As per Relevance of the word addresses, we have this rfc below:
Network Working Group Y.
Request for Comments: 1918 Cisco
Obsoletes: 1627, 1597 B.
BCP: 5 Chrysler Corp
Category: Best Current Practice D.
RIPE
G. J. de
RIPE
E.
Silicon Graphics, Inc
February 1996
Address Allocation for Private
Status of this
This document specifies an Internet Best Current Practices for
Internet Community, and requests discussion and suggestions
improvements. Distribution of this memo is unlimited
1.
For the purposes of this document, an enterprise is an
autonomously operating a network using TCP/IP and in
determining the addressing plan and address assignments within
network
This document describes address allocation for private internets.
allocation permits full network layer connectivity among all
inside an enterprise as well as among all public hosts of
enterprises. The cost of using private internet address space is
potentially costly effort to renumber hosts and networks
public and private
2.
With the proliferation of TCP/IP technology worldwide,
outside the Internet itself, an increasing number of non-
enterprises use this technology and its addressing capabilities
sole intra-enterprise communications, without any intention to
directly connect to other enterprises or the Internet itself
The Internet has grown beyond anyone's expectations.
exponential growth continues to introduce new challenges.
challenge is a concern within the community that globally
address space will be exhausted. A separate and far more
concern is that the amount of routing overhead will grow beyond
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
within the community to find long term solutions to both of
problems. Meanwhile it is necessary to revisit address
procedures, and their impact on the Internet routing system
To contain growth of routing overhead, an Internet Provider obtains
block of address space from an address registry, and then assigns
its customers addresses from within that block based on each
requirement. The result of this process is that routes to
customers will be aggregated together, and will appear to
providers as a single route [RFC1518], [RFC1519]. In order for
aggregation to be effective, Internet providers encourage
joining their network to use the provider's block, and thus
their computers. Such encouragement may become a requirement in
future
With the current size of the Internet and its growth rate it is
longer realistic to assume that by virtue of acquiring
unique IP addresses out of an Internet registry an organization
acquires such addresses would have Internet-wide IP connectivity
the organization gets connected to the Internet. To the contrary,
is quite likely that when the organization would connect to
Internet to achieve Internet-wide IP connectivity the
would need to change IP addresses (renumber) all of its public
(hosts that require Internet-wide IP connectivity), regardless
whether the addresses used by the organization initially
globally unique or not
It has been typical to assign globally unique addresses to all
that use TCP/IP. In order to extend the life of the IPv4
space, address registries are requiring more justification than
before, making it harder for organizations to acquire
address space [RFC1466].
Hosts within enterprises that use IP can be partitioned into
categories
Category 1: hosts that do not require access to hosts in
enterprises or the Internet at large; hosts
this category may use IP addresses that
unambiguous within an enterprise, but may
ambiguous between enterprises
Category 2: hosts that need access to a limited set of
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
category an unrestricted external access (
Rekhter, et al Best Current Practice [Page 2]
RFC 1918 Address Allocation for Private Internets February 1996
via IP connectivity) may be unnecessary and
undesirable for privacy/security reasons. Just
hosts within the first category, such hosts may
IP addresses that are unambiguous within
enterprise, but may be ambiguous
enterprises
Category 3: hosts that need network layer access outside
enterprise (provided via IP connectivity); hosts
the last category require IP addresses that
globally unambiguous
We will refer to the hosts in the first and second categories
"private". We will refer to the hosts in the third category
"public".
Many applications require connectivity only within one enterprise
do not need external (outside the enterprise) connectivity for
majority of internal hosts. In larger enterprises it is often easy
identify a substantial number of hosts using TCP/IP that do not
network layer connectivity outside the enterprise
Some examples, where external connectivity might not be required
are
- A large airport which has its arrival/departure
individually addressable via TCP/IP. It is very
that these displays need to be directly accessible
other networks
- Large organizations like banks and retail chains
switching to TCP/IP for their internal communication.
numbers of local workstations like cash registers,
machines, and equipment at clerical positions rarely
to have such connectivity
- For security reasons, many enterprises use
layer gateways to connect their internal network to
Internet. The internal network usually does not
direct access to the Internet, thus only one or
gateways are visible from the Internet. In this case,
internal network can use non-unique IP network numbers
- Interfaces of routers on an internal network usually do
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
The Internet Assigned Numbers Authority (IANA) has reserved
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
"20-bit block", and to the third as "16-bit" block. Note that (
pre-CIDR notation) the first block is nothing but a single class
network number, while the second block is a set of 16
class B network numbers, and third block is a set of 256
class C network numbers
An enterprise that decides to use IP addresses out of the
space defined in this document can do so without any
with IANA or an Internet registry. The address space can thus be
by many enterprises. Addresses within this private address space
only be unique within the enterprise, or the set of enterprises
choose to cooperate over this space so they may communicate with
other in their own private internet
As before, any enterprise that needs globally unique address space
required to obtain such addresses from an Internet registry.
enterprise that requests IP addresses for its external
will never be assigned addresses from the blocks defined above
In order to use private address space, an enterprise needs
determine which hosts do not need to have network layer
outside the enterprise in the foreseeable future and thus could
classified as private. Such hosts will use the private address
defined above. Private hosts can communicate with all other
inside the enterprise, both public and private. However, they
have IP connectivity to any host outside of the enterprise. While
having external (outside of the enterprise) IP connectivity
hosts can still have access to external services via
gateways (e.g., application layer gateways).
All other hosts will be public and will use globally unique
space assigned by an Internet Registry. Public hosts can
with other hosts inside the enterprise both public and private
can have IP connectivity to public hosts outside the enterprise
Public hosts do not have connectivity to private hosts of
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
of IP address, changes to the appropriate DNS entries, and changes
configuration files on other hosts that reference the host by
address
Because private addresses have no global meaning, routing
about private networks shall not be propagated on inter-
links, and packets with private source or destination
should not be forwarded across such links. Routers in networks
using private address space, especially those of Internet
providers, are expected to be configured to reject (filter out
routing information about private networks. If such a router
such information the rejection shall not be treated as a
protocol error
Indirect references to such addresses should be contained within
enterprise. Prominent examples of such references are DNS
Records and other information referring to internal
addresses. In particular, Internet service providers should
measures to prevent such leakage
4. Advantages and Disadvantages of Using Private Address
The obvious advantage of using private address space for the
at large is to conserve the globally unique address space by
using it where global uniqueness is not required
Enterprises themselves also enjoy a number of benefits from
usage of private address space: They gain a lot of flexibility
network design by having more address space at their disposal
they could obtain from the globally unique pool. This
operationally and administratively convenient addressing schemes
well as easier growth paths
For a variety of reasons the Internet has already
situations where an enterprise that has not been connected to
Internet had used IP address space for its hosts without getting
space assigned from the IANA. In some cases this address space
been already assigned to other enterprises. If such an
would later connects to the Internet, this could potentially
very serious problems, as IP routing cannot provide
operations in presence of ambiguous addressing. Although in
Internet Service Providers should guard against such mistakes
the use of route filters, this does not always happen in practice
Using private address space provides a safe choice for
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
actually reduce an enterprise's flexibility to access the Internet
Once one commits to using a private address, one is committing
renumber part or all of an enterprise, should one decide to
IP connectivity between that part (or all of the enterprise) and
Internet. Usually the cost of renumbering can be measured
counting the number of hosts that have to transition from private
public. As was discussed earlier, however, even if a network
globally unique addresses, it may still have to renumber in order
acquire Internet-wide IP connectivity
Another drawback to the use of private address space is that it
require renumbering when merging several private internets into
single private internet. If we review the examples we list in
2, we note that companies tend to merge. If such companies prior
the merge maintained their uncoordinated internets using
address space, then if after the merge these private internets
be combined into a single private internet, some addresses within
combined private internet may not be unique. As a result, hosts
these addresses would need to be renumbered
The cost of renumbering may well be mitigated by development
deployment of tools that facilitate renumbering (e.g. Dynamic
Configuration Protocol (DHCP)). When deciding whether to use
addresses, we recommend to inquire computer and software
about availability of such tools. A separate IETF effort (
Working Group) is pursuing full documentation of the requirements
procedures for renumbering
5. Operational
One possible strategy is to design the private part of the
first and use private address space for all internal links. Then
public subnets at the locations needed and design the
connectivity
This design does not need to be fixed permanently. If a group of
or more hosts requires to change their status (from private to
or vice versa) later, this can be accomplished by renumbering
the hosts involved, and changing physical connectivity, if needed.
locations where such changes can be foreseen (machine rooms, etc.),
it is advisable to configure separate physical media for public
private subnets to facilitate such changes. In order to avoid
network disruptions, it is advisable to group hosts with
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
the equipment concerned, it is advisable to use the 24-bit
(class A network) of private address space and make an
plan with a good growth path. If subnetting is a problem, the 16-
block (class C networks), or the 20-bit block (class B networks)
private address space can be used
One might be tempted to have both public and private addresses on
same physical medium. While this is possible, there are pitfalls
such a design (note that the pitfalls have nothing to do with the
of private addresses, but are due to the presence of multiple
subnets on a common Data Link subnetwork). We advise caution
proceeding in this area
It is strongly recommended that routers which connect enterprises
external networks are set up with appropriate packet and
filters at both ends of the link in order to prevent packet
routing information leakage. An enterprise should also filter
private networks from inbound routing information in order to
itself from ambiguous routing situations which can occur if routes
the private address space point outside the enterprise
It is possible for two sites, who both coordinate their
address space, to communicate with each other over a public network
To do so they must use some method of encapsulation at their
to a public network, thus keeping their private addresses private
If two (or more) organizations follow the address
specified in this document and then later wish to establish
connectivity with each other, then there is a risk that
uniqueness would be violated. To minimize the risk it is
recommended that an organization using private IP addresses
randomly from the reserved pool of private addresses, when
sub-blocks for its internal allocation
If an enterprise uses the private address space, or a mix of
and public address spaces, then DNS clients outside of the
should not see addresses in the private address space used by
enterprise, since these addresses would be ambiguous. One way
ensure this is to run two authority servers for each DNS
containing both publically and privately addressed hosts. One
would be visible from the public address space and would contain
the subset of the enterprise's addresses which were reachable
public addresses. The other server would be reachable only from
private network and would contain the full set of data, including
private addresses and whatever public addresses are reachable
private network. In order to ensure consistency, both servers
be configured from the same data of which the publically visible
Rekhter, et al Best Current Practice [Page 7]
RFC 1918 Address Allocation for Private Internets February 1996
only contains a filtered version. There is certain degree
additional complexity associated with providing these capabilities
6. Security
Security issues are not addressed in this memo
7.
With the described scheme many large enterprises will need only
relatively small block of addresses from the globally unique
address space. The Internet at large benefits through conservation
globally unique address space which will effectively lengthen
lifetime of the IP address space. The enterprises benefit from
increased flexibility provided by a relatively large private
space. However, use of private addressing requires that
organization renumber part or all of its enterprise network, as
connectivity requirements change over time
8.
We would like to thank Tony Bates (MCI), Jordan Becker (ANS), Hans
Werner Braun (SDSC), Ross Callon (BayNetworks), John Curran (
Planet), Vince Fuller (BBN Planet), Tony Li (cisco Systems),
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
(PSG), Erik Fair (Apple Computer), Dave Crocker (
Consulting), Tom Kessler (SGI), Dave Piscitello (Core Competence),
Matt Crawford (FNAL), Michael Patton (BBN), and Paul Vixie (
Software Consortium) for their review and constructive comments
9.
[RFC1466] Gerich, E., "Guidelines for Management of IP
Space", RFC 1466, Merit Network, Inc., May 1993.
[RFC1518] Rekhter, Y., and T. Li, "An Architecture for IP
Allocation with CIDR", RFC 1518, September 1993.
[RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "
Inter-Domain Routing (CIDR): an Address Assignment
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'
Yakov
Cisco
170 West Tasman
San Jose, CA,
Phone: +1 914 528 0090
Fax: +1 408 526-4952
EMail: yakov@cisco.
Robert G
Chrysler
CIMS: 424-73-00
25999 Lawrence
Center Line, MI 48015
Phone: +1 810 758 8212
Fax: +1 810 758 8173
EMail: rgm3@is.chrysler.
Daniel
RIPE Network Coordination
Kruislaan 409
1098 SJ Amsterdam, the
Phone: +31 20 592 5065
Fax: +31 20 592 5090
EMail: Daniel.Karrenberg@ripe.
Geert Jan de
RIPE Network Coordination
Kruislaan 409
1098 SJ Amsterdam, the
Phone: +31 20 592 5065
Fax: +31 20 592 5090
EMail: GeertJan.deGroot@ripe.
Eliot
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.
Rekhter, et al Best Current Practice [Page 9]
if you see any problems within the linking, don't worry be happy,
this is version 0.1 of the Relevance System and you gotta expect some crappy subroutines sometimes,
just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
RFC documents can be found at I.E.T.F.
Relevance System Copyright © 2002 Spectrum WorldResearch
other technical nosh by ServerMasters Corporation
collaboration of BobX
