service Providers (ISPs)
   adopted the practice of utilising private (or non-globally unique)
   [RFC1918] IP addresses for the infrastructure links and in some cases
   the loopback interfaces within their networks.  The reasons for this
   approach centered on conservation of address space (i.e., scarcity of
   public IPv4 address space) and security of the core network (also
   known as core hiding).

   However, a number of technical and operational issues occurred as a
   result of using private (or non-globally unique) IP addresses, and
   virtually all these ISPs moved away from the practice.  Tier 1 ISPs
   are considered the benchmark of the industry and as of the time of
   writing, there is no known tier 1 ISP that utilises the practice of
   private addressing within their core network.

   The following sections will discuss the various issues associated
   with deploying private [RFC1918] IP addresses within ISP core
   networks.

   The intent of this document is not to suggest that private IP
   addresses can not be used with the core of an SP network, as some
   providers use this practice and operate successfully.  The intent is
   to outline the potential issues or effects of such a practice.

   Note:  The practice of ISPs using "squat" address space (also known
   as "stolen" space) has many of the same, plus some additional, issues