without any further information. The disadvantage is that because only three sizes are available, networks were usually too big or too small for most organizations to use. The smallest allocation and routing block contained 28 = 256 addresses — larger than necessary for personal or department networks, but too small for most enterprises. The next larger block contained 216 = addresses—too large to be used efficiently even by large organizations. But for network users who needed more than addresses, the only other size (224) gave them far too many, more than 16 million. This led to inefficiencies in address use as well as inefficiencies in routing, because it required a large number of allocated class-C networks with individual route announcements, being geographically dispersed with little opportunity for route aggregation. Within a decade after the invention of the Domain Name System (DNS), it became apparent that the old classful network scheme was not scalable. This led to the development of subnetting and CIDR. The formerly meaningful class distinctions based on the top 3 address bits were removed, and the new system was described as being classless, in contrast to the old system, which became known as classful. Routing protocols were revised to carry not just IP addresses, but also their matching subnet masks. Implementing CIDR required every host and router on the Internet to be reprogrammed in small ways—no small feat at a time when the Internet was entering a period of rapid growth. In 1993, the Internet Engineering Task Force published a new set of standards, and , to define this new scheme for allocating of IP address blocks and routing IPv4 packets. An updated version, , was published in 2006. After a period of experimentation with various alternatives, Classless Inter-Domain Routing was based on variable-length subnet masking (VLSM), which allows each network to be divided into various power-of-two-sized subnets, so that each network or subnet can be sized appropriately for local needs. Variable-length subnet masks were mentioned as one alternative in . Techniques for grouping addresses for common operations were based on the concept of cluster addressing, first proposed by Carl-Herbert Rokitansky. CIDR notation CIDR notation is a compact representation of an IP address and its associated network mask. The notation was invented by Phil Karn in the 1980s. CIDR notation specifies an IP address, a slash ('/') character, and a decimal number. The decimal number is the count