Additional Private IPv4 Space Issues
draft-azinger-additional-private-ipv4-space-issues-00

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Abstract

When a private network or internetwork grows very large it is sometimes not possible to address it using private IPv4 address space. This document describes the problems faced by those networks, the available options and the issues involved in assigning a new block of private IPv4 address space.

Table of Contents

1.  Introduction
2.  Large networks
3.  Network Address Translation
4.  Available Options
    4.1.  Unique globally scoped IPv6 unicast addresses
    4.2.  Unique locally scoped IPv6 unicast addresses
    4.3.  Address transfers or leases from organizations with available address space
    4.4.  Using unannounced address space allocated to another organization
    4.5.  Unique IPv4 space registered by an RIR
    4.6.  Unique IPv4 space shared by a group of operators
5.  Other Options
    5.1.  Redefining Additional Unicast Space as Private Address Space
    5.2.  Potential Consequences of Not Redefining Additional Unicast Space as Private Address Space
    5.3.  Redefining Future Use Space as Unicast Address Space
6.  Security Considerations
7.  IANA Considerations
8.  References
    8.1.  Normative References
    8.2.  Informative References
Appendix A.  Acknowledgments
§  Authors' Addresses

1.  Introduction

[RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) sets aside three blocks of IPv4 address space for use in private networks: 192.168.0.0/16, 172.16.0.0/12 and 10.0.0.0/8. These blocks can be used simultaneously in multiple, separately managed networks without registration or coordination with IANA or any Internet registry. Very large networks can find that they need to connect more interfaces than the number of addresses available in these three ranges. It has occasionally been suggested that additional private IPv4 address space should be reserved for use by these networks. Although such an action might address some of the needs for these very large network operators it is not without consequences, particularly as we near the date when the IANA free pool will be fully allocated.

2.  Large networks

The main categories of very large networks using private address space are: cable operators, wireless (cell phone) operators, private internets and VPN service providers. In the case of the first two categories, the complete address space reserved in [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) tends to be used by a single organization. In the case of private internets and VPN service providers there are multiple independently managed and operated networks and the difficulty is in avoiding address clashes.

3.  Network Address Translation

The address space set aside in [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) is a finite resource which can be used to provide limited Internet access via Network Address Translation (NAT). A discussion of the advantages and disadvantages of NATs is outside the scope of this document. Nonetheless, it must be acknowledged that NAT is adequate in some situations and not in others. For instance, it is often technically feasible to use NAT or even multiple layers of NAT within the networks operated by residential users or corporations where peer to peer communication is not needed. Where peer to peer communication is needed or where services or applications do not work properly behind NAT, globally unique address space is required.

In many cases it is possible to use multiple layers of NAT to re-use parts of the address space defined in [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.). In particular, the tendency for low-cost CPEs to use 192.168.0.0/16 as the default address range for the LAN allows providers to make full use of 172.16.0.0/12 and 10.0.0.0/8.

4.  Available Options

When a network operator has exhausted the private address space set aside in [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) but needs to continue operating a single routing domain a number of options are available. These include:

4.1.  Unique globally scoped IPv6 unicast addresses

Using unique, globally scoped IPv6 unicast addresses is the preferred option as it removes any concerns about address scarcity. In some cases implementing a new network protocol on a very large network takes more time than is available, based on network growth and the proportion of private space that has already been used. In these cases, there is a call for additional private address space that can be shared by all network operators.

4.2.  Unique locally scoped IPv6 unicast addresses

Using the unique, locally scoped IPv6 unicast addresses defined in [RFC4193] (Hinden, R. and B. Haberman, “Unique Local IPv6 Unicast Addresses,” October 2005.) is another approach and does not require coordination with an Internet registry. Although the addresses defined in [RFC4193] (Hinden, R. and B. Haberman, “Unique Local IPv6 Unicast Addresses,” October 2005.) are probabilistically unique, network operators on private internets and those providing VPN services might not want to use them because there is a very low probability of non-unique locally assigned global IDs being generated by the algorithm. Also, in the case of private internets, it can be very challenging to coordinate the introduction of a new network protocol to support the internet's continued growth.

4.3.  Address transfers or leases from organizations with available address space

The Regional Internet Registries (RIRs) have recently been developing policies to allow organizations with available address space to transfer such designated space to other organizations. In other cases, leases might be arranged. This approach is only viable for operators of very large networks if enough address space is made available for transfer or lease and if the very large networks are able to pay the costs of these transfers. It is not possible to know how much address space will become available in this way, when it will be available and how much it will cost. For these reasons, address transfers will not be an attractive proposition to many large network operators. Leases might not be attractive to some organizations if both parties cannot agree a suitable length of time. Also, the leasor might worry about its own unanticipated needs for additional IPv4 address space.

4.4.  Using unannounced address space allocated to another organization

Some network operators have considered using IP address space which is allocated to another organizatiobn but is not publicly visible in BGP routing tables. This is not a preferred option as the fact that an address block is not visible from one view does not mean that it is not visible from another. It is also possible that the registrant of the address block might want to increase its visibility to other networks in the future, causing problems for anyone using it unofficially. In some cases there might also be legal risks involved in using address space officially allocated to another organization.

4.5.  Unique IPv4 space registered by an RIR

The policy framework shared by the RIRs does not discriminate based on what an address is used to do, just on how efficiently the assigned addresses are used. Unique IPv4 addresses registered by an RIR are potentially available to organizations whose networks have used all the addresses set aside in [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.). Nonetheless, network operators are naturally disinclined to request unique IPv4 addresses for a purpose that could be met with private addresses were it not for the size of the network. Addresses assigned in this way are not available for anyone else to use and so their registration denies them to new entrants, including potential customers.

4.6.  Unique IPv4 space shared by a group of operators

Where a group of networks find themselves in a position where they each need a large amount of IPv4 address space from an RIR in addition to that defined in [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) they might cooperatively agree to all use the same address space to number their networks. The clear benefit to this approach is that it significantly reduces the potential demand on the pool of unallocated IPv4 address space.

This approach has the potential to create an unofficial new private address range without proper scrutiny.

5.  Other Options

Another set of options can also be considered.

5.1.  Redefining Additional Unicast Space as Private Address Space

It would be possible to re-designate a portion of the current global unicast IPv4 address space as private unicast address space. Doing this could benefit a number of operators of large network for the short period before they complete their IPv6 roll-out. However, this benefit incurs a cost by reducing the pool of global unicast addresses available to end users.

When considering re-designating a portion of the current global unicast IPv4 address space as private unicast address space it is important to consider how much space would be used and for how long it would be sufficient. Not all of the large networks making full of of the space defined in [RFC1918] (Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, “Address Allocation for Private Internets,” February 1996.) would have their needs met with a single /8. In 2005, [I‑D.hain‑1918bis] (Hain, T., “Expanded Address Allocation for Private Internets,” February 2005.) suggested reserving three /8s for this purpose.

A further consideration is which of the currently unallocated IPv4 unicast /8 blocks should be used for this purpose. Using address space which is known to be used unofficially is tempting. For instance, 1.0.0.0/8, which was proposed in [I‑D.hain‑1918bis] (Hain, T., “Expanded Address Allocation for Private Internets,” February 2005.) is known to be used by a number of different users. These include networks making use of HIP LSIs [RFC4423] (Moskowitz, R. and P. Nikander, “Host Identity Protocol (HIP) Architecture,” May 2006.), [WIANA] (WIANA, “The Wireless Internet Assigned Numbers Authority,” .), [anoNet] (anoNet, “anoNet: Cooperative Chaos,” .) and others. There is anecdotal [VEGODA] (Vegoda, L., “Awkward /8 Assignments,” .) and research [WESSELS] (Wessels, D., “Searching for Evidence of Unallocated Address Space Usage in DITL 2008 Data,” .) evidence to suggest that several other IPv4 /8s are used in this fashion.

Although new IPv4 /8s are allocated approximately once a month, they are not easy to bring into use because network operators are slow to change their filter configurations. This is despite long-running awareness campaigns [CYMRU] (Greene, B., “The Bogon Reference,” .), [LEWIS] (Lewis, J., “This system has been setup for testing purposes for 69/8 address space,” .) and active work [ripe‑351] (Karrenberg, D., “De-Bogonising New Address Blocks,” .) to notify people whose filters are not changed in a timely fashion. Updating code that recognises private address space in deployed software and infrastructure systems is likely to be far more difficult as many systems have these ranges hard-coded and cannot be quickly changed with a new configuration file.

5.2.  Potential Consequences of Not Redefining Additional Unicast Space as Private Address Space

If additional private address space is not defined and the large network operators affected by this problem are not able to solve their problems with IPv6 address space or by segmenting their networks into multiple routing domains, those networks will need unique IPv4 addresses. It is possible and even likely that a single network could consume a whole IPv4 /8 in a year. At the time of writing there are just 28 unallocated IPv4 /8s, so it would not take many such requests to make a major dent in the available IPv4 address space.

5.3.  Redefining Future Use Space as Unicast Address Space

There have also been proposals to re-designate the former Class E space (240.0.0.0/4) as unicast address space. [I‑D.wilson‑class‑e] (Wilson, P., Michaelson, G., and G. Huston, “Redesignation of 240/4 from "Future Use" to "Private Use",” September 2008.) suggests that it should be privately scoped while [I‑D.fuller‑240space] (Fuller, V., “Reclassifying 240/4 as usable unicast address space,” March 2008.) does not propose a scope. Both proposals note that existing deployed equipment may not be able to use addresses from 240.0.0.0/4. Potential users would need to be sure of the status of the equipment on their network and the networks with which they intend to communicate.

It is not immediately clear how useful 240.0.0.0/4 could be in practice. While [I‑D.fuller‑240space] (Fuller, V., “Reclassifying 240/4 as usable unicast address space,” March 2008.) documents the status of several popular desktop and server operating systems, the status of the most widely deployed routers and switches is less clear and it is possible that 240.0.0.0/4 might only be useful in very large, new greenfield deployments where full control of all deployed systems is available. However, in such cases it might well be easier to deploy an IPv6 network.

6.  Security Considerations

This document has no security implications.

7.  IANA Considerations

This document makes no request of IANA.

8.  References

8.1. Normative References

8.2. Informative References

Appendix A.  Acknowledgments

The authors would also like to thank Ron Bonica, Michelle Cotton, Lee Howard and Barbara Roseman for their assistance in early discussions of this document.

Authors' Addresses