Blockchain//Chain ID

- Unique integer identifier for each blockchain network — Mainnet=1, Sepolia=11155111, custom=anything.

Unique integer identifier for each blockchain network — Mainnet=1, Sepolia=11155111, custom=anything.

Included in transaction signatures: a tx signed for chainId 1 is invalid on chainId 5.

Metamask uses chainId + RPC URL to know which blockchain it's talking to.

Immutable after network launch — changing chainId in genesis.json after the first block breaks everything.

Part of the P2P handshake: nodes reject peers with different chain IDs during initial connection.