# Part II: Blockchain Fundamentals

### Preface

This part lays out the common groundwork of blockchain: from the chained data structure—blocks → Merkle trees → block headers → hash-pointer links—to the roles of hashing, asymmetric cryptography, and digital signatures in identity and integrity. It then covers P2P topology, node roles, and gossip propagation, along with the security assumptions and attack costs of PoW/PoS/BFT consensus. Next, it transitions to the smart-contract lifecycle (authoring → compiling → deployment → invocation → state transition and recording) and typical use cases (DeFi, supply chain, NFTs, governance, insurance). From a modular perspective, it clarifies the division of labor across L0/L1/L2/L3 (execution/settlement/consensus/data availability) and explains how Ethereum gas prices form, including EIP-1559 and CALLDATA costs. Finally, using EIP-4844/Proto-Danksharding as a bridge, it explains how the “impossible trilemma” constrains scaling roadmaps and how L2 provides a practical workaround—establishing a shared frame of reference for the engineering narrative in the OnlyDX chapters.

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