What byte order means and why it exists
A value larger than one byte — a 16-bit, 32-bit, or 64-bit integer — has to be stored as a sequence of individual bytes in memory or in a file, and there are two sensible orders to put them in: most significant byte first ("big-endian," the same order you'd naturally read the number aloud) or least significant byte first ("little-endian," reversed). Neither is more correct than the other; they're opposite conventions solving the same problem, and different processor architectures and file formats committed to different ones for reasons rooted in hardware design decisions made decades ago. The term itself comes from Danny Cohen's 1980 paper "On Holy Wars and a Plea for Peace," which borrowed the big-endian/little-endian labels from Gulliver's Travels, where a war breaks out over which end of a boiled egg to crack first.
How the conversion actually works
Converting little-endian hex to decimal means reversing the byte order first, then reading the reversed bytes as an ordinary big-endian number. Take the hex string 01000000, four bytes:01 00 00 00. Reversed, that's 00 00 00 01, which reads as decimal 1. If the same four bytes were already in big-endian order, they'd represent a much larger number — 16,777,216 — so getting the byte order wrong doesn't produce a slightly-off answer, it produces a completely different value. The conversion in the other direction — decimal to little-endian hex — does the reverse: convert the number to ordinary big-endian hex bytes, then reverse their order.
Which architectures use which order
x86 and x86-64 processors (the dominant architecture in desktops, laptops, and most servers) are little-endian. ARM processors, used in most mobile devices and increasingly in servers and desktops, are bi-endian — capable of either mode — but are overwhelmingly configured as little-endian in practice, largely for compatibility with the existing little-endian software ecosystem. Network protocols historically standardized on big-endian, often called "network byte order" for exactly this reason — TCP/IP headers, for instance, are defined big-endian regardless of the endianness of the machines sending or receiving them, which is why networking code frequently includes explicit byte-order conversion calls (like C's htons/ntohs functions) when moving a value between host memory and a network packet.
A recognizable worked example
The hex value 0xDEADBEEF is a well-known placeholder pattern used by programmers to mark uninitialized or freed memory in a debugger, chosen because it's a valid hex value that reads as recognizable "words" and is unlikely to occur by coincidence. Its bytes in order are DE AD BE EF. If those four bytes were instead read out of memory on a little-endian system without accounting for byte order — for instance, dumped in the same left-to-right sequence they appear in a hex editor, then naively read as one big-endian number — the reversed byte sequence EF BE AD DE would decode to 3,735,928,559 in decimal, the same underlying value as 0xDEADBEEF, just approached from the opposite byte order. Recognizing that a "wrong" decoded value is actually the right value with swapped byte order is a common debugging move once byte order is a suspect.
Endianness within a byte doesn't exist
It's worth being precise about what byte order does and doesn't affect: endianness governs the order of whole bytes within a multi-byte value, not the order of bits within a single byte. A single hex byte like0xA3 means the same thing regardless of the system's endianness — there's no such thing as a "little-endian byte." Confusion sometimes arises because bit numbering conventions (which bit is "first") are a separate, much less standardized topic from byte ordering, and the two get conflated in casual discussion even though only byte order is what this tool, or the term "little-endian," actually refers to.
Where this actually causes bugs
Byte-order mistakes show up most often when reading a binary file format or network packet without checking which endianness it specifies, when porting code between architectures that differ in endianness, or when interpreting a raw memory or hex dump by eye. Many binary file formats build an explicit byte-order marker into their header specifically to avoid this ambiguity — Unicode's UTF-16 and UTF-32 encodings, for example, can begin with a byte-order mark that tells a reader which endianness the rest of the file uses, precisely because there's no way to infer it reliably from the data alone.