'''CIDR notation''' is a compact representation of an IP address and its associated network mask. The notation was invented by [[Phil Karn]] in the 1980s.<ref name=kantor>{{cite web |title=Re: Stupid Question maybe? |url=https://seclists.org/nanog/2018/Dec/334 |publisher=North American Network Operators Group |author=Brian Kantor |date=December 2018 |quote=rjyi/24 is certainly cleaner than 255.255.255.0. I seem to remember it was Phil Karn who in the early 80's suggested that expressing subnet masks as the number of bits from the top end of the address word was efficient, since subnet masks were always a series of ones followd by zeros with no interspersing, which was incorporated (or independently invented) about a decade later as CIDR a.b.c.d/n notation in RFC1519.}}</ref><ref name=simpson>{{cite web |title=Re: Stupid Question maybe? |url=https://seclists.org/nanog/2018/Dec/368 |publisher=North American Network Operators Group |author=William Simpson |date=December 2018 |quote=Actually, Brian is correct. Phil was w-a-y ahead of the times. But I don't remember him talking about it until the late '80s.}}</ref> CIDR notation specifies an IP address, a [[slash (punctuation)|slash]] ('/') character, and a decimal number. The decimal number is the count of consecutive leading ''1''-bits (from left to right) in the network mask. Each 1-bit denotes a bit of the address range which must remain identical to the given IP address. The IP address in CIDR notation is always represented according to the standards for IPv4 or IPv6.
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255.255.255.0. I seem to remember it was Phil Karn who in the early 80's suggested that expressing subnet masks as the number of bits from the top end of the address word was efficient, since subnet masks were always a series of ones followd by zeros with no interspersing, which was incorporated (or independently invented) about a decade later as CIDR a.b.c.d/n notation in RFC1519.}}</ref><ref name=simpson>{{cite web |title=Re: Stupid Question maybe? |url=https://seclists.org/nanog/2018/Dec/368 |publisher=North American Network Operators Group |author=William Simpson |date=December 2018 |quote=Actually, Brian is correct. Phil was w-a-y ahead of the times. But I don't remember him talking about it until the late '80s.}}</ref> CIDR notation specifies an IP address, a [[slash (punctuation)|slash]] ('/') character, and a decimal number. The decimal number is the count of consecutive leading ''1''-bits (from left to right) in the network mask. Each 1-bit denotes a bit of the address range which must remain identical to the given IP address. 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 {{IPaddr|10.0.0.1|8}}), or it may be the beginning address of an entire network (using a host identifier of 0, as in {{IPaddr|10.0.0.0|8}} or its equivalent {{IPaddr|10|8}}). CIDR notation can even be used with no IP address at all, e.g. when referring to a {{IPaddr||24}} as a generic description of an IPv4 network that has a 24-bit prefix and 8-bit host numbers.
