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Historically, the IP address space was divided into three main '[[classful network|classes of network]]', where each class had a fixed network size. The class, and hence the length of the subnet mask and the number of hosts on the network, could always be determined from the most significant bits of the IP address. Without any other way of specifying the length of a subnet mask, [[routing|routing protocols]] necessarily used the class of the IP address specified in route advertisements to determine the size of the routing prefixes to be set up in the [[routing table]]s.
==Key Concepts==
CIDR is principly a [[bitwise]], prefix-based standard for the interpretation of IP addresses. The [[Internet Assigned Numbers Authority]] (IANA) issues to [[Regional Internet Registry|Regional Internet Registries]] (RIRs) blocks of IP addresses which all posess 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.
==Example==
[[Image:CIDR_Address.png|right|414px]]
In the late 1990s, the IP address 208.130.29.33 (since reassigned) was used by the <tt>www.freesoft.org</tt> web server. An analysis of this address identified three CIDR prefixes. 208.128.0.0/11, a large block containing over 2 million addresses, had been assigned by [[IANA]] to [[MCI]]. Automation Research Systems, a [[Virginia]] [[Value-added reseller|VAR]], leased an Internet connection from MCI and was assigned the 208.130.28.0/22 sub-prefix, capable of addressing just over 1000 devices. ARS used a /24 block for its public web servers, of which 208.130.29.33 was one.
All of these CIDR prefixes would be used, at different locations in the network. Outside of MCI's network, the 208.128.0.0/11 prefix would be used to direct to MCI traffic bound not only for 208.130.29.33, but also for any of the roughly two million IP addresses with the same initial 11 bits. Within MCI's network, 208.130.28.0/22 would be visible, directing traffic to the leased line serving this client. Only within the Automation Research Systems corporate network would the 208.130.29.0/24 prefix be used.
==CIDR and masks==
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