RFC 2036 (rfc2036) - Page 3 of 9
Observations on the use of Components of the Class A Address Space within the Internet
Alternative Format: Original Text Document
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
For network service providers within the deployed Internet the
implications from this recommendation to deploy prefixes from the
Class A address space add more pressure to the requirement to
uniformly deploy class-less routing protocols. While this is already
a mandatory requirement for any domain which operates without a
default route (ie. the provider carries full Internet routing and
effectively calculates default), other providers currently can use
an imported default route and operate within a class-full routing
configuration. This mode of operation is sub-optimal, in so far as
the task of aggregating routes falls on peer network service
providers performing proxy aggregation of contiguous class-full
address blocks.
In deploying components of the Class A the use of proxy aggregation
is no longer sufficient. Where a domain sees a default route and a
subnet of a Class A route the routing structure, in a class-full
configuration, may not necessarily follow the default route to reach
other parts of the Class A network not covered by the advertised
Class A subnet route.
Accordingly for Network Service Providers operating within the
Internet domain the deployment of components of the Class A space
entails a requirement to deploy class-less routing protocols, even in
the presence of a default route. It is noted that this absolute
requirement is not the case at present.
Consideration of Non-Transit Network Configurations
For disconnected network environments, where the network domain is
operated with no links to any peer networking domain, such networks
can continue to use class-full interior routing protocols with subnet
support. Allocation of addresses using prefix blocks from the Class A
space within such environments is possible without adding any
additional routing or address deployment restrictions on the network
domain.
Huston Informational