What is the difference between mainnet, testnet, and local network?

(1) Mainnet — the live Ethereum network with real ETH and real economic consequences. Transactions have actual value; mistakes are irreversible. Deploy here only after thorough testing. Ethereum Mainnet (chainId: 1); (2) Testnets — public test networks using valueless test ETH (obtained free from faucets). Used for final pre-production testing in a realistic network environment. Current Ethereum testnets: Sepolia (PoS, recommended for application testing) and Goerli (being deprecated). Testnets are slower than local networks but simulate mainnet conditions; (3) Local network — runs entirely on your machine. Tools: Hardhat Network (built into Hardhat), Anvil (Foundry's local node), Ganache (deprecated). Benefits: instant transactions, free ETH (pre-funded accounts), deterministic block mining, stack trace debugging, mainnet forking. Development workflow: local network → testnet → mainnet. Layer 2 testnets also exist: Arbitrum Sepolia, Optimism Sepolia, Polygon Mumbai/Amoy for testing L2-specific behavior.