Network Working Group
Request for Comments: 5494
Updates: 826, 951, 1044, 1329, 2131
Category: Standards Track
J. Arkko
C. Pignataro
2132, 2176, 2225, 2834, 2835, Cisco Systems
3315, 4338, 4361, 4701 April 2009

IANA Allocation Guidelines for the Address Resolution Protocol (ARP)

Status of This Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright © 2009 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document ( Please review these documents carefully, as they describe your rights and restrictions with respect to this document.


This document specifies the IANA guidelines for allocating new values in the Address Resolution Protocol (ARP). This document also reserves some numbers for experimentation purposes. The changes also affect other protocols that employ values from the ARP name spaces.

1. Introduction

This document specifies the IANA guidelines [RFC5226] for allocating new values for various fields in the Address Resolution Protocol (ARP) [RFC0826]. The change is also applicable to extensions of ARP that use the same message format, such as [RFC0903], [RFC1931], and [RFC2390].

The change also affects other protocols that employ values from the ARP name spaces. For instance, the ARP hardware address type (ar$hrd) number space is also used in the "htype" (hardware address type) fields in the Bootstrap Protocol (BOOTP) [RFC0951] and Dynamic Host Configuration Protocol (DHCP) [RFC2131], as well as in the "hardware type" field in the DHCP Unique Identifiers in DHCPv6 [RFC3315]. These protocols are therefore affected by the update in the IANA rules. Other affected specifications include the specialized address resolution mechanisms in:

   o  DHCP options [RFC2132] [RFC4361]
   o  ATM (Asynchronous Transfer Mode) ARP [RFC2225]
   o  HARP (High-Performance Parallel Interface ARP) [RFC2834] [RFC2835]
  • Dual MAC (Media Access Control) FDDI (Fiber Distributed Data Interface) ARP [RFC1329]
  • MAPOS (Multiple Access Protocol over Synchronous Optical Network/ Synchronous Digital Hierarchy) ARP [RFC2176]
   o  FC (Fibre Channel) ARP [RFC4338]
  • DNS DHCID Resource Record [RFC4701]

The IANA guidelines are given in Section 2. Previously, no IANA guidance existed for such allocations. The purpose of this document is to allow IANA to manage number assignments based on these guidelines in a consistent manner.

This document also reserves some numbers for experimentation purposes. These numbers are given in Section 3.

2. IANA Considerations

The following rules apply to the fields of ARP:

ar$hrd (16 bits) Hardware address space

Requests for ar$hrd values below 256 or for a batch of more than one new value are made through Expert Review [RFC5226].

Note that certain protocols, such as BOOTP and DHCPv4, employ these values within an 8-bit field. The expert should determine that a need to allocate the new values exists and that the existing values are insufficient to represent the new hardware address types. The expert should also determine the applicability of the request and assign values higher than 255 for requests that do not apply to BOOTP/DHCPv4. Similarly, the expert should assign 1-octet values for requests that apply to BOOTP/DHCPv4, as for example the "IPsec tunnel" with value 31 [RFC3456]. Conversely, ARP-only uses, without a foreseeable reason to use the same value in BOOTP/DHCPv4, should favor 2-octet values.

Requests for individual new ar$hrd values that do not specify a value, or where the requested value is greater than 255, are made through First Come First Served [RFC5226]. The assignment will always result in a 2-octet value.

ar$pro (16 bits) Protocol address space

These numbers share the Ethertype space. The Ethertype space is administered as described in [RFC5342].

ar$op (16 bits) Opcode

Requests for new ar$op values are made through IETF Review or IESG Approval [RFC5226].

3. Allocations Defined in This Document

When testing new protocol extension ideas, it is often necessary to use an actual constant in order to use the new function, even when testing in a closed environment. This document reserves the following numbers for experimentation purposes in ARP:

  • Two new ar$hrd values are allocated for experimental purposes: HW_EXP1 (36) and HW_EXP2 (256). Note that these two new values were purposely chosen so that one would be below 256 and the other would be above 255, and so that there would be different values in the least and most significant octets.
  • Two new values for the ar$op are allocated for experimental purposes: OP_EXP1 (24) and OP_EXP2 (25).

Note that Appendix B.2 of [RFC5342] lists two Ethertypes that can be used for experimental purposes.

In addition, for both ar$hrd and ar$op, the values 0 and 65535 are marked as reserved. This means that they are not available for allocation.

4. Security Considerations

This specification does not change the security properties of the affected protocols.

However, a few words are necessary about the use of the experimental code points defined in Section 3. Potentially harmful side effects from the use of the experimental values need to be carefully evaluated before deploying any experiment across networks that the owner of the experiment does not entirely control. Guidance given in [RFC3692] about the use of experimental values needs to be followed.

5. Acknowledgments

The lack of any current rules has come up as new values were requested from IANA, who contacted the IESG for advice. The author would like to thank Michelle Cotton in particular for bringing up this issue. The author would also like to thank Brian Carpenter, Thomas Narten, Scott Bradner, Donald Eastlake, Andrew G. Malis, Brian Haberman, Robert Sparks, Larry Zhu, and Dave Thaler for feedback.

6. References

6.1. Normative References

   [RFC0826]  Plummer, D., "Ethernet Address Resolution Protocol: Or
              converting network protocol addresses to 48.bit Ethernet
              address for transmission on Ethernet hardware", STD 37,
              RFC 826, November 1982.
   [RFC0951]  Croft, B. and J. Gilmore, "Bootstrap Protocol", RFC 951,
              September 1985.
   [RFC1044]  Hardwick, K. and J. Lekashman, "Internet Protocol on
              Network System's HYPERchannel: Protocol specification",
              STD 45, RFC 1044, February 1988.
   [RFC1329]  Kuehn, P., "Thoughts on Address Resolution for Dual MAC
              FDDI Networks", RFC 1329, May 1992.
   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.
   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, March 1997.
   [RFC2176]  Murakami, K. and M. Maruyama, "IPv4 over MAPOS Version 1",
              RFC 2176, June 1997.
   [RFC2225]  Laubach, M. and J. Halpern, "Classical IP and ARP over
              ATM", RFC 2225, April 1998.
   [RFC2834]  Pittet, J., "ARP and IP Broadcast over HIPPI-800",
              RFC 2834, May 2000.
   [RFC2835]  Pittet, J., "IP and ARP over HIPPI-6400 (GSN)", RFC 2835,
              May 2000.
   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.
   [RFC3692]  Narten, T., "Assigning Experimental and Testing Numbers
              Considered Useful", BCP 82, RFC 3692, January 2004.
   [RFC4338]  DeSanti, C., Carlson, C., and R. Nixon, "Transmission of
              IPv6, IPv4, and Address Resolution Protocol (ARP) Packets
              over Fibre Channel", RFC 4338, January 2006.
   [RFC4361]  Lemon, T. and B. Sommerfeld, "Node-specific Client
              Identifiers for Dynamic Host Configuration Protocol
              Version Four (DHCPv4)", RFC 4361, February 2006.
   [RFC4701]  Stapp, M., Lemon, T., and A. Gustafsson, "A DNS Resource
              Record (RR) for Encoding Dynamic Host Configuration
              Protocol (DHCP) Information (DHCID RR)", RFC 4701,
              October 2006.
   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.
   [RFC5342]  Eastlake. , D., "IANA Considerations and IETF Protocol
              Usage for IEEE 802 Parameters", BCP 141, RFC 5342,
              September 2008.

6.2. Informative References

   [RFC0903]  Finlayson, R., Mann, T., Mogul, J., and M. Theimer,
              "Reverse Address Resolution Protocol", STD 38, RFC 903,
              June 1984.
   [RFC1931]  Brownell, D., "Dynamic RARP Extensions for Automatic
              Network Address Acquisition", RFC 1931, April 1996.
   [RFC2390]  Bradley, T., Brown, C., and A. Malis, "Inverse Address
              Resolution Protocol", RFC 2390, September 1998.
   [RFC3456]  Patel, B., Aboba, B., Kelly, S., and V. Gupta, "Dynamic
              Host Configuration Protocol (DHCPv4) Configuration of
              IPsec Tunnel Mode", RFC 3456, January 2003.

Appendix A. Changes from the Original RFCs

This document specifies only the IANA rules associated with various fields in ARP. The specification of these rules also affects the allocation of corresponding fields in protocols listed in Section 1 that share the registry. This document does not make any changes in the operation of these protocols themselves.

Authors' Addresses

   Jari Arkko
   Jorvas  02420
   Carlos Pignataro
   Cisco Systems
   7200-12 Kit Creek Road
   Research Triangle Park, NC  27709