RFC 2317 (rfc2317) - Page 2 of 10

Classless IN-ADDR

RFC 2317           Classless IN-ADDR.ARPA delegation          March 1998


   One of the problems encountered when assigning a longer prefix (less
   address space) is that it seems impossible for such an organization
   to maintain its own reverse ("IN-ADDR.ARPA") zone autonomously.  By
   use of the reverse delegation method described below, the most
   important objection to assignment of longer prefixes to unrelated
   organizations can be removed.

   Let us assume we have assigned the address spaces to three different
   parties as follows:

           192.0.2.0/25   to organization A
           192.0.2.128/26 to organization B
           192.0.2.192/26 to organization C

   In the classical approach, this would lead to a single zone like
   this:

   $ORIGIN 2.0.192.in-addr.arpa.
   ;
   1               PTR     host1.A.domain.
   2               PTR     host2.A.domain.
   3               PTR     host3.A.domain.
   ;
   129             PTR     host1.B.domain.
   130             PTR     host2.B.domain.
   131             PTR     host3.B.domain.
   ;
   193             PTR     host1.C.domain.
   194             PTR     host2.C.domain.
   195             PTR     host3.C.domain.

   The administration of this zone is problematic.  Authority for this
   zone can only be delegated once, and this usually translates into
   "this zone can only be administered by one organization."  The other
   organizations with address space that corresponds to entries in this
   zone would thus have to depend on another organization for their
   address to name translation.  With the proposed method, this
   potential problem can be avoided.

4. Classless IN-ADDR.ARPA delegation

   Since a single zone can only be delegated once, we need more points
   to do delegation on to solve the problem above.  These extra points
   of delegation can be introduced by extending the IN-ADDR.ARPA tree
   downwards, e.g. by using the first address or the first address and
   the network mask length (as shown below) in the corresponding address





Eidnes, et. al.          Best Current Practice