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Network Working Group                                        P. Ferguson
Request for Comments: 2071                           cisco Systems, Inc.
Category: Informational                                     H. Berkowitz
                                                       PSC International
                                                            January 1997

                     Network Renumbering Overview:
               Why would I want it and what is it anyway?

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.


   The PIER [Procedures for Internet/Enterprise Renumbering] working
   group is compiling a series of documents to assist and instruct
   organizations in their efforts to renumber.  However, it is becoming
   apparent that, with the increasing number of new Internet Service
   Providers (ISP's) and organizations getting connected to the Internet
   for the first time, the concept of network renumbering needs to be
   further defined.  This document attempts to clearly define the
   concept of network renumbering and discuss some of the more pertinent
   reasons why an organization would have a need to do so.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.   Background . . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.   Network Renumbering Defined. . . . . . . . . . . . . . . . .  3
   4.   Reasons for Renumbering. . . . . . . . . . . . . . . . . . .  3
   5.   Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   6.   Security Considerations  . . . . . . . . . . . . . . . . . . 12
   7.   Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . 12
   8.   References . . . . . . . . . . . . . . . . . . . . . . . . . 13
   9.   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14

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1. Introduction

   The popularity of connecting to the global Internet over the course
   of the past several years has spawned new problems; what most people
   casually refer to as "growing pains" can be attributed to more basic
   problems in understanding the requirements for Internet connectivity.
   However, the reasons why organizations may need to renumber their
   networks can greatly vary. We'll discuss these issues in some amount
   of detail below.  It is not within the intended scope of this
   document to discuss renumbering methodologies, techniques, or tools.

2. Background

   The ability for any network or interconnected devices, such as
   desktop PCs or workstations, to obtain connectivity to any potential
   destination in the global Internet is reliant upon the possession of
   unique IP host addresses [1].  A duplicate host address that is being
   used elsewhere in the Internet could best be described as
   problematic, since the presence of duplicate addresses would cause
   one of the destinations to be unreachable from some origins in the
   Internet.  It should be noted, however, that globally unique IP
   addresses are not always necessary, and is dependent on the
   connectivity requirements [2].

   However, the recent popularity in obtaining Internet connectivity has
   made these types of connectivity dependencies unpredictable, and
   conventional wisdom in the Internet community dictates that the
   various address allocation registries, such as the InterNIC, as well
   as the ISP's, become more prudent in their address allocation
   strategies.  In that vein, the InterNIC has defined address
   allocation policies [3] wherein the majority of address allocations
   for end-user networks are accommodated by their upstream ISP, except
   in cases where dual- or multihoming and very large blocks of
   addresses are required.  With this allocation policy becoming
   standard current practice, it presents unique problems regarding the
   portability of addresses from one provider to another.

   As a practical matter, end users cannot assume they "own" address
   allocations, if their intention is to be to have full connectivity to
   the global Internet. Rather, end users will "borrow" part of the
   address space of an upstream provider's allocation. The larger
   provider block from which their space is suballocated will have been
   assigned in a manner consistent with global Internet routing.

   Not having "permanent" addresses does not mean users will not have
   unique identifiers. Such identifiers are typically Domain Name System
   (DNS) [4] names for endpoints such as servers and workstations.
   Mechanisms such as the Dynamic Host Configuration Protocol (DHCP) [5]

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   can help automate the assignment and maintenance of host names, as
   well as the 'borrowed' addresses required for routing-level

   The PIER Working Group is developing procedures and guidelines for
   detailed renumbering of specific technologies, such as routers [6].
   PIER WG documents are intended to suggest methods both for making
   existing networks prepared for convenient renumbering, as well as for
   operational transition to new addressing schemes.

   Also, in many instances, organizations who have never connected to
   the Internet, yet have been using arbitrary blocks of addresses since
   their construction, have different and unique challenges.

3. Network Renumbering Defined

   In the simplest of definitions, the exercise of renumbering a network
   consists of changing the IP host addresses, and perhaps the network
   mask, of each device within the network that has an address
   associated with it. This activity may or may not consist of all
   networks within a particular domain, such as FOO.EDU, or networks
   which comprise an entire autonomous system.

   Devices which may need to be renumbered, for example, are networked
   PC's, workstations, printers, file servers, terminal servers, and
   routers. Renumbering a network may involve changing host parameters
   and configuration files which contain IP addresses, such as
   configuration files which contain addresses of DNS and other servers,
   addresses contained in SNMP [7] management stations, and addresses
   configured in access control lists. While this is not an all-
   inclusive list, the PIER working group is making efforts to compile
   documentation to identify these devices in a more detailed fashion.

   Network renumbering need not be sudden activity, either; in most
   instances, an organization's upstream service provider(s) will allow
   a grace period where both the "old" addresses and the "new" addresses
   may be used in parallel.

4. Reasons for Renumbering

   The following sections discuss particular reasons which may
   precipitate network renumbering, and are not presented in any
   particular order of precedence.  They are grouped into reasons that
   primarily reflect decisions made in the past, operational
   requirements of the present, or plans for the future.

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   Some of these requirements reflect evolution in the organization's
   mission, such as a need to communicate with business partners, or to
   work efficiently in a global Internet.  Other requirements reflect
   changes in network technologies.

4.1  Past

   Many organizations implemented IP-based networks not for connectivity
   to the Internet, but simply to make use of effective data
   communications mechanisms.  These organizations subsequently found
   valid reasons to connect to other organizations or the Internet in
   general, but found the address structures they chose incompatible
   with overall Internet practice.

   Other organizations connected early to the Internet, but did so at a
   time when address space was not scarce.  Yet other organizations
   still have no requirement to connect to the Internet, but have legacy
   addressing structures that do not scale to adequate size.

4.1.1  Initial addressing using non-unique addresses

   As recently as two years ago, many organizations had no intention of
   connecting to the Internet, and constructed their corporate or
   organizational network(s) using unregistered, non-unique network
   addresses.  Obviously, as most problems evolve, these same
   organizations determined that Internet connectivity had become a
   valuable asset, and subsequently discovered that they could no longer
   use the same unregistered, non-unique network addresses that were
   previously deployed throughout their organization.  Thus, the labor
   of renumbering to valid network addresses is now upon them, as they
   move to connect to the global Internet.

   While obtaining valid, unique addresses is certainly required to
   obtain full Internet connectivity in most circumstances, the number
   of unique addresses required can be significantly reduced by the
   implementation of Network Address Translation (NAT) devices [8] and
   the use of private address space, as specified in [9].  NAT reduces
   not only the number of required unique addresses, but also localizes
   the changes required by renumbering.

   It should also be noted that NAT technology may not always be a
   viable option, depending upon scale of addressing, performance or
   topological constraints.

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4.1.2  Legacy address allocation

   There are also several instances where organizations were originally
   allocated very large amounts of address space, such as traditional
   "Class A" or "Class B" allocations, while the actual address
   requirements are much less than the total amount of address space
   originally allocated.  In many cases, these organizations could
   suffice with a smaller CIDR allocation, and utilize the allocated
   address space in a more efficient manner.  As allocation requirements
   become more stringent, mechanisms to review how these organizations
   are utilizing their address space could, quite possibly, result in a
   request to return the original allocation to a particular registry
   and renumber with a more appropriately sized address block.

4.1.3  Limitations of Bridged Internetworks

   Bridging has a long and distinguished history in legacy networks.  As
   networks grow, however, traditional bridged networks reach
   performance- and stability-related limits, including (but not limited
   to) broadcast storms.

   Early routers did not have the speed to handle the needs of some
   large networks.  Some organizations were literally not able to move
   to routers until router forwarding performance improved to be
   comparable to bridges.  Now that routers are of comparable or
   superior speed, and offer more robust features, replacing bridged
   networks becomes reasonable.

   IP addresses assigned to pure bridged networks tend not to be
   subnetted, yet subnetting is a basic approach for router networks.
   Introducing subnetting is a practical necessity in moving from
   bridging to routing.

   Special cases of bridging are realized in workgroup switching
   systems, discussed below.

4.1.4  Limitations of Legacy Routing Systems

   Other performance problems might come from routing mechanisms that
   advertise excessive numbers of routing updates (e.g., RIP, IGRP).
   Likewise, appropriate replacement protocols (e.g., OSPF, EIGRP, S-IS)
   will work best with a structured addressing system that encourages

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4.1.5  Limitations of System Administration Methodologies

   There can be operational limits to growth based on the difficulty of
   adds, moves and changes.  As enterprise networks grow, it may be
   necessary to delegate portions of address assignment and maintenance.
   If address space has been assigned randomly or inefficiently, it may
   be difficult to delegate portions of the address space.

   It is not unusual for organizational networks to grow sporadically,
   obtaining an address prefix here and there, in a non-contiguous
   fashion.  Depending on the number of prefixes that an organization
   acquires over time, it may become increasingly unmanageable or demand
   higher levels of maintenance and administration when individual
   prefixes are acquired in this way.

   Reasonable IP address management may in general simplify continuing
   system administration; a good numbering plan is also a good
   renumbering plan.  Renumbering may force a discipline into system
   administration that will reduce long-term support costs.

   It has been observed "...there is no way to renumber a network
   without an inventory of the hosts (absent DHCP). On a large network
   that needs a database, plus tools and staff to maintain the
   database."[10] It can be argued that a detailed inventory of router
   configurations is even more essential.

4.2  Present

   Organizations now face needs to connect to the global Internet, or at
   a minimum to other organizations through bilateral private links.

   Certain new transmission technologies have tended to redefine the
   basic notion of an IP subnet.  An IP numbering plan needs to work
   with these new ideas. Legacy bridged networks and leading-edge
   workgroup switched networks may very well need changes in the
   subnetting structure.  Renumbering needs may also develop due to the
   characteristics of new WAN technologies, especially nonbroadcast
   multi-access (NBMA) services such as Frame-Relay and Asynchronous
   Transfer Mode (ATM).

   Increased use of telecommuting by mobile workers, and in small and
   home offices, need on-demand WAN connectivity, using modems or ISDN.
   Effective use of demand media often requires changes in numbering and

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4.2.1   Change in organizational structure or network topology

   As companies grow, through mergers, acquisitions and reorganizations,
   the need may arise for realignment and modification of the various
   organizational network architectures.  The connectivity of disparate
   corporate networks present unique challenges in the realm of
   renumbering, since one or more individual networks may have to be
   blended into a much larger architecture consisting a different IP
   address prefix altogether.

4.2.2  Inter-Enterprise Connectivity

   Even if they do not connect to the general Internet, enterprises may
   interconnect to other organizations which have independent numbering
   systems. Such connectivity can be as simple as bilateral dedicated
   circuits. If both enterprises use unregistered or private address
   space, they run the risk of using duplicate addresses.

   In such cases, one or both organizations may need to renumber into
   different parts of the private address space, or obtain unique
   registered addresses.

4.2.3   Change of Internet Service Provider

   As mentioned previously in Section 2, it is increasingly becoming
   current practice for organizations to have their IP addresses
   allocated by their upstream ISP.  Also, with the advent of Classless
   Inter Domain Routing (CIDR) [11], and the considerable growth in the
   size of the global Internet table, Internet Service Providers are
   becoming more and more reluctant to allow customers to continue using
   addresses which were allocated by the ISP, when the customer
   terminates service and moves to another ISP.  The prevailing reason
   is that the ISP was previously issued a CIDR block of contiguous
   address space, which can be announced to the remainder of the
   Internet community as a single prefix. (A prefix is what is referred
   to in classless terms as a contiguous block of IP addresses.)  If a
   non-customer advertises a specific component of the CIDR block, then
   this adds an additional routing entry to the global Internet routing
   table.  This is what is commonly referred to as "punching holes" in a
   CIDR block. Consequently, there are usually no routing anomalies in
   doing this since a specific prefix is always preferred over an
   aggregate route.  However, if this practice were to happen on a large
   scale, the growth of the global routing table would become much
   larger, and perhaps too large for current backbone routers to
   accommodate in an acceptable fashion with regards to performance of
   recalculating routing information and sheer size of the routing table
   itself.  For obvious reasons, this practice is highly discouraged by
   ISP's with CIDR blocks, and some ISP's are making this a contractual

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   issue, so that customers understand that addresses allocated by the
   ISP are non-portable.

   It is noteworthy to mention that the likelihood of being forced to
   renumber in this situation is inversely proportional to the size of
   the customer's address space.  For example, an organization with a
   /16 allocation may be allowed to consider the address space
   "portable", while an organization with multiple non-contiguous /24
   allocations may not.  While the scenarios may be vastly different in
   scope, it becomes an issue to be decided at the discretion of the
   initial allocating entity, and the ISP's involved; the major deciding
   factor being whether or not the change will fragment an existing CIDR
   block and whether it will significantly contribute to the overall
   growth of the global Internet routing tables.

   It should also be noted that (contrary to opinions sometimes voiced)
   this form of renumbering is a technically necessary consequence of
   changing ISP's, rather than a commercial or political mandate.

4.2.3  Internet Global Routing

   Even large organizations, now connected to the Internet with
   "portable" address space, may find their address allocation too
   small. Current registry guidelines require that address space usage
   be justified by an engineering plan. Older networks may not have
   efficiently utilized existing address space, and may need to make
   their existing structures more efficient before new address
   allocations can be made.

4.2.4  Internal Use of LAN Switching

   Introducing workgroup switches may introduce subtle renumbering
   needs.  Fundamentally, workgroup switches are specialized, high-
   performance bridges, which make their main forwarding decisions based
   on Layer 2 (MAC) address information. Even so, they rarely are
   independent of Layer 3 (IP) address structure.  Pure Layer 2
   switching has a "flat" address space that will need to be renumbered
   into a hierarchical, subnetted space consistent with routing.

   Introducing single switches or stacks of switches may not have
   significant impact on addressing, as long as it is understood that
   each system of switches is a single broadcast domain. Each broadcast
   domain should map to a single IP subnetwork.

   Virtual LANs (VLANs) further extend the complexity of the role of
   workgroup switches. It is generally true that moving an end station
   from one switch port to another within the same VLAN will not cause
   major changes in addressing. Many overview presentations of this

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   technology do not make it clear that moving the same end station
   between different VLANs will move the end station into another IP
   subnet, requiring a significant address change.

   Switches are commonly managed by SNMP applications. These network
   management applications communicate with managed devices using IP.
   Even if the switch does not do IP forwarding, it will itself need IP
   addresses if it is to be managed. Also, if the clients and servers in
   the workgroup are managed by SNMP, they will also require IP
   addresses. The workgroup, therefore, will need to appear as one or
   more IP subnetworks.

   Increasingly, internetworking products are not purely Layer 2 or
   Layer 3 devices. A workgroup switch product often includes a routing
   function, so the numbering plan must support both flat Layer 2 and
   hierarchical Layer 3 addressing.

4.2.4  Internal Use of NBMA Cloud Services

   "Cloud" services such as frame relay often are more economical than
   traditional services. At first glance, when converting existing
   enterprise networks to NBMA, it might appear that the existing subnet
   structure should be preserved, but this is often not the case.

   Many organizations often  began by treating the "cloud" as a single
   subnet, but experience has shown it is often better to treat the
   individual virtual circuits as separate subnets, which appear as
   traditional point-to-point circuits.  When the individual point-to-
   point VCs become separate subnets, efficient address utilization
   requires the use of long prefixes (i.e., 30 bit) for these subnets.
   In practice, obtaining 30 bit prefixes means the logical network
   should support variable length subnet masks (VLSM).  VLSMs are the
   primary method in which an assigned prefix can be subnetted
   efficiently for different media types. This is accomplished by
   establishing one or more prefix lengths for LAN media with more than
   two hosts, and subdividing one or more of these shorter prefixes into
   longer /30 prefixes that minimize address loss.

   There are alternative ways to configure routing over NBMA, using
   special mechanisms to exploit or simulate point-to-multipoint VCs.
   These often have a significant performance impact, and may be less
   reliable because a single routing point of failure is created.
   Motivations for such alternatives tend to include:

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      1.  A desire not to use VLSM. This is often founded in fear
          rather than technology.

      2.  Router implementation issues that limit the number of subnets
          or interfaces a given router can support.

      3.  An inherently point-to-multipoint application (e.g., remote
          hosts to a data center). In such cases, some of the
          limitations are due to the dynamic routing protocol in use.
          In such "hub-and-spoke" implementations, static routing can
          be preferable from a performance and flexibility standpoint,
          since it does not produce routing protocol chatter and is
          unaffected by split horizon constraints (namely, the inability
          to build an adjacency with a peer within the same IP

4.2.5  Expansion of Dialup Services

   Dialup services, especially public Internet access providers, are
   experiencing explosive growth. This success represents a particular
   drain on the available address space, especially with a commonly used
   practice of assigning unique addresses to each customer.

   In this case, individual users announce their address to the access
   server using PPP's IP control protocol (IPCP) [12]. The server may
   validate the proposed address against some type of user
   identification, or simply make the address active in a subnet to
   which the access server (or set of bridged access servers) belongs.

   The preferred technique is to allocate dynamic addresses to the user
   from a pool of addresses available to the access server.

4.2.6  Returning non-contiguous prefixes for an aggregate

   In many instances, an organization can return their current, non-
   contiguous prefix allocations for a contiguous block of address space
   of equal or greater size, which can be accommodated with CIDR.  Also,
   many organizations have begun to deploy classless interior routing
   protocols within their domains that make use of route summarization
   and other optimized routing features, effectively reducing the total
   number of routes being propagated within their internal network(s),
   and making it much easier to administer and maintain.

   Hierarchical routing protocols such as OSPF scale best when the
   address assignment of a given network reflects the topology, and the
   topology of the network can often be fluid. Given that the network is
   fluid, even the best planned address assignment scheme, given time,
   will diverge from the actual topology. While not required, some

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   organization may choose to gain the benefit of both technical and
   administrative scalability of their IGP by periodically renumbering
   to have address assignments reflect the network topology. Patrick
   Henry once said "the tree of liberty must from time to time be
   watered with the blood of patriots." In the Internet, routing trees
   of the best-planned networks need from time to time be watered with
   at least the sweat of network administrators.  Improving aggregation
   is also highly encouraged to reduce the size of not only the global
   Internet routing table, but also the size and scalability of interior
   routing within the enterprise.

4.3  Future

   Emerging new protocols will most definitely affect addressing plans
   and numbering schemes.

4.3.1  Internal Use of Switched Virtual Circuit Services

   Services such as ATM virtual circuits, switched frame relay, etc.,
   present challenges not considered in the original IP design.  The
   basic IP decision in forwarding a packet is whether the destination
   is local or remote, in relation to the source host's subnet. Address
   resolution mechanisms are used to find the medium address of the
   destination in the case of local destinations, or to find the medium
   address of the router in the case of remote routers.

   In these new services, there are cases where it is far more effective
   to "cut-through" a new virtual circuit to the destination. If the
   destination is on a different subnet than the source, the cut-through
   typically is to the egress router that serves the destination subnet.
   The advantage of cut-through in such a case is that it avoids the
   latency of multiple router hops, and reduces load on "backbone"
   routers. The cut-through decision is usually made by an entry router
   that is aware of both the routed and switched environments.

   This entry router communicates with a address resolution server using
   the Next Hop Resolution Protocol (NHRP) [13]. This server maps the
   destination network address to either a next-hop router (where cut-
   through is not appropriate) or to an egress router reached over the
   switched service. Obviously, the data base in such a server may be
   affected by renumbering. Clients may have a hard-coded address of the
   server, which again may need to change.  While the NHRP protocol
   specifications are still evolving at the time of this writing,
   commercial implementations based on drafts of the protocol standard
   are in use.

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4.3.2  Transitioning to IP version 6

   Of course, when IPv6 [14] deployment is set in motion, and as
   methodologies are developed to transition to IPv6, renumbering will
   also be necessary, but perhaps not immediately mandatory.  To aid in
   the transition to IPv6, mechanisms to deploy dual- IPv4/IPv6 stacks
   on network hosts should also become available. It is also envisioned
   that Network Address Translation (NAT) devices will be developed to
   assist in the IPv4 to IPv6 transition, or perhaps supplant the need
   to renumber the majority of interior networks altogether, but that is
   beyond the scope of this document. At the very least, DNS hosts will
   need to be reconfigured to resolve new host names and addresses, and
   routers will need to be reconfigured to advertise new prefixes.

   IPv6 address allocation will be managed by the Internet Assigned
   Numbers Authority (IANA) as set forth in [15].

5. Summary

   As indicated by the Internet Architecture Board (IAB) in [16], the
   task of renumbering networks is becoming more widespread and
   commonplace.  Although there are numerous reasons why an organization
   would desire, or be required to renumber, there are equally as many
   reasons why address allocation should be done with great care and
   forethought at the onset, in order to minimize the impact that
   renumbering would have on the organization. Even with the most
   forethought and vision, however, an organization cannot foresee the
   possibility for renumbering. The best advice, in this case, is to be
   prepared, and get ready for renumbering.

6. Security Considerations

   Although no obvious security issues are discussed in this document,
   it stands to reason that renumbering certain devices can defeat
   security systems designed and based on static IP host addresses.
   Care should be exercised by the renumbering entity to ensure that all
   security systems deployed with the network(s) which may need to be
   renumbered be given special consideration and significant forethought
   to provide continued functionality and adequate security.

7. Acknowledgments

   Special acknowledgments to Yakov Rekhter [cisco Systems, Inc.], Tony
   Bates [cisco Systems, Inc.] and Brian Carpenter [CERN] for their
   contributions and editorial critique.

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8. References

 [1] Gerich, E., "Unique Addresses are Good", RFC 1814, IAB, July 1995.

 [2] Crocker, D., "To Be `On' the Internet", RFC 1775, March 1995.

 [3] Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D., and J.
     BCP 12, RFC 2050, November 1996.

 [4] Mockapetris, P., "Domain Names - Concepts and Facilities",
     and  "Domain Names - Implementation and Specification",
     STD 13, RFCs 1034, 1035, November 1987.

 [5] Droms, R., "Dynamic Host Configuration Protocol", RFC 1541,
     October 1993.

 [6] Berkowitz, H., "Router Renumbering Guide", RFC 2072,
     January 1997.

 [7] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "A Simple
     Network Management Protocol (SNMP)", STD 15, RFC 1157,
     May 1990.

 [8] Egevang,, K., and P. Francis, "The IP Network Address Translator
     (NAT)", RFC 1631, May 1994.

 [9] Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G-J., and E.
     Lear, "Address Allocation for Private Internets", RFC 1918,
     February 1996.

 [10] Messages to PIER list on CERN renumbering; Brian Carpenter, CERN.
      Available in PIER WG mailing list archives.

 [11] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless
      Inter-Domain Routing (CIDR): an Address Assignment and
      Aggregation Strategy", RFC 1519, October 1993.

 [12] McGregor, G., "The PPP Internet Protocol Control Protocol
      (IPCP)", RFC 1332, May 1992.

 [13] Luciani, J., Katz, D., Piscitello, D., and Cole, B., "NBMA Next
      Hop Resolution Protocol (NHRP)", Work in Progress.

 [14] Deering, S., and R. Hinden, "Internet Protocol, Version 6 (IPv6)
      Specification", RFC 1883, December 1995.

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 [15] IAB and IESG, "IPv6 Address Allocation Management", RFC 1881,
      December 1995.

 [16] Carpenter, B., and Y. Rekhter, "Renumbering Needs Work", RFC 1900,
      February 1996.

9. Authors' Addresses

   Paul Ferguson
   cisco Systems, Inc.
   1875 Campus Commons Road
   Suite 210
   Reston, VA 22091

   Phone: (703) 716-9538
   Fax: (703) 716-9599
   EMail: pferguso@cisco.com

   Howard C. Berkowitz
   PSC International
   1600 Spring Hill Road
   Vienna, VA 22182

   Phone (703) 998-5819
   Fax:  (703) 998-5058
   EMail:  hcb@clark.net

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