Network Working Group Y. Rekhter
Request for Comments: 1597 T.J. Watson Research Center, IBM Corp.
Category: Informational B. Moskowitz
Chrysler Corp.
D. Karrenberg
RIPE NCC
G. de Groot
RIPE NCC
March 1994

Address Allocation for Private Internets

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.

1. IntrodUCtion

This RFCdescribes methods to preserve IP address space by not
allocating globally unique IP addresses to hosts private to an
enterprise while still permitting full network layer connectivity
between all hosts inside an enterprise as well as between all public
hosts of different enterprises. The authors hope, that using these
methods, significant savings can be made on allocating IP address
space.

For the purposes of this memo, 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.

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 current practice is to assign globally unique addresses to all
hosts that use TCP/IP. There is a growing concern that the finite IP
address space might become exhausted. Therefore, the guidelines for
assigning IP address space have been tightened in recent years [1].
These rules are often more conservative than enterprises would like,
in order to implement and operate their networks.

Hosts within enterprises that use IP can be partitioned into three
categories:

- hosts that do not require Access to hosts in other enterprises
or the Internet at large;

- hosts that need access to a limited set of outside services
(e.g., E-mail, FTP, netnews, remote login) which can be handled
by application layer gateways;

- hosts that need network layer access outside the enterprise
(provided via IP connectivity);

- hosts within the first category may use IP addresses that are
unambiguous within an enterprise, but may be ambiguous between
enterprises.

For many hosts in the second category an unrestricted external access
(provided 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.

Only hosts in the last category require IP addresses that are
globally unambiguous.

Many applications require connectivity only within one enterprise and
do not even need external 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 organisations 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 (e.g., firewalls) 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 firewall hosts are
visible from the Internet. In this case, the internal network
can use non-unique IP numbers.

- If two enterprises communicate over their own private link,
usually only a very limited set of hosts is mutually reachable
from the other enterprise over this link. Only those hosts need
globally unique IP numbers.

- Interfaces of routers on an internal network usually do not
need to be directly accessible from outside the enterprise.

3. Private Address Space

The Internet Assigned Numbers Authority (IANA) has reserved the
following three blocks of the IP address space for private networks:

10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

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 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 255 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.

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. Such hosts will be
called private hosts, and 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 external host. While not having external network

layer connectivity private hosts can still have access to external
services via application layer relays.

All other hosts will be called 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 external public hosts.
Public hosts do not have connectivity to private hosts of other
enterprises.

Moving a host from private to public or vice versa involves a change
of 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 between 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. When such an enterprise
later connects to the Internet, it could potentially create very

serious problems, as IP routing cannot provide correct operations in
presence of ambiguous addressing. Using private address space
provides a safe choice for such enterprises, avoiding clashes once
outside connectivity is needed.

One could argue that the potential need for renumbering represents a
significant drawback of using the addresses out of the block
allocated for private internets. However, we need to observe that
the need is only "potential", since many hosts may never move into
the third category, and an enterprise may never decide to
interconnect (at IP level) with another enterprise.

But even if renumbering has to happen, we have to observe that with
Classless Inter-Domain Routing (CIDR) an enterprise that is connected
to the Internet may be encouraged to renumber its public hosts, as it
changes its Network Service Providers. Thus renumbering is likely to
happen more often in the future, regardless of whether an enterprise
does or does not use the addresses out of the block allocated for
private networks. Tools to facilitate renumbering (e.g., DHCP) would
certainly make it less of a concern.

Also observe that the clear division of public and private hosts and
the resulting need to renumber makes uncontrolled outside
connectivity more difficult, so to some extend the need to renumber
could be viewed as an advantage.

5. Operational Considerations

A recommended 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 is not fixed permanently. If a number of hosts require
to change status later this can be accomplished by renumbering only
the hosts involved and installing another physical subnet if
required.

If a suitable subnetting scheme can be designed and is supported by
the equipment concerned, it is advisable to use the 24-bit block of
private address space and make an addressing plan with a good growth
path. If subnetting is a problem, the 16-bit class C block, which
consists of 255 contiguous class C network numbers, can be used.

Using multiple IP (sub)nets on the same physical medium has many
pitfalls. We recommend to avoid it unless the operational problems
are well understood and it is proven that all equipment supports this
properly.

Moving a single host between private and public status will involve a
change of address and in most cases physical connectivity. In
locations where such changes can be foreseen (machine rooms etc.) it
may be advisable to configure separate physical media for public and
private subnets to facilitate such changes.

Changing the status of all hosts on a whole (sub)network can be done
easily and without disruption for the enterprise network as a whole.
Consequently it is advisable to group hosts whose connectivity needs
might undergo similar changes in the future on their own subnets.

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.

Groups of organisations which foresee a big need for mutual
communication can consider forming an enterprise by designing a
common addressing plan supported by the necessary organisational
arrangements like a registry.

If two sites of the same enterprise need to be connected using an
external service provider, they can consider using an IP tunnel to
prevent packet leaks form the private network.

A possible approach to avoid leaking of DNS RRs is to run two
nameservers, one external server authoritative for all globally
unique IP addresses of the enterprise and one internal nameserver
authoritative for all IP addresses of the enterprise, both public and
private. In order to ensure consistency both these servers should be
configured from the same data of which the external nameserver only
receives a filtered version.

The resolvers on all internal hosts, both public and private, query
only the internal nameserver. The external server resolves queries
from resolvers outside the enterprise and is linked into the global
DNS. The internal server forwards all queries for information
outside the enterprise to the external nameserver, so all internal
hosts can access the global DNS. This ensures that information about
private hosts does not reach resolvers and nameservers outside the
enterprise.

6. References

[1] Gerich, E., "Guidelines for Management of IP Address Space", RFC
1466, Merit Network, Inc., May 1993.

7. Security Considerations

While using private address space can improve security, it is not a
substitute for dedicated security measures.

8. 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.

9. Acknowledgments

We would like to thank Tony Bates (RIPE NCC), Jordan Becker (ANS),
Hans-Werner Braun (SDSC), Ross Callon (Wellfleet), John Curran
(NEARNET), Vince Fuller (Barrnet), Tony Li (cisco Systems), Anne Lord
(RIPE NCC), Milo Medin (NSI), Marten Terpstra (RIPE NCC), and Geza
Turchanyi (RIPE NCC) for their review and constructive comments.

10. Authors' Addresses

Yakov Rekhter
T.J. Watson Research Center, IBM Corp.
P.O. Box 218
Yorktown Heights, NY, 10598

Phone: +1 914 945 3896
Fax: +1 914 945 2141
EMail: yakov@watson.ibm.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: 3858921@mcimail.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