Observations on the use of Components of the Class A Address Space within the Internet
RFC 2036

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Network Working Group                                          G. Huston
Request for Comments: 2036                              Telstra Internet
Category: Informational                                     October 1996

          Observations on the use of Components of the Class A
                   Address Space within the Internet

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.

Abstract

   This document is a commentary on the recommendation that IANA
   commence allocation of the presently unallocated components of the
   Class A address space to registries, for deployment within the
   Internet as class-less address blocks.

   The document examines the implications for service providers and end
   clients within this environment. The document notes the major
   conclusion that widespread adoption of class-less routing protocols
   is required, within a relatively rapid timeframe for this
   recommendation to be effective.

Introduction

   The Address Lifetime Expectancy (ALE) Working Group of the IETF has
   recorded the allocation of Internet addresses from the unallocated
   address pool. ALE has noted that the existing practice of drawing
   addresses from the Class C space (192/3 address prefix) will result
   in near to medium term exhaustion of this section of the unallocated
   address pool. The largest remaining pool is in the Class A space,
   where some 25% of Internet addresses (the upper half of the Class A
   space) remain, to date, unallocated.

   This document is a commentary on the potential recommendation that
   the Internet Assigned Numbers Authority (IANA), through delegated
   registries, commence allocation of the presently unallocated
   components of the Class A  address space to registries, for
   deployment within the Internet through the mechanism of allocation of
   class-less address prefixes.

   The deployment of class-less address prefixes from the Class A space
   within the Internet will require some changes to the routing
   structure within Internet component network domains. The motivation

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RFC 2036        Components of the Class A Address Space     October 1996

   for, and nature of, such changes as they effect network domains and
   network service providers are outlined in this document.

Current Practice with Address Allocations

   To date the allocation of class-less network prefixed address blocks
   has followed a conservative practice of using address allocations
   which are compatible superblocks of Class C addresses, while the
   allocation of addresses within the space of Class A and Class B
   networks has continued to be aligned with the class-based prefix
   structure.

   Within this address allocation environment for non-transit network
   domains there is accordingly the option to continue to use address
   deployment strategies which involve fixed subnet address structures
   within contiguous areas, and use Class-full interior routing
   protocols. In the situation where variable length subnet masks or
   disconnected subnets are deployed within the network domain's routing
   structure, interior routing protocols which use subnet-based routing
   of Class-full networks can still be successfully deployed and the end
   network has the option of using an explicit or implicit sink subnet
   default route. Where such non-transit network domains are connected
   to the Internet infrastructure the boundary exchange between the
   non-transit network and the network service provider (this term is
   used as a synonym for a transit network domain, which provides a
   traffic transit service to other non-transit and peer transit network
   domains) is either a class-full advertisement of routes, or an
   aggregated address advertisement where the aggregate is a superblock
   of the deployed component class-full networks. At the boundary points
   of the non-transit network it is a requirement that the non-transit
   network's subnet default route (if used explicitly) not be directed
   to the network service provider's domain, to avoid a routing loop at
   the domain boundary point.

   For network service providers the interior routing protocol can use
   either aggregated routing or explicit class-full routing within this
   environment. At the network service provider's boundary peering
   points the strongly recommended practice is to advertise aggregated
   routes to transit peers, which in turn may be further aggregated
   across the Internet, within the parameters of permissible policies.

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RFC 2036        Components of the Class A Address Space     October 1996

Implications of Address Allocation from the Class A space

Network Service Providers Must Use Class-less Routing