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  • From: Bill Manning
  • Date: Mon Apr 17 11:30:15 2000

a bit late but here is #2
                                                         Bill Manning
                                                        17 April 2000 

            Documenting Special Use IPv4 Address Blocks
		that have been registered with IANA

1. Status of this Memo

This draft, file name draft-manning-dsua-02.txt, is intended to become
something that might be of use to those who are interested in the
operational requirements of an IPv4 based network.  Distribution of 
this document is unlimited. Comments should be sent to the author.

This document is an Internet-Draft and is NOT offered in accordance with 
Section 10 of RFC2026, and the author does not provide the IETF with any 
rights other than to publish as an Internet-Draft.

Internet-Drafts are working documents of the Internet Engineering Task 
Force (IETF), its areas, and its working groups.  Note that other groups 
may also distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months and 
may be updated, replaced, or obsoleted by other documents at any time.  It 
is inappropriate to use Internet-Drafts as reference material or to cite 
them other than as "work in progress."

The list of current Internet-Drafts can be accessed at

The list of Internet-Draft Shadow Directories can be accessed at


This document lists the existent special use prefixes from the IPv4 space 
that have been registered with the IANA and provides some suggestions for 
operational procedures when these prefixes are encountered.  This document 
does not address IPv4 space that is reserved for future delegation in the 
operational Internet.

The current list of special use prefixes:
	all D/E space

2.1 Prefix Discussion: has a number of unique properties, many of which were built into
the protocol stacks used throughout the Internet. or the all-zeros
address has been used and is still recognized as the historical broadcast
address. This use or restriction is deprecated and modern code will treat
broadcast correctly as an all-ones value within the subnet. It is fairly 
common practice to use to encode the idea of "default".

Also, many stacks will allow the system administrator to encode IP addresses
of the form, with the presumption that historical, "natural" masks
apply and so this would represent a host that carries the local value of
x.x.160.57 within the /16 net-block that is in use on that media. These 
properties suggest that a prudent network manager & system admin will treat as a special use net-block. Router and Host requirements documents 
and implementations treat this range with special use constraints. is earmarked for what is called "loop-back". This construct is 
to allow a node to test/validate its IP stack.  Most software only uses
a single value from this range, for loop-back purposes.  It
is treated with the same levels of restriction by router and host requirements
and implementations so it is difficult to use any other addresses within 
this block for anything other than node specific applications, generally 
bootstrapping.  All in all a tremendous waste of IP space. Good thing we'll 
not likely need it. is listed as the TEST-NET. This prefix is earmarked for use in 
documentation and example code. Network operations and End System 
administrators should ensure that this prefix is not coded into systems
or routed through any infrastructure.  Since it has the appearance of a
"normal" prefix, special precautions should be taken to ensure that this
prefix is not propagated in either the Internet or any private networks
that use the IP protocols.  Often used in conjunction with
or in vendor and protocol documentation.,, are the prefixes called out
in RFC 1918. They are only for use in private networks that wish to use
the IP protocols. Network operations and End System administrators should 
ensure that applications do not use these ranges as source or destination
addresses for any packets that traverse the Internet infrastructure.  Since 
they have the appearance of "normal" prefixes, special precautions should be 
taken to ensure that they are not propagated in the Internet. has been ear-marked as the IP range to use for end node 
auto-configuration when a DHCP server may not be found. As such, network
operations and administrators should be VERY aggressive in ensuring that
neither route advertisements nor packet forwarding should occur across
any media boundaries. This is true for the Internet as well as any
private networks that use the IP protocols. End node administrators
should be aware that some vendors will auto-configure and add this
prefix to the nodes forwarding table. This will cause problems with
sites that run router discovery or deprecated routing protocols such as

Class D & E space. These are parts of the IPv4 space that retain some context
of class-fullness. They are used for identification of multicast and a range
left unspecified.  This extract from RFC 1166 covers these ranges.

      The fourth type of address, class D, is used as a multicast
      address [13].  The four highest-order bits are set to 1-1-1-0.

                            1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       |1 1 1 0|                  multicast address                    |

                                 Class D Address

      Note:  No addresses are allowed with the four highest-order bits
      set to 1-1-1-1.  These addresses, called "class E", are reserved.

As a side note, at least one vendor has hijacked an address range for
use by its printservers. That range is and the specific
address that they use is  This is not a valid delegation
to this vendor and its use argues for re-constitution of this service
into the link-local range or configurable with site delegated space.

3. DNS considerations:

None of these address prefixes is to be used or visible on the public Internet.
In fact, some of these prefixes must not appear outside the machine. To 
encourage honesty, most of these prefixes have been mapped to authoritative
servers in the DNS. This encourages people to ensure that when used, these 
prefixes are coded with local-scope DNS and there will be no "leakage" to the 
global Internet.

4. Access Control suggestions:

In todays network, it is prudent to control access. In the case of these
special use prefixes, it is generally a good idea to filter them so they
do not propagate. After all, you don't want someone else's use of these 
prefixes to taint your environment. All of these address classes should be 
invalid as source addresses (except where negotiated in advance), and very 
few should be permitted as destination addresses (Multicast for example, 
should be permitted as a designation, just not as a source).  An example of 
one form of access control is listed below:

access-list 100 deny   ip host any
access-list 100 deny   ip
access-list 100 deny   ip
access-list 100 deny   ip
access-list 100 deny   ip
access-list 100 deny   ip
access-list 100 deny   ip
access-list 100 deny   ip any
access-list 100 permit ip any any

5. Security Considerations:

Use of most of these special use prefixes open up significant opportunities 
for anonymity and ambiguity. People, being what they are, will hide behind 
ambiguous or nebulous identities to do things that are antisocial and 
downright hostile. It would be nice to have better authentication methods
in play than an IP address which has lost its global uniqueness.

6. References:

[DHC-IPV4-AUTOCONFIG] - R. Troll, Automatically Choosing an IP Address 
in an Ad-Hoc IPv4 Network, Internet draft, 
draft-ietf-dhc-ipv4-autoconfig-04.txt, October 1998

[RFC1918]  Y. Rekhter, Address Allocation for Private Internets,
February 1996, RFC 1918

[RFC1122] R. Braden,  Requirements for Internet Hosts -- Communication Layers,
October 1989, RFC 1122

[RFC1166] S.Kirkpatrick, INTERNET NUMBERS, July 1990, RFC 1166

[RFC1812] F. Baker, Requirements for IP Version 4 Routers,
June 1995, RFC 1812

[RFC2267] P. Ferguson, D. Senie, Network Ingress Filtering: 
Defeating Denial of Service Attacks which employ IP Source Address Spoofing,
January 1998, RFC 2267

[NET-TEST] Netname: IANA, Netnumber:, Coordinator: 
Internet Assigned Numbers Authority, 1993

[LOOPBACK] Netname: LOOPBACK, Netnumber:, Coordinator:
Internet Assigned Numbers Authority, 1972

[RESERVED-1] Netname RESERVED-1, Netblock: -,
Coordinator: Internet Assigned Numbers Authority, 1972

8. Author's Address

	Bill Manning
	4676 Admiralty Way, #1001
	Marina del Rey, CA. 90292
	[email protected]
"When in doubt, Twirl..."  -anon