IPv4 Address Converter

The 32 bits behind the dots

An IPv4 address is a single unsigned 32-bit integer. The familiar dotted-quad form is only a display convention: 192.168.1.1 is four 8-bit chunks written in decimal and joined with dots, which is far easier to read than the 3232235777 it actually is. Every notation on this page is the same number wearing different clothes, so converting between them never loses information.

The decimal form is that integer written out in full. The hexadecimal form groups the 32 bits into 8 hex digits, which is convenient because each pair of digits maps to exactly one octet — 0xC0A80101 splits cleanly into C0, A8, 01, 01. The binary form makes the bit boundaries visible, which is what you want when you are reasoning about netmasks.

Where each notation shows up

Integer form appears wherever an address needs to be stored or compared cheaply. Databases use it because INT UNSIGNED sorts and range-scans faster than a string, so WHERE ip BETWEEN x AND y becomes an index seek. Log pipelines and GeoIP tables ship ranges as integer pairs for the same reason.

Hexadecimal shows up in packet dumps, kernel structures and older config formats. Binary is mostly a teaching and debugging aid: line up an address against its netmask in binary and the network/host split stops being abstract.

Reverse DNS and in-addr.arpa

Forward DNS maps a name to an address. Reverse DNS does the opposite, and because DNS delegates authority left to right while IPv4 gets more specific left to right, the octets have to be reversed. 192.168.1.1 becomes 1.1.168.192.in-addr.arpa, and a PTR record at that name gives the hostname. Mail servers check this routinely — a missing or mismatched PTR is a common reason legitimate mail gets scored as spam.