CIDR is principally a [[bitwise]], prefix-based standard for the interpretation of IP addresses. TheIt facilitates [[Internetrouting]] Assignedby Numbersallowing Authority]]blocks (IANA)of issuesaddresses to [[Regionalbe Internetgrouped Registry|Regionaltogether Internetinto single [[routing Registriestable]] (RIRs)entries. These groups, commonly called '''CIDR blocks''', are identified using a syntax similar to that of IP addresses: whicha allfour-part possessdotted-decimal theaddress, samefollowed initialby bitsa slash, then a number from 0 to 32: ''A.B.C.D/N''. The RIRsdotted-decimal portion is interpreted, eachlike responsiblean forIP address, as a single,32-bit large,binary geographicnumber areathat has been broken into four 8-bit bytes. The number following the slash is the '''prefix length''', thenthe subdividenumber theseof blockssignificant intobits, smallercounting blocksfrom andthe issueleft-hand themside toof the ISPsaddress. ThisWhen processspeaking canin beabstract repeatedterms, severalthe timesdotted-decimal atportion differentis levelssometimes ofomitted, thus a ''/20'' is a CIDR block with a 20-bit (unspecified) delegationprefix.
The [[Internet Assigned Numbers Authority]] (IANA) issues to [[Regional Internet Registry|Regional Internet Registries]] (RIRs) blocks of IP addresses which all possess the same initial bits. The RIRs, each responsible for a single, large, geographic area, then subdivide these blocks into smaller blocks and issue them to ISPs. This process can be repeated several times at different levels of delegation. Typically, an [[Internet Service Provider]] will possess one or more CIDR blocks of IP addresses.
