At some point, it is expected that this plan will eventually consume all of the remaining class C address space. As of this writing, the upper half of the class A address space has already been reserved for future expansion. This section describes how the CIDR plan can be used to utilize this portion of the class A space efficiently. It is expected that this contingency would only be used if no long term solution has become apparent by the time that the class C address space is consumed.
Fundamentally, there are two differences between using a class A address and a block of class C's. First, the configuration of DNS becomes somewhat more complicated than it is without the aggregation of class A subnets. The second difference is that the routers within the class A address would need to support and use a classless IGP.
Maintenance of DNS with a subnetted class A is somewhat painful. As part of the mechanism for providing reverse address lookups, DNS maintains a "IN-ADDR.ARPA" reverse domain. This is configured by reversing the dotted decimal network number, appending "IN-ADDR.ARPA" and using this as a type of pseudo-domain. Individual hosts then end up pointing back to a host name. Thus, for example, 131.108.1.111 has a DNS record "111.1.108.131.IN-ADDR.ARPA." Since the pseudo- domains can only be delegated on a byte boundary, this becomes painful if a stub domain receives a block of address space that does not fall on a byte boundary. The solution in this case is to enumerate all of the possible byte combinations involved. This is painful, but workable. This is discussed further below.
Routing within a class A used for CIDR is also an interesting challenge. The usual case will be that a domain will be assigned a portion of the class A address space. The domain can either use an IGP which allows variable length subnets or it can pick a single subnet mask to be used throughout the domain. In the latter case, difficulties arise because other domains have been allocated other parts of the class A address space and may be using a different subnet mask. If the domain is itself a transit, it may also need to allocate some portion of its space to a client, which might also use a different subnet mask. The client would then need routing information about the remainder of the class A.
If the client's IGP does not support variable length subnet masks, this could be done by advertising the remainder of the class A's address space in appropriately sized subnets. However, unless the client has a very large portion of the class A space, this is likely to result in a large number of subnets (for example, a mask of 255.255.255.0 would require a total of 65535 subnets, including those allocated to the client). For this reason, it may be preferable to simply use an IGP that supports variable length subnet masks within the client's domain.
Similarly, if a transit has been assigned address space from a class A network number, it is likely that it was not assigned the entire class A, and that other transit domains will get address space from this class A. In this case, the transit would also have to inject routing information about the remainder of the class A into it's IGP. This is analogous to the situation above, with the same complications. For this reason, we recommend that the use of a class A for CIDR only be attempted if IGP's with variable length subnet mask support be used throughout the class A. Note that the IGP's need not support supernetting, as discussed above.
Note that the technique here could also apply to class B addresses. However, the limited number of available class B addresses and their usage for multihomed networks suggests that this address space should only be reserved for those large single organizations that warrant this type of address. [2]