rfc2036.txt

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   is multiply homed within network service providers.

   There are also additional constraints placed on the non-transit
   network domain where the network has exterior connections to other
   peer networks. Even in the case where the network domain uses a
   class-less interior routing protocol, there is the additional
   consideration that this requirement for use of a class-less routing
   domain is transitive to other connected network domains. An second
   network domain, externally connected to the class-less domain routing
   part of the Class A space, will interpret the boundary reachability
   advertisement as a complete Class A network advertisement, if using
   class-full routing. Even if both network domains are connected to the
   same network provider the provider's default routing  advertisement
   default to the class-full domain will be overridden by the assumed
   class A advertisement through the domain-to-domain connection,
   leading to unintended traffic diversion. The diversion occurs in this
   case as the traffic directed to parts of the Class A network which
   are not deployed within the first domain will transit the first
   domain before entering the network service provider's domain.

   It is also possible to have configurations with unintended routing
   holes. An example of such a configuration is two stub clients of
   different network service providers, both using class-less interior
   routing (X and Y), both directly connected to a third network domain



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RFC 2036        Components of the Class A Address Space     October 1996


   (Z), which uses class-full interior routing, which is configured as a
   transit between X and Y. X's advertisement of a component of a Class
   A to Z will be assumed by Z to be a complete Class A network, and as
   such will be advertised to Y, overriding Y's default route received
   from the network service provider. Y will pass all Class A addressed
   traffic to Z, who will in turn pass it to X. As X is configured as a
   non-transit stub network X must discard all non-locally addressed
   traffic.

   Thus reasonable operational practice would be to ensure that if a
   network domain deploys a component of the Class A address space, the
   network domain is configured to use class-less interior routing
   protocols, and the network has a single exterior connection to a
   class-less network provider domain, with the boundary configured as a
   class-less routing exchange. Multiply homed network domains do infer
   a common requirement of class-less routing exchanges and interior
   class-less routing protocols across all peer connected network
   domains.

   It is possible to propose that multi homed network domains should
   probably not get subnets of a class A for these reasons, although
   with an increasing diversity of network service providers instances
   of multi-homed network domains may become more prevalent, and the
   requirement to transition to an interior class-less routing structure
   as a consequence of moving to a multi-homed configuration may not be
   explicitly apparent to all network domains.

Potential Guidelines for Allocation of an Address Prefix from the Class
   A Address Space

   To summarise the possible guidelines for allocation from the Class A
   space, such addresses should only be assigned to network domains
   which:

    - have no exterior connection (in which case the domain can use
      either class-full or class-less interior routing protocols without
      further implication),

    or

    - are a component of a private internet domain which uses class-full
      routing exchanges and no other part of the same Class A is
      assigned into the domain (this is probably an unlikely scenario
      given a probable direction to use the Class A space as the major
      resource for the unallocated pool of addresses for allocation),






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RFC 2036        Components of the Class A Address Space     October 1996


    or

    - have a single default exterior connection to a class-less routing
      domain, use class-full routing  protocols and explicitly direct a
      subnet default route to the exterior connection,

    or

    - use class-less interior routing protocols and connect only to
      other network domains which also use class-less interior routing
      protocols.

   It is a reasonable objective to nominate a transition objective to
   the final configuration (uniform use of class-less routing domains
   within the Internet) which would enable deployment of components of
   the Class A space uniformly across the Internet.

Related Potential Activities

   Given the pressures on the remaining Class C address space in the
   unallocated address pool, it is noted that there would be widespread
   deployment of components of the remaining Class A space in class-less
   allocation guidelines. There is a consequent requirement for
   widespread deployment of class-less interior routing protocols in
   order to ensure continued correct operation of the routed Internet.
   This is a more significant transition than that deployed to date with
   the network service providers' deployment of Class-less Inter-Domain
   Routing (CIDR) protocols, in that there is a necessary transition to
   deploy Class-less Interior Routing Protocols (CIRP) within a large
   number of network domains which are currently configured with class-
   full routing.

   However this would appear to be a necessary task if we wish to
   continue to utilise a pool of globally unique Internet addresses to
   allocate to new systems and networks, but one requiring significant
   effort considering the space of the routing transition required to
   make this work.

   There are a number of directed activities which can assist in this
   transition:

    - The network registries commence initial class-less allocation from
      the unallocated Class A space to those entities who either:

      o  operate a CIRP environment, and either have no external
         connectivity, or are singly homed to a network service provider
         using a CIDR environment, with no other exterior connections,




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RFC 2036        Components of the Class A Address Space     October 1996


      or

      o  operate a class-full routing protocol, and either have no
         external connectivity, or are singly homed to a network service
         provider using a CIDR environment, with no other exterior
         connections, and are willing to point the subnet default route
         towards the network service provider.

    - In deploying the Class A space there is a requirement within the
      vendors' product sets to allow explicit configuration of whether
      the router operates in a class-less or class-full mode, with
      correct behaviour of the default route in each case. Class-full
      mode of operation must also allow explicit configuration of
      subnet default behaviour as to whether to follow the default
      route, or to operate a subnet default sink.

    - There is a similar, but longer term, activity within the host
      configuration environment to support a mode of address
      configuration which uses a local network prefix and host address,
      possibly in addition to the current configuration mode of class-
      full network, subnet and host address

    - Internet Service Providers also must support full class-less
      configurations in both interior routing configurations and
      interdomain peering routing exchanges, and provide support to
      client network domains operating a class-less boundary routing
      exchange configuration and be able to undertake proxy aggregation
      as permitted.

Security Considerations

   Correct configuration of the routing environment of the Internet is
   essential to the secure operation of the Internet.

   The potential use of the Class A space raises no additional
   considerations in this area.















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RFC 2036        Components of the Class A Address Space     October 1996


References

   [CIDR]
        Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless Inter-
        Domain Routing (CIDR): an Address Assignment and Aggregation
        Strategy", RFC 1519, BARRnet, cisco, MERIT, OARnet, September
        1993.

Author's Address

      Geoff Huston
      Telstra Internet
      Locked Bag 5744
      Canberra  ACT  2601
      Australia

      phone: [phone]
      email: [e-mail]

































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