Historically, Internet registries and Internet service providers allocated IP addresses in blocks of 256 (for Class C) or larger octet-based blocks for classes B and A. By definition, each block fell upon an octet boundary. The structure of the reverse DNS domain was based on this definition. However, with the introduction of Classless Inter-Domain Routing, IP addresses were allocated in much smaller blocks, and hence the original design of pointer records was impractical, since autonomy of administration of smaller blocks could not be granted. RFC 2317 devised a methodology to address this problem by using CNAME records.