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Understanding Classless Inter-domain Routing (CIDR)

An example of Classless Inter-domain Routing (CIDR), showing how the address allocations for different-sized address block down to the individual IP address is implemented

In the example opposite, the Technology Training website is hosted on a machine with IP address The hosting is in fact provided as a service by a hosting company called UKServers. A simple reverse DNS lookup against the host name reveals the IP address of, as shown opposite.

The Internet Assigned Numbers Authority maintains a list of network prefixes and the private organisations and the registries to which they have been assigned. In this example, the list reveals that the block 217/8 has been assigned to the European Regional Internet Registry (RIPE). Réseaux IP Européens (RIPE) has in turn allocated a block of IP addresses within this range to a company called UK Solutions, the range defined by

in other words the range –

equivalent in size to 32 contiguous class C networks. A smaller block from this allocation has been allocated to UKServers for their public hosting business, identified on the RIPE database as

or the equivalent of half of a traditional class C network. The Technology Training Limited web server has been allocated an address within this block,

as can be seen by using a simple ping command to the host name, or by using one of the web-based whois tools which are widely available.

To understand the CIDR notation used in this article better, and the use of IP subnet masks, see the related article

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