Little Endian vs. Big Endian

One of the most confusing aspects of Bitcoin data serialization is the way multi-byte numbers are stored in memory. This is known as Endianness. Bitcoin uses both "Big Endian" and "Little Endian" depending on what part of the protocol you are looking at.

1. What is Endianness?

Imagine you have the hex number 12 34 56 78. How should you write it in a file?

• Big Endian (BE): Write it "Natural" order: 12 34 56 78. The "Big" end comes first.

• Little Endian (LE): Write it "Reverse" byte order: 78 56 34 12. The "Little" end comes first.

2. Why does Bitcoin use Little Endian?

Satoshi Nakamoto developed Bitcoin on x86 computers (Intel CPUs).

• Intel CPUs are Little Endian internally.

• To make the code faster and simpler, Satoshi chose to store many numbers (like transaction amounts, versions, and block heights) in the CPU's native Little Endian format.

3. The "Block Hash" Flip

This is the most common pitfall for new developers.

• Internal Order (LE): This is how the computer sees the hash.

• Display Order (BE): This is how block explorers (like Mempool.space) show the hash.

Example: If a block hash starts with many zeros in an explorer: 00000000000000000005... In the raw data of the block file, those zeros are actually at the End: ...05000000000000000000

4. What is flipped?

• Flipped (Little Endian): TXIDs, Block Hashes, Transaction Amounts (Satoshis), Block Version numbers.

• Natural (Big Endian): IP Addresses (in network packets), DER Signatures, and raw Private Keys.

5. Byte-Level vs. Bit-Level

Important: Endianness only applies to the Order of Bytes, not the order of bits inside a byte.

• Hex 0x1A is still 0x1A. You don't flip the 1 and the A.

• You only flip the groups of two hex characters.

Data Type	Endianness	Human Order	Raw Byte Order
Amount	Little	100	64 00 00 00
TXID	Little	Starts with 00	Ends with 00
IP Address	Big	1.2.3.4	01 02 03 04

In the next section, we will discuss Byte Serialization in Raw Transactions.