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Issues with Private IP Addressing in the Internet
RFC 6752

Document Type RFC - Informational (September 2012)
Author Anthony Kirkham
Last updated 2015-10-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Ron Bonica
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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


   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

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
   ( 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.

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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

   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
   (or effects) as that of using private IP address space within core
   networks.  The term "squat IP address space" refers to the practice

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   of an ISP using address space for its own infrastructure/core network
   addressing that has been officially allocated by an RIR (Regional
   Internet Registry) to another provider, but that provider is not
   currently using or advertising within the Internet.  Squat addressing
   is not discussed further in this document.  It is simply noted as an
   associated issue.

2.  Conservation of Address Space

   One of the original intents for the use of private IP addressing
   within an ISP core was the conservation of IP address space.  When an
   ISP is allocated a block of public IP addresses (from an RIR), this
   address block was traditionally split in order to dedicate some
   portion for infrastructure use (i.e., for the core network) and the
   other portion for customer (subscriber) or other address pool use.
   Typically, the number of infrastructure addresses needed is
   relatively small in comparison to the total address count.  So unless
   the ISP was only granted a small public block, dedicating some
   portion to infrastructure links and loopback addresses (/32) is
   rarely a large enough issue to outweigh the problems that are
   potentially caused when private address space is used.

   Additionally, specifications and equipment capability improvements
   now allow for the use of /31 subnets [RFC3021] for link addresses in
   place of the original /30 subnets -- further minimising the impact of
   dedicating public addresses to infrastructure links by only using two
   (2) IP addresses per point-to-point link versus four (4),

   The use of private addressing as a conservation technique within an
   Internet Service Provider (ISP) core can cause a number of technical
   and operational issues or effects.  The main effects are described

3.  Effects on Traceroute

   The single biggest effect caused by the use of private addressing
   [RFC1918] within an Internet core is the fact that it can disrupt the
   operation of traceroute in some situations.  This section provides
   some examples of the issues that can occur.

   A first example illustrates the situation where the traceroute
   crosses an Autonomous System (AS) boundary, and one of the networks
   has utilised private addressing.  The following simple network is
   used to show the effects.

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              AS64496                 EBGP                AS64497
                    IBGP Mesh <--------------->  IBGP Mesh

R1 Pool -                                                      R6 Pool -                                

                               .9          .10
    .1       .2  .5       .6    ------------    .6      .5  .2      .1
  R1-----------R2-----------R3--|          |--R4----------R5----------R6

  R1 Loopback:                    R4 Loopback:
  R2 Loopback:                    R5 Loopback:
  R3 Loopback:                    R6 Loopback:

   Using this example, performing the traceroute from AS64497 to
   AS64496, we can see the private addresses of the infrastructure links
   in AS64496 are returned.

   Type escape sequence to abort.
   Tracing the route to

     1 40 msec 20 msec 32 msec
     2 16 msec 20 msec 20 msec
     3 20 msec 20 msec 32 msec
     4 20 msec 20 msec 20 msec
     5 20 msec 20 msec 20 msec

   This effect in itself is often not a problem.  However, if anti-
   spoofing controls are applied at network perimeters, then responses
   returned from hops with private IP addresses will be dropped.  Anti-
   spoofing refers to a security control where traffic with an invalid
   source address is discarded.  Anti-spoofing is further described in
   [BCP38] and [BCP84].  Additionally, any [RFC1918] filtering
   mechanism, such as those employed in most firewalls and many other
   network devices can cause the same effect.

   The effects are illustrated in a second example below.  The same
   network as in example 1 is used, but with the addition of anti-
   spoofing deployed at the ingress of R4 on the R3-R4 interface (IP

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   Type escape sequence to abort.
   Tracing the route to

     1 24 msec 20 msec 20 msec
     2 20 msec 52 msec 44 msec
     3 44 msec 20 msec 32 msec
     4  *  *  *
     5  *  *  *
     6  *  *  *
     7  *  *  *
     8  *  *  *
     9  *  *  *
    10  *  *  *
    11  *  *  *
    12  *  *  *

   In a third example, a similar effect is caused.  If a traceroute is
   initiated from a router with a private (source) IP address, located
   in AS64496 and the destination is outside of the ISP's AS (AS64497),
   then in this situation, the traceroute will fail completely beyond
   the AS boundary.

   R1# traceroute
   Type escape sequence to abort.
   Tracing the route to

     1 20 msec 20 msec 20 msec
     2 52 msec 24 msec 40 msec
     3  *  *  *
     4  *  *  *
     5  *  *  *
     6  *  *  *

   While it is completely unreasonable to expect a packet with a private
   source address to be successfully returned in a typical SP
   environment, the case is included to show the effect as it can have
   implications for troubleshooting.  This case will be referenced in a
   later section.

   In a complex topology, with multiple paths and exit points, the
   provider will lose its ability to trace paths originating within its
   own AS, through its network, to destinations within other ASes.  Such
   a situation could be a severe troubleshooting impediment.

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   For completeness, a fourth example is included to show that a
   successful traceroute can be achieved by specifying a public source
   address as the source address of the traceroute.  Such an approach
   can be used in many operational situations if the router initiating
   the traceroute has at least one public address configured.  However,
   the approach is more cumbersome.

   Protocol [ip]:
   Target IP address:
   Source address:
   Numeric display [n]:
   Timeout in seconds [3]:
   Probe count [3]:
   Minimum Time to Live [1]:
   Maximum Time to Live [30]: 10
   Port Number [33434]:
   Loose, Strict, Record, Timestamp, Verbose[none]:
   Type escape sequence to abort.
   Tracing the route to

     1 0 msec 4 msec 0 msec
     2 0 msec 4 msec 0 msec
     3 [AS 64497] 0 msec 4 msec 0 msec
     4 [AS 64497] 0 msec 0 msec 4 msec
     5 [AS 64497] 0 msec 0 msec 4 msec

   It should be noted that some solutions to this problem have been
   proposed in [RFC5837], which provides extensions to ICMP to allow the
   identification of interfaces and their components by any combination
   of the following:  ifIndex, IPv4 address, IPv6 address, name, and
   MTU.  However, at the time of this writing, little or no deployment
   was known to be in place.

4.  Effects on Path MTU Discovery

   The Path MTU Discovery (PMTUD) process was designed to allow hosts to
   make an accurate assessment of the maximum packet size that can be
   sent across a path without fragmentation.  Path MTU Discovery is
   utilised by IPv4 [RFC1191], IPv6 [RFC1981], and some tunnelling
   protocols such as Generic Routing Encapsulation (GRE) and IPsec.

   The PMTUD mechanism requires that an intermediate router can reply to
   the source address of an IP packet with an ICMP reply that uses the
   router's interface address.  If the router's interface address is a
   private IP address, then this ICMP reply packet may be discarded due
   to unicast reverse path forwarding (uRPF) or ingress filtering,

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   thereby causing the PMTUD mechanism to fail.  If the PMTUD mechanism
   fails, this will cause transmission of data between the two hosts to
   fail silently due to the traffic being black-holed.  As a result, the
   potential for application-level issues may be created.

5.  Unexpected Interactions with Some NAT Implementations

   Private addressing is legitimately used within many enterprise,
   corporate, or government networks for internal network addressing.
   When users on the inside of the network require Internet access, they
   will typically connect through a perimeter router, firewall, or
   network proxy that provides Network Address Translation (NAT) or
   Network Address Port Translation (NAPT) services to a public

   Scarcity of public IPv4 addresses is forcing many service providers
   to make use of NAT.  CGN (Carrier-Grade NAT) will enable service
   providers to assign private [RFC1918] IPv4 addresses to their
   customers rather than public, globally unique IPv4 addresses.  NAT444
   will make use of a double NAT process.

   Unpredictable or confusing interactions could occur if traffic such
   as traceroute, PMTUD, and possibly other applications were launched
   from the NAT IPv4 'inside address', and it passed over the same
   address range in the public IP core.  While such a situation would be
   unlikely to occur if the NAT pools and the private infrastructure
   addressing were under the same administration, such a situation could
   occur in the more typical situation of a NATed corporate network
   connecting to an ISP.  For example, say is used to
   internally number the corporate network.  A traceroute or PMTUD
   request is initiated inside the corporate network from say
   The packet passes through a NAT (or NAPT) gateway, then over an ISP
   core numbered from the same range.  When the responses are delivered
   back to the originator, the returned packets from the privately
   addressed part of the ISP core could have an identical source and
   destination address of

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            NAT Pool -

    .1       .2  .14     .13  .1           .2  .6      .5  .2      .1
                                                          R6 Loopback:


   Type escape sequence to abort.
   Tracing the route to

     1 0 msec 0 msec 0 msec
     2 0 msec 4 msec 0 msec
     3 0 msec 4 msec 0 msec        <<<<
     4 4 msec 0 msec 4 msec
     5 0 msec 0 msec 0 msec

   This duplicate address space scenario has the potential to cause
   confusion among operational staff, thereby making it more difficult
   to successfully debug networking problems.

   Certainly a scenario where the same [RFC1918] address space becomes
   utilised on both the inside and outside interfaces of a NAT/NAPT
   device can be problematic.  For example, the same private address
   range is assigned by both the administrator of a corporate network
   and its ISP.  Some applications discover the outside address of their
   local Customer Premises Equipment (CPE) to determine if that address
   is reserved for special use.  Application behaviour may then be based
   on this determination.  "IANA-Reserved IPv4 Prefix for Shared Address
   Space" [RFC6598] provides further analysis of this situation.

   To address this scenario and others, "IANA-Reserved IPv4 Prefix for
   Shared Address Space" [RFC6598] allocated a dedicated /10 address
   block for the purpose of Shared CGN (Carrier Grade NAT) Address
   Space:  The purpose of Shared CGN Address Space is to
   number CPE (Customer Premise Equipment) interfaces that connect to
   CGN devices.  As explained in [RFC6598], [RFC1918] addressing has
   issues when used in this deployment scenario.

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6.  Interactions with Edge Anti-Spoofing Techniques

   Denial-of-Service (DOS) attacks and Distributed Denial-of-Service
   (DDoS) attacks can make use of spoofed source IP addresses in an
   attempt to obfuscate the source of an attack.  Network Ingress
   Filtering [RFC2827] strongly recommends that providers of Internet
   connectivity implement filtering to prevent packets using source
   addresses outside of their legitimately assigned and advertised
   prefix ranges.  Such filtering should also prevent packets with
   private source addresses from egressing the AS.

   Best security practices for ISPs also strongly recommend that packets
   with illegitimate source addresses should be dropped at the AS
   perimeter.  Illegitimate source addresses includes private [RFC1918]
   IP addresses, addresses within the provider's assigned prefix ranges,
   and bogons (legitimate but unassigned IP addresses).  Additionally,
   packets with private IP destination addresses should also be dropped
   at the AS perimeter.

   If such filtering is properly deployed, then traffic either sourced
   from or destined for privately addressed portions of the network
   should be dropped, hence the negative consequences on traceroute,
   PMTUD, and regular ping-type traffic.

7.  Peering Using Loopbacks

   Some ISPs use the loopback addresses of Autonomous System Border
   Routers (ASBRs) for peering, in particular, where multiple
   connections or exchange points exist between the two ISPs.  Such a
   technique is used by some ISPs as the foundation of fine-grained
   traffic engineering and load balancing through the combination of IGP
   metrics and multi-hop BGP.  When private or non-globally reachable
   addresses are used as loopback addresses, this technique is either
   not possible or considerably more complex to implement.

8.  DNS Interaction

   Many ISPs utilise their DNS to perform both forward and reverse
   resolution for infrastructure devices and infrastructure addresses.
   With a privately numbered core, the ISP itself will still have the
   capability to perform name resolution of its own infrastructure.
   However, others outside of the autonomous system will not have this
   capability.  At best, they will get a number of unidentified
   [RFC1918] IP addresses returned from a traceroute.

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   It is also worth noting that in some cases, the reverse resolution
   requests may leak outside of the AS.  Such a situation can add load
   to public DNS servers.  Further information on this problem is
   documented in "AS112 Nameserver Operations" [RFC6304].

9.  Operational and Troubleshooting Issues

   Previous sections of this document have noted issues relating to
   network operations and troubleshooting.  In particular, when private
   IP addressing within an ISP core is used, the ability to easily
   troubleshoot across the AS boundary may be limited.  In some cases,
   this may be a serious troubleshooting impediment.  In other cases, it
   may be solved through the use of alternative troubleshooting

   The key point is that the flexibility of initiating an outbound ping
   or traceroute from a privately numbered section of the network is
   lost.  In a complex topology, with multiple paths and exit points
   from the AS, the provider may be restricted in its ability to trace
   paths through the network to other ASes.  Such a situation could be a
   severe troubleshooting impediment.

   For users outside of the AS, the loss of the ability to use a
   traceroute for troubleshooting is very often a serious issue.  As
   soon as many of these people see a row of "* * *" in a traceroute
   they often incorrectly assume that a large part of the network is
   down or inaccessible (e.g., behind a firewall).  Operational
   experience in many large providers has shown that significant
   confusion can result.

   With respect to [RFC1918] IP addresses applied as loopbacks, in this
   world of acquisitions, if an operator needed to merge two networks,
   each using the same private IP ranges, then the operator would likely
   need to renumber one of the two networks.  In addition, assume an
   operator needed to compare information such as NetFlow / IP Flow
   Information Export (IPFIX) or syslog, between two networks using the
   same private IP ranges.  There would likely be an issue as the unique
   ID in the collector is, in most cases, the source IP address of the
   UDP export, i.e., the loopback address.

10.  Security Considerations

   One of the arguments often put forward for the use of private
   addressing within an ISP is an improvement in the network security.
   It has been argued that if private addressing is used within the
   core, the network infrastructure becomes unreachable from outside the
   provider's autonomous system, hence protecting the infrastructure.
   There is legitimacy to this argument.  Certainly, if the core is

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   privately numbered and unreachable, it potentially provides a level
   of isolation in addition to what can be achieved with other
   techniques, such as infrastructure Access Control Lists (ACLs), on
   their own.  This is especially true in the event of an ACL
   misconfiguration, something that does commonly occur as the result of
   human error.

   There are three key security gaps that exist in a privately addressed
   IP core.

   1.  The approach does not protect against reflection attacks if edge
       anti-spoofing is not deployed.  For example, if a packet with a
       spoofed source address corresponding to the network's
       infrastructure address range is sent to a host (or other device)
       attached to the network, that host will send its response
       directly to the infrastructure address.  If such an attack was
       performed across a large number of hosts, then a successful
       large-scale DoS attack on the infrastructure could be achieved.
       This is not to say that a publicly numbered core will protect
       from the same attack; it won't.  The key point is that a
       reflection attack does get around the apparent security offered
       in a privately addressed core.

   2.  Even if anti-spoofing is deployed at the AS boundary, the border
       routers will potentially carry routing information for the
       privately addressed network infrastructure.  This can mean that
       packets with spoofed addresses, corresponding to the private
       infrastructure addressing, may be considered legitimate by edge
       anti-spoofing techniques (such as Unicast Reverse Path Forwarding
       - Loose Mode) and forwarded.  To avoid this situation, an edge
       anti-spoofing algorithm (such as Unicast Reverse Path Forwarding
       - Strict Mode) would be required.  Strict approaches can be
       problematic in some environments or where asymmetric traffic
       paths exist.

   3.  The approach on its own does not protect the network
       infrastructure from directly connected customers (i.e., within
       the same AS).  Unless other security controls, such as access
       control lists (ACLs), are deployed at the ingress point of the
       network, customer devices will normally be able to reach, and
       potentially attack, both core and edge infrastructure devices.

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11.  Alternate Approaches to Core Network Security

   Today, hardware-based ACLs, which have minimal to no performance
   impact, are now widespread.  Applying an ACL at the AS perimeter to
   prevent access to the network core may be a far simpler approach and
   provide comparable protection to using private addressing; such a
   technique is known as an infrastructure ACL (iACL).

   In concept, iACLs provide filtering at the edge network, which allows
   traffic to cross the network core but not to terminate on
   infrastructure addresses within the core.  Proper iACL deployment
   will normally allow required network management traffic to be passed,
   such that traceroutes and PMTUD can still operate successfully.  For
   an iACL deployment to be practical, the core network needs to have
   been addressed with a relatively small number of contiguous address
   blocks.  For this reason, the technique may or may not be practical.

   A second approach to preventing external access to the core is IS-IS
   core hiding.  This technique makes use of a fundamental property of
   the IS-IS protocol, which allows link addresses to be removed from
   the routing table while still allowing loopback addresses to be
   resolved as next hops for BGP.  The technique prevents parties
   outside the AS from being able to route to infrastructure addresses,
   while still allowing traceroutes to operate successfully.  IS-IS core
   hiding does not have the same practical requirement for the core to
   be addressed from a small number of contiguous address blocks as with
   iACLs.  From an operational and troubleshooting perspective, care
   must be taken to ensure that pings and traceroutes are using source
   and destination addresses that exist in the routing tables of all
   routers in the path, i.e., not hidden link addresses.

   A third approach is the use of either an MPLS-based IP VPN or an
   MPLS-based IP Core where the 'P' routers (or Label Switch Routers) do
   not carry global routing information.  As the core 'P' routers (or
   Label Switch Routers) are only switching labeled traffic, they are
   effectively not reachable from outside of the MPLS domain.  The 'P'
   routers can optionally be hidden so that they do not appear in a
   traceroute.  While this approach isolates the 'P' routers from
   directed attacks, it does not protect the edge routers ('PE' routers
   or Label Edge Routers (LERs)).  Obviously, there are numerous other
   engineering considerations in such an approach; we simply note it as
   an option.

   These techniques may not be suitable for every network.  However,
   there are many circumstances where they can be used successfully
   without the associated effects of privately addressing the core.

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12.  References

12.1.  Normative References

   [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", May 2000.

   [BCP84]    Baker, F. and P. Savola, "Ingress Filtering for Multihomed
              Networks", March 2004.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

12.2.  Informative References

   [RFC3021]  Retana, A., White, R., Fuller, V., and D. McPherson,
              "Using 31-Bit Prefixes on IPv4 Point-to-Point Links",
              RFC 3021, December 2000.

   [RFC5837]  Atlas, A., Bonica, R., Pignataro, C., Shen, N., and JR.
              Rivers, "Extending ICMP for Interface and Next-Hop
              Identification", RFC 5837, April 2010.

   [RFC6304]  Abley, J. and W. Maton, "AS112 Nameserver Operations",
              RFC 6304, July 2011.

   [RFC6598]  Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
              M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
              Space", BCP 153, RFC 6598, April 2012.

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Appendix A.  Acknowledgments

   The author would like to thank the following people for their input
   and review:  Dan Wing (Cisco Systems), Roland Dobbins (Arbor
   Networks), Philip Smith (APNIC), Barry Greene (ISC), Anton Ivanov
   (, Ryan Mcdowell (Cisco Systems), Russ White (Cisco
   Systems), Gregg Schudel (Cisco Systems), Michael Behringer (Cisco
   Systems), Stephan Millet (Cisco Systems), Tom Petch (BT Connect), Wes
   George (Time Warner Cable), and Nick Hilliard (INEX).

   The author would also like to acknowledge the use of a variety of
   NANOG mail archives as references.

Author's Address

   Anthony Kirkham
   Palo Alto Networks
   Level 32, 101 Miller St
   North Sydney, New South Wales  2060

   Phone:  +61 7 33530902

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