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The telephone does not match the names. I contacted the several of them. Some do not work.




Network Working Group

Request for Comments: 1918

Obsoletes: 1627, 1597

BCP: 5

Category: Best Current Practice


Y. Rekhter

Cisco Systems

B. Moskowitz

Crysler Corp.

D. Karrenberg


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



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



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



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



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



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,



- 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 www.google.com 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: - (10/8 prefix) - (172.16/12 prefix) - (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







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



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



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




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 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

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It appears that there are private networks that have no country disignation. This I did not know.




IPv4 Blocks for Private Internets

The IPv4 blocks - (10/8 prefix) - (172.16/12 prefix) - (192.168/16 prefix)


are reserved as private address space, specified in the RFC 1918. This means everybody can set up an internal network using addresses in these blocks, but packets originating from those addresses should never appear on the internet.


Consequently, the administrators of those internal networks are not obliged to register themselves with a whois database such as the RIPE Database.


Thus, if you have received packets originating from these addresses, there is either a misconfiguration somewhere or the packets that you received contain a spoofed IP address, or they have been sent from within your own network.


RFC 1918 can be found at ftp://ftp.ripe.net/rfc/rfc1918.txt

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Guest Black Sun

In Internet terminology a private network is typically a network that uses private IP address space, following the agreed standard of RFC 1918. Computers may be allocated addresses from this address space when it is necessary for them to communicate with other computing devices on an internal (not public Internet) network.




Wiki also has an external link to:


RFC 1918 – "Address Allocation for Private Internets"




The Internet Engineering Task Force (IETF) has the same copy as you. The IETF is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual. The IETF Mission Statement is documented in RFC 3935.

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I found this.


This document provides the engineering-level definition of “IPv6 Capable” products necessary for interoperable use throughout the US Department of Defense (DoD).


RFC Status: Info – Informational


RFC Editor by searching on the RFC number or keywords.


In addition to new emerging standards, existing standards are frequently updated by RFCs that extend or elaborate the standards, and on occasion standards may be rendered obsolete by revised RFCs.


In IETF practice, once published, an RFC is never updated; the technical material it defines can only be changed by publication of another RFC. The RFC Editor web page tracks all RFCs, and relates them to other RFCs that update or obsolete them.


The obsolescence and replacement of RFCs by new RFCs complicates a simple and clear definition of the mandatory requirements in this Standard Profiles document.


There will be a period of time during which commercially available products may support either or both of the versions of the standard. In some cases the requirement is to support the function, preferably complying with the emerging replacement RFC but at least according to the previously published RFC. In these situations, the old and new standards will be discussed together in this document with exceptions or conditions noted, to provide clear guidance to vendors for implementation and testing.




I want to conact them, but I think that is a private channel.

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Spammer address:


Network IP address lookup:


whois query for


Results returned from whois.arin.net:



OrgName: Internet Assigned Numbers Authority


Address: 4676 Admiralty Way, Suite 330

City: Marina del Rey

StateProv: CA

PostalCode: 90292-6695

Country: US


NetRange: -



NetHandle: NET-192-168-0-0-1

Parent: NET-192-0-0-0-0

NetType: IANA Special Use



Comment: This block is reserved for special purposes.

Comment: Please see RFC 1918 for additional information.

Comment: http://www.arin.net/reference/rfc/rfc1918.txt

RegDate: 1994-03-15

Updated: 2007-11-27


OrgAbuseHandle: IANA-IP-ARIN

OrgAbuseName: Internet Corporation for Assigned Names and Number

OrgAbusePhone: +1-310-301-5820

OrgAbuseEmail: abuse@iana.org


OrgTechHandle: IANA-IP-ARIN

OrgTechName: Internet Corporation for Assigned Names and Number

OrgTechPhone: +1-310-301-5820

OrgTechEmail: abuse@iana.org

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