Berkeley Unix notation for an Internet address, consisting of one to four numbers (a "dotted quad") in hexadecimal (leading 0x), octal (leading 0), or (usually) decimal. It represents a 32-bit address. Each leading number represents eight bits of the address (high byte first) and the last number represents the rest. E.g. address 0x25.32.0xab represents 0x252000ab. By far the most common form is four decimal numbers, e.g. 18.104.22.168.
Many programs accept an address in dot notation in place of a hostname.