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==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: {{IPaddr||24|netmask=dotted}}. 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 {{IETF RFC|950}}<ref name="RFC 950 2.1"/> to specify non-contiguous bits until {{IETF RFC|4632}}<ref name="RFC 4632"/>{{rp|at=Section 5.1}} stated that the mask must be left contiguous. Given this constraint, a subnet mask and CIDR notation serve exactly the same function.
==CIDR blocks==
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