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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 form, each number is an 8-bit octet from 0 to 255, and the address is 32 bits long.
How a dotted-quad address is written
RFC 3986 defines an IPv4 literal in URI syntax as four decimal octets separated by dots. Its grammar is IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet. Each dec-octet must be in the range 0–255. The example 192.0.2.1 illustrates the format; it is not offered here as a claim about a live host or service.
Each octet represents one byte. The components are written most-significant byte first, as RFC 7373 describes for IPv4 dotted-quad format.
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Not every string a system resolver or API accepts is a valid IPv4 literal under the strict four-octet URI grammar. Some implementations accept legacy alternatives, such as fewer than four components or numbers written in octal or hexadecimal. RFC 6943 discusses these looser forms and contrasts them with the strict four-part decimal form used by inet_pton().
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This difference matters when validating input: acceptance by one parser does not guarantee acceptance by another. Use the syntax required by the relevant protocol or API rather than assuming all parsers interpret an address string the same way.
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Standards references
- RFC 3986, section 3.2.2, defines dotted-decimal notation for an IPv4 literal in URI syntax.
- RFC 7373, section 4.9, specifies dotted-quad format with the most significant byte first.
- RFC 6943, section 3.1.1, discusses loose legacy address syntax and its distinction from strict forms.
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