of consecutive leading 1-bits (from left to right) in the network mask. The number can also be thought of as the width (in bits) of the network prefix. The IP address in CIDR notation is always represented according to the standards for IPv4 or IPv6. The address may denote a specific interface address (including a host identifier, such as ), or it may be the beginning address of an entire network (using a host identifier of 0, as in or its equivalent ). CIDR notation can even be used with no IP address at all, e.g. when referring to a as a generic description of an IPv4 network that has a 24-bit prefix and 8-bit host numbers. For example: represents the IPv4 address and its associated network prefix , or equivalently, its subnet mask , which has 24 leading 1-bits. the IPv4 block represents the 1024 IPv4 addresses from to . the IPv6 block represents the block of IPv6 addresses from to . represents the IPv6 loopback address. Its prefix length is 128 which is the number of bits in the address. In IPv4, what is now called CIDR notation came into wide use only after the implementation of CIDR. It does not appear in the original CIDR standards, which instead used a dotted-decimal subnet mask after the slash; for example, . Describing the network prefix's width as a single number () was easier for network administrators to conceptualize and to mentally calculate, so it gradually became incorporated into later standards documents and into network configuration interfaces. The number of addresses inside a network or subnet may be calculated as 2address length − prefix length, where address length is 128 for IPv6 and 32 for IPv4. For example, in IPv4, the prefix length gives: 232−29 = 23 = 8 addresses. Subnet masks A subnet mask is a bitmask that encodes the prefix length associated with an IPv4 address or network in quad-dotted notation: 32 bits, starting with a number of 1-bits equal to the prefix length, ending with 0-bits, and encoded in four-part dotted-decimal format: . A subnet mask encodes the same information as a prefix length but predates the advent of CIDR. In CIDR notation, the prefix bits are always contiguous. Subnet masks were allowed by to specify non-contiguous bits until stated that the mask must be left contiguous. Given this constraint, a subnet mask and CIDR notation serve