single ISP are encouraged by IETF recommendations to obtain IP address space directly from their ISP. Networks served by multiple ISPs, on the other hand, may obtain provider-independent address space directly from the appropriate RIR. For example, in the late 1990s, the IP address (since reassigned) was used by www.freesoft.org. An analysis of this address identified three CIDR prefixes. , a large CIDR block containing over 2 million addresses, had been assigned by ARIN (the North American RIR) to MCI. Automation Research Systems (ARS), a Virginia VAR, leased an Internet connection from MCI and was assigned the block, capable of addressing just over 1000 devices. ARS used a block for its publicly accessible servers, of which was one. All of these CIDR prefixes would be used, at different locations in the network. Outside MCI's network, the prefix would be used to direct to MCI traffic bound not only for , but also for any of the roughly two million IP addresses with the same initial 11 bits. Within MCI's network, would become visible, directing traffic to the leased line serving ARS. Only within the ARS corporate network would the prefix have been used. IPv4 CIDR blocks In common usage, the first address in a subnet, all binary zero in the host identifier, is reserved for referring to the network itself, while the last address, all binary one in the host identifier, is used as a broadcast address for the network; this reduces the number of addresses available for hosts by 2. As a result, a network, with one binary digit in the host identifier, would be unusable, as such a subnet would provide no available host addresses after this reduction. creates an exception to the "host all ones" and "host all zeros" rules to make networks usable for point-to-point links. addresses (single-host network) must be accessed by explicit routing rules, as there is no room in such a network for a gateway. In routed subnets larger than or , the number of available host addresses is usually reduced by two, namely the largest address, which is reserved as the broadcast address, and the smallest address, which identifies the network itself. IPv6 CIDR blocks The large address size used in IPv6 permitted implementation of worldwide route summarization and guaranteed sufficient address pools at each site. The standard subnet size for IPv6 networks is a block, which is required for the operation