// GLOSSARY -- BLOCKCHAIN & CRYPTO FUNDAMENTALS

What is Chain ID?

1 min read Updated

A chain ID is a unique numeric identifier for a blockchain network — preventing replay attacks by ensuring transactions signed for one chain can't be executed on another.

WHY IT MATTERS

Chain IDs are essential for multi-chain security. Without them, a transaction signed for Ethereum could be replayed on Polygon (same address format, same key). Chain ID is included in the transaction signature, binding it to a specific network.

Common chain IDs: Ethereum Mainnet (1), Goerli (5), Sepolia (11155111), Polygon (137), Arbitrum (42161), Optimism (10), Base (8453). Wallets use chain IDs to ensure you're connected to the intended network.

EIP-155 formalized chain ID inclusion in transaction signatures, making replay protection mandatory for all modern Ethereum transactions.

FREQUENTLY ASKED QUESTIONS

Where do I find chain IDs?
chainlist.org maintains a comprehensive directory. Major chains: Ethereum=1, Polygon=137, Arbitrum=42161, Base=8453, Optimism=10.
What is a replay attack?
Submitting a transaction from one chain on another chain where it would also be valid. Chain ID prevents this by making signatures chain-specific.
Can two chains have the same ID?
They shouldn't — it creates replay risk. But it's possible with careless chain launches. Always verify chain ID before interacting with unfamiliar networks.

FURTHER READING

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