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Shorter CIDR prefixes match more addresses, while longer prefixes match fewer. In the case of overlaid CIDR blocks, an address can match multiple CIDR prefixes of different lengths. CIDR is also used for IPv6 addresses and the syntax semantic is identical. The prefix length can range from 0 to 128, due to the larger number of bits in the address.
Special address blocks Address block Address range Number of addresses Scope Description 0.0.0.0/8 ... (CIDR) Top-level domain § Reserved domains; References
Each / 8 block contains 256 3 = 2 24 = 16,777,216 addresses, which covers the whole range of the last three delimited segments of an IP address. This means that 256 /8 address blocks fit into the entire IPv4 space.
RFC 1918 name IP address range Number of addresses Largest CIDR block (subnet mask) Host ID size Mask bits Classful description [Note 1]; 24-bit block: 10.0.0.0 – 10.255.255.255: 16 777 216
Any user may use any of the reserved blocks. Typically, a network administrator will divide a block into subnets; for example, many home routers automatically use a default address range of 192.168.0.0 through 192.168.0.255 (192.168.0.0 / 24).
Since the introduction of the CIDR system, IANA has typically allocated address space in the size of /8 prefix blocks for IPv4 and /23 to/12 prefix blocks from the 2000::/3 IPv6 block to requesting regional registries as needed. Since the exhaustion of the Internet Protocol Version 4 address space, no further IPv4 address space is allocated by ...
This template accepts IPv4 or IPv6 addresses as input and displays minimum-sized blocks of addresses that cover all of the inputs. The result uses CIDR notation and can be used by an administrator to block a range of IP addresses. The template can be used by editing any page, inserting the template, and previewing the result.
CIDR was designed to permit repartitioning of any address space so that smaller or larger blocks of addresses could be allocated to users. The hierarchical structure created by CIDR is managed by the Internet Assigned Numbers Authority (IANA) and the regional Internet registries (RIRs).