Classful arrangement architecture served its purpose in the startup date of the Internet, but it lacked scalability in the face of the accelerated amplification of the arrangement in the 1990s. The chic arrangement of the abode amplitude was replaced with Classless Inter-Domain Acquisition (CIDR) in 1993. CIDR is based on variable-length subnet appearance (VLSM) to acquiesce allocation and acquisition based on arbitrary-length prefixes.
Today, debris of classful arrangement concepts action alone in a bound ambit as the absence agreement ambit of some arrangement software and accouterments apparatus (e.g. netmask), and in the abstruse abracadabra acclimated in arrangement administrators' discussions.
IPv4 clandestine addresses
Early arrangement design, if all-around end-to-end connectivity was envisioned for communications with all Internet hosts, advised that IP addresses be abnormally assigned to a accurate computer or device. However, it was begin that this was not consistently all-important as clandestine networks developed and accessible abode amplitude bare to be conserved.
Computers not affiliated to the Internet, such as branch machines that acquaint alone with anniversary added via TCP/IP, charge not accept globally different IP addresses. Three ranges of IPv4 addresses for clandestine networks were aloof in RFC 1918. These addresses are not baffled on the Internet and appropriately their use charge not be accommodating with an IP abode registry.
Today, if needed, such clandestine networks about affix to the Internet through arrangement abode adaptation (NAT).