rfc1918

Network Working Group Y. Rekhter
Request for Comments: 1918 Cisco Systems
Obsoletes: 1627, 1597 B. Moskowitz BCP: 5 Chrysler Corp. Category: Best Current Practice D. Karrenberg RIPE NCC G. J. de Groot RIPE NCC E. Lear Silicon Graphics, Inc.

February 1996

Address Allocation for Private Internets

Status of this Memo

This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for improvements. Distribution of this memo is unlimited.

1. Introduction
   For the purposes of this document, an enterprise is an entity

autonomously operating a network using TCP/IP and in particular

determining the addressing plan and address assignments within that
network. This document describes address allocation for private internets. The

allocation permits full network layer connectivity among all hosts

inside an enterprise as well as among all public hosts of different
enterprises. The cost of using private internet address space is the

potentially costly effort to renumber hosts and networks between
public and private.

2. Motivation
   With the proliferation of TCP/IP technology worldwide, including outside the Internet itself, an increasing number of non-connected

enterprises use this technology and its addressing capabilities for
sole intra-enterprise communications, without any intention to ever directly connect to other enterprises or the Internet itself. The Internet has grown beyond anyone’s expectations. Sustained exponential growth continues to introduce new challenges. One

challenge is a concern within the community that globally unique
address space will be exhausted. A separate and far more pressing

concern is that the amount of routing overhead will grow beyond the
Rekhter, et al Best Current Practice [Page 1]

RFC 1918 Address Allocation for Private Internets February 1996
capabilities of Internet Service Providers. Efforts are in progress

within the community to find long term solutions to both of these
problems. Meanwhile it is necessary to revisit address allocation procedures, and their impact on the Internet routing system. To contain growth of routing overhead, an Internet Provider obtains a block of address space from an address registry, and then assigns to

its customers addresses from within that block based on each customer
requirement. The result of this process is that routes to many customers will be aggregated together, and will appear to other providers as a single route [RFC1518], [RFC1519]. In order for route aggregation to be effective, Internet providers encourage customers joining their network to use the provider’s block, and thus renumber their computers. Such encouragement may become a requirement in the future.

With the current size of the Internet and its growth rate it is no

longer realistic to assume that by virtue of acquiring globally

unique IP addresses out of an Internet registry an organization that

acquires such addresses would have Internet-wide IP connectivity once
the organization gets connected to the Internet. To the contrary, it

is quite likely that when the organization would connect to the

Internet to achieve Internet-wide IP connectivity the organization

would need to change IP addresses (renumber) all of its public hosts
(hosts that require Internet-wide IP connectivity), regardless of

whether the addresses used by the organization initially were
globally unique or not.

It has been typical to assign globally unique addresses to all hosts
that use TCP/IP. In order to extend the life of the IPv4 address space, address registries are requiring more justification than ever before, making it harder for organizations to acquire additional address space [RFC1466].

Hosts within enterprises that use IP can be partitioned into three
categories: Category 1: hosts that do not require access to hosts in other enterprises or the Internet at large; hosts within

this category may use IP addresses that are
unambiguous within an enterprise, but may be ambiguous between enterprises. Category 2: hosts that need access to a limited set of outside services (e.g., E-mail, FTP, netnews, remote login) which can be handled by mediating gateways (e.g., application layer gateways). For many hosts in this

category an unrestricted external access (provided
Rekhter, et al Best Current Practice [Page 2]

RFC 1918 Address Allocation for Private Internets February 1996

via IP connectivity) may be unnecessary and even
undesirable for privacy/security reasons. Just like hosts within the first category, such hosts may use

IP addresses that are unambiguous within an
enterprise, but may be ambiguous between enterprises. Category 3: hosts that need network layer access outside the enterprise (provided via IP connectivity); hosts in

the last category require IP addresses that are
globally unambiguous.

We will refer to the hosts in the first and second categories as
"private". We will refer to the hosts in the third category as "public".

Many applications require connectivity only within one enterprise and

do not need external (outside the enterprise) connectivity for the
majority of internal hosts. In larger enterprises it is often easy to

identify a substantial number of hosts using TCP/IP that do not need
network layer connectivity outside the enterprise. Some examples, where external connectivity might not be required, are:

• A large airport which has its arrival/departure displays
  individually addressable via TCP/IP. It is very unlikely

that these displays need to be directly accessible from
other networks.

• Large organizations like banks and retail chains are
  switching to TCP/IP for their internal communication. Large numbers of local workstations like cash registers, money machines, and equipment at clerical positions rarely need to have such connectivity.
• For security reasons, many enterprises use application

layer gateways to connect their internal network to the
Internet. The internal network usually does not have direct access to the Internet, thus only one or more gateways are visible from the Internet. In this case, the internal network can use non-unique IP network numbers.

• Interfaces of routers on an internal network usually do not
  need to be directly accessible from outside the enterprise. Rekhter, et al Best Current Practice [Page 3]

RFC 1918 Address Allocation for Private Internets February 1996

3. Private Address Space

The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private internets: 10.0.0.0 - 10.255.255.255 (10/8 prefix) 172.16.0.0 - 172.31.255.255 (172.16/12 prefix) 192.168.0.0 - 192.168.255.255 (192.168/16 prefix) We will refer to the first block as "24-bit block", the second as "20-bit block", and to the third as "16-bit" block. Note that (in

pre-CIDR notation) the first block is nothing but a single class A
network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 256 contiguous class C network numbers.

An enterprise that decides to use IP addresses out of the address

space defined in this document can do so without any coordination
with IANA or an Internet registry. The address space can thus be used by many enterprises. Addresses within this private address space will only be unique within the enterprise, or the set of enterprises which

choose to cooperate over this space so they may communicate with each
other in their own private internet. As before, any enterprise that needs globally unique address space is required to obtain such addresses from an Internet registry. An

enterprise that requests IP addresses for its external connectivity
will never be assigned addresses from the blocks defined above. In order to use private address space, an enterprise needs to

determine which hosts do not need to have network layer connectivity

outside the enterprise in the foreseeable future and thus could be
classified as private. Such hosts will use the private address space defined above. Private hosts can communicate with all other hosts inside the enterprise, both public and private. However, they cannot have IP connectivity to any host outside of the enterprise. While not

having external (outside of the enterprise) IP connectivity private

hosts can still have access to external services via mediating
gateways (e.g., application layer gateways).

All other hosts will be public and will use globally unique address
space assigned by an Internet Registry. Public hosts can communicate

with other hosts inside the enterprise both public and private and
can have IP connectivity to public hosts outside the enterprise.

Public hosts do not have connectivity to private hosts of other
enterprises. Rekhter, et al Best Current Practice [Page 4]

RFC 1918 Address Allocation for Private Internets February 1996

Moving a host from private to public or vice versa involves a change
of IP address, changes to the appropriate DNS entries, and changes to

configuration files on other hosts that reference the host by IP
address. Because private addresses have no global meaning, routing information

about private networks shall not be propagated on inter-enterprise
links, and packets with private source or destination addresses should not be forwarded across such links. Routers in networks not using private address space, especially those of Internet service providers, are expected to be configured to reject (filter out) routing information about private networks. If such a router receives

such information the rejection shall not be treated as a routing
protocol error.

Indirect references to such addresses should be contained within the
enterprise. Prominent examples of such references are DNS Resource

Records and other information referring to internal private
addresses. In particular, Internet service providers should take measures to prevent such leakage.

4. Advantages and Disadvantages of Using Private Address Space

The obvious advantage of using private address space for the Internet

at large is to conserve the globally unique address space by not
using it where global uniqueness is not required.

Enterprises themselves also enjoy a number of benefits from their
usage of private address space: They gain a lot of flexibility in

network design by having more address space at their disposal than
they could obtain from the globally unique pool. This enables

operationally and administratively convenient addressing schemes as
well as easier growth paths.

For a variety of reasons the Internet has already encountered

situations where an enterprise that has not been connected to the

Internet had used IP address space for its hosts without getting this
space assigned from the IANA. In some cases this address space had been already assigned to other enterprises. If such an enterprise would later connects to the Internet, this could potentially create very serious problems, as IP routing cannot provide correct operations in presence of ambiguous addressing. Although in principle

Internet Service Providers should guard against such mistakes through
the use of route filters, this does not always happen in practice.

Using private address space provides a safe choice for such
enterprises, avoiding clashes once outside connectivity is needed. Rekhter, et al Best Current Practice [Page 5]

RFC 1918 Address Allocation for Private Internets February 1996

A major drawback to the use of private address space is that it may
actually reduce an enterprise’s flexibility to access the Internet. Once one commits to using a private address, one is committing to renumber part or all of an enterprise, should one decide to provide

IP connectivity between that part (or all of the enterprise) and the
Internet. Usually the cost of renumbering can be measured by

counting the number of hosts that have to transition from private to
public. As was discussed earlier, however, even if a network uses globally unique addresses, it may still have to renumber in order to acquire Internet-wide IP connectivity.

Another drawback to the use of private address space is that it may

require renumbering when merging several private internets into a
single private internet. If we review the examples we list in Section 2, we note that companies tend to merge. If such companies prior to

the merge maintained their uncoordinated internets using private
address space, then if after the merge these private internets would be combined into a single private internet, some addresses within the combined private internet may not be unique. As a result, hosts with these addresses would need to be renumbered.

The cost of renumbering may well be mitigated by development and
deployment of tools that facilitate renumbering (e.g. Dynamic Host Configuration Protocol (DHCP)). When deciding whether to use private addresses, we recommend to inquire computer and software vendors about availability of such tools. A separate IETF effort (PIER

Working Group) is pursuing full documentation of the requirements and
procedures for renumbering.

5. Operational Considerations

One possible strategy is to design the private part of the network
first and use private address space for all internal links. Then plan

public subnets at the locations needed and design the external
connectivity. This design does not need to be fixed permanently. If a group of one

or more hosts requires to change their status (from private to public
or vice versa) later, this can be accomplished by renumbering only the hosts involved, and changing physical connectivity, if needed. In locations where such changes can be foreseen (machine rooms, etc.),

it is advisable to configure separate physical media for public and
private subnets to facilitate such changes. In order to avoid major network disruptions, it is advisable to group hosts with similar connectivity needs on their own subnets. Rekhter, et al Best Current Practice [Page 6]

RFC 1918 Address Allocation for Private Internets February 1996

If a suitable subnetting scheme can be designed and is supported by
the equipment concerned, it is advisable to use the 24-bit block (class A network) of private address space and make an addressing plan with a good growth path. If subnetting is a problem, the 16-bit block (class C networks), or the 20-bit block (class B networks) of private address space can be used.

One might be tempted to have both public and private addresses on the
same physical medium. While this is possible, there are pitfalls to

such a design (note that the pitfalls have nothing to do with the use
of private addresses, but are due to the presence of multiple IP subnets on a common Data Link subnetwork). We advise caution when proceeding in this area.

It is strongly recommended that routers which connect enterprises to

external networks are set up with appropriate packet and routing

filters at both ends of the link in order to prevent packet and
routing information leakage. An enterprise should also filter any

private networks from inbound routing information in order to protect

itself from ambiguous routing situations which can occur if routes to
the private address space point outside the enterprise. It is possible for two sites, who both coordinate their private address space, to communicate with each other over a public network.

To do so they must use some method of encapsulation at their borders
to a public network, thus keeping their private addresses private.

If two (or more) organizations follow the address allocation

specified in this document and then later wish to establish IP
connectivity with each other, then there is a risk that address uniqueness would be violated. To minimize the risk it is strongly

recommended that an organization using private IP addresses choose
randomly from the reserved pool of private addresses, when allocating sub-blocks for its internal allocation. If an enterprise uses the private address space, or a mix of private and public address spaces, then DNS clients outside of the enterprise

should not see addresses in the private address space used by the
enterprise, since these addresses would be ambiguous. One way to

ensure this is to run two authority servers for each DNS zone
containing both publically and privately addressed hosts. One server

would be visible from the public address space and would contain only
the subset of the enterprise’s addresses which were reachable using public addresses. The other server would be reachable only from the private network and would contain the full set of data, including the

private addresses and whatever public addresses are reachable the
private network. In order to ensure consistency, both servers should

be configured from the same data of which the publically visible zone
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RFC 1918 Address Allocation for Private Internets February 1996
only contains a filtered version. There is certain degree of additional complexity associated with providing these capabilities.

6. Security Considerations
   Security issues are not addressed in this memo.

7. Conclusion

With the described scheme many large enterprises will need only a

relatively small block of addresses from the globally unique IP
address space. The Internet at large benefits through conservation of

globally unique address space which will effectively lengthen the
lifetime of the IP address space. The enterprises benefit from the

increased flexibility provided by a relatively large private address
space. However, use of private addressing requires that an organization renumber part or all of its enterprise network, as its connectivity requirements change over time.

8. Acknowledgments
   We would like to thank Tony Bates (MCI), Jordan Becker (ANS), Hans Werner Braun (SDSC), Ross Callon (BayNetworks), John Curran (BBN Planet), Vince Fuller (BBN Planet), Tony Li (cisco Systems), Anne Lord (RIPE NCC), Milo Medin (NSI), Marten Terpstra (BayNetworks), Geza Turchanyi (RIPE NCC), Christophe Wolfhugel (Pasteur Institute), Andy Linton (connect.com.au), Brian Carpenter (CERN), Randy Bush (PSG), Erik Fair (Apple Computer), Dave Crocker (Brandenburg Consulting), Tom Kessler (SGI), Dave Piscitello (Core Competence), Matt Crawford (FNAL), Michael Patton (BBN), and Paul Vixie (Internet Software Consortium) for their review and constructive comments.

9. References
   [RFC1466] Gerich, E., "Guidelines for Management of IP Address Space", RFC 1466, Merit Network, Inc., May 1993. [RFC1518] Rekhter, Y., and T. Li, "An Architecture for IP Address Allocation with CIDR", RFC 1518, September 1993. [RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC 1519, September 1993. Rekhter, et al Best Current Practice [Page 8]

RFC 1918 Address Allocation for Private Internets February 1996

10. Authors’ Addresses

Yakov Rekhter

Cisco systems
170 West Tasman Drive San Jose, CA, USA Phone: +1 914 528 0090 Fax: +1 408 526-4952 EMail: yakov@cisco.com

Robert G Moskowitz

Chrysler Corporation
CIMS: 424-73-00 25999 Lawrence Ave Center Line, MI 48015 Phone: +1 810 758 8212 Fax: +1 810 758 8173 EMail: rgm3@is.chrysler.com

Daniel Karrenberg

RIPE Network Coordination Centre

Kruislaan 409
1098 SJ Amsterdam, the Netherlands Phone: +31 20 592 5065 Fax: +31 20 592 5090 EMail: Daniel.Karrenberg@ripe.net

Geert Jan de Groot

RIPE Network Coordination Centre

Kruislaan 409
1098 SJ Amsterdam, the Netherlands Phone: +31 20 592 5065 Fax: +31 20 592 5090 EMail: GeertJan.deGroot@ripe.net

Eliot Lear

Mail Stop 15-730
Silicon Graphics, Inc. 2011 N. Shoreline Blvd. Mountain View, CA 94043-1389 Phone: +1 415 960 1980 Fax: +1 415 961 9584 EMail: lear@sgi.com Rekhter, et al Best Current Practice [Page 9]