Block header

The block header, as stated before, consists of the metadata of the block. This holds the information that's needed to link the blocks in the blockchain:

Figure 1.4: Structure of a block header

Each block header will have the components outlined in Figure 1.4. These are the minimum fields required in a permissionless blockchain, such as Bitcoin, to efficiently create a block that can be appended to an immutable blockchain. The Previous Block Hash field is a reference to the last block created. The Merkle Root is the value of the Merkle hash tree; it summarizes all the transactions in the block. Timestamp, Difficulty Target, and Nonce are used by the PoW consensus algorithm to solve the hash puzzle. We'll be revisiting these concepts in more depth throughout the book.

Note: Unlike in a permissionless blockchain, where consensus algorithms are used to generate blocks, permissioned blockchains use the signature of the block creator to represent the block identity. However, blocks in permissioned blockchains maintain previous block identifiers, just as permissionless blockchains do.
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