endianness

Endianness is the order in which bytes are arranged to represent multi-byte numeric values in memory or in a data stream. In big-endian systems, the most significant byte is stored at the lowest memory address; in little-endian systems, the least significant byte is stored first. For example, the 32-bit value 0x12345678 is stored as 12 34 56 78 in big-endian and as 78 56 34 12 in little-endian.

Some architectures can operate in either mode or are configured for a particular mode. The x86 family uses little-endian storage for all multi-byte values. ARM and POWER can be run in either little- or big-endian configurations, depending on the system. Network protocols define a common byte order—network byte order—which is big-endian—to simplify interoperation between heterogeneous systems.

Endianness matters when data is written to a file, sent over a network, or exchanged between processes on different architectures. If the sender and receiver use different endianness, the byte order must be converted. In C and many languages, conversion helpers such as htonl, htons, ntohl, and ntohs are provided; for 64-bit values, similar functions exist. Floating-point numbers also consist of bytes, and their byte order matters when exchanged across systems, though conversion can be less portable than for integers.

Endianness is a property of memory layout rather than of the data type itself. It has practical implications for data interoperability, portability, and performance, and is a common concern in systems programming and networked applications. The terms big-endian and little-endian originate from Gulliver’s Travels, where rival groups argued over which end of an egg to crack.