Every valid Bitcoin block requires a header section that distinguishes that it is compatible for the Bitcoin blockchain. The header is also what identifies an individual block from any other block. Blocks in the blockchain are layered sequentially, starting with the genesis block.
All Bitcoin blocks are on the Bitcoin blockchain are 80-bytes long, with the following breakdown:
- 4-byte long Bitcoin version number
- 32-byte block hash from previous block
- 32-byte long merkle root
- 4-byte long timestamp of the block
- 4-byte difficulty target for the block.
- 4-byte nonce used by miners.