A number bigger than a byte is stored as several bytes. The order those bytes are written in is called endianness, and two orders mean the same bytes read as two numbers.
About 2 min of reading
Read bytes in both orders.
Read four bytes big-endian.
Before you run it, say out loud what it will print. Then open the answer.
Read the same four bytes little-endian.
Before you run it, say out loud what it will print. Then open the answer.
Pack big and unpack little to see the swap.
Before you run it, say out loud what it will print. Then open the answer.
Swap a value to network order.
Before you run it, say out loud what it will print. Then open the answer.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
A file or protocol states its byte order. Reading it with the wrong order is not a crash, it is a wrong number, which is why it is easy to miss.
Most modern computers are little-endian. Formats are often big-endian. The gap between the two is where byte-order bugs come from.
import struct
print(struct.unpack(">I", b"\x00\x00\x01\x00")[0])
print(struct.unpack("<I", b"\x00\x00\x01\x00")[0])The same four bytes read big-endian and little-endian give different numbers. The bytes are identical.
import struct
print(struct.pack(">H", 256))
print(struct.pack("<H", 256))Big-endian writes the big end first, little-endian the small end first. Most formats say which they use.
import struct
print(struct.unpack("<H", struct.pack(">H", 256))[0])Packing big and unpacking little swaps the bytes, so 256 comes back as 1. That is a byte-order bug.
import socket
print(socket.htons(1))Network formats are big-endian, so a little-endian machine swaps on send. The helper functions do that swap for you.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.