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A dotted-quad address is an IPv4 address written as four decimal numbers separated by periods, for example 192.0.2.1. In the standard four-part format, each number is an 8-bit octet from 0 to 255, making the complete address 32 bits long.
How dotted-quad notation works
“Quad” refers to the address’s four parts; “dotted” describes the periods between them. Each part represents one byte, and the bytes are written from most significant to least significant. RFC 3986 defines this form for an IPv4 literal in a URI as four decimal numbers from 0 to 255 separated by dots. RFC 3986, section 3.2.2
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For example, 192.0.2.1 has four octets: 192, 0, 2, and 1. It illustrates the notation, not a claim that a particular host or service is available at that address.
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A strict dotted-quad IPv4 address has exactly four decimal components, each within the range 0–255. The format is also called dotted-decimal notation. RFC 7373 describes IPv4 addresses as being represented in “dotted-quad format, most significant byte first.” RFC 7373, section 4.9
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Why some systems accept other-looking addresses
Not every parser uses the same rules. Some system resolvers or APIs accept legacy representations with fewer than four parts, or numbers written in bases such as octal or hexadecimal. Those forms are not the strict four-decimal-part IPv4 literal defined for URI syntax by RFC 3986. RFC 6943 discusses these looser forms and contrasts them with the strict syntax used by inet_pton(). RFC 6943, section 3.1.1
When validating an address, use the grammar required by the relevant protocol or API. A string accepted by one resolver is not necessarily a valid IPv4 literal in a URI or accepted by another parser.
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