Crypto glossary
Chain ID
A chain ID is the number that identifies an EVM network, such as 1 for Ethereum, and is signed into transactions so they cannot be replayed elsewhere.
EIP-155, activated on Ethereum in 2016, added the chain ID to the data that is signed in every transaction. Because the identifier is part of the signature, a transaction signed for Ethereum mainnet is invalid on any network with a different ID. Each EVM chain picks a unique number: Ethereum is 1, BNB Smart Chain 56, Polygon PoS 137, Arbitrum One 42161 and Base 8453.
Wallets use the chain ID to know which network they are connected to, to add new networks through an RPC URL and to warn users when a dApp asks them to switch. Typed signatures under EIP-712 also include it in their domain, protecting off-chain approvals and orders from cross-chain reuse.
Chain IDs are coordinated by convention through a public registry rather than enforced by any authority, so collisions on small or test networks do happen. A malicious website can propose a fake network with a familiar name and a hostile RPC, so users should check the chain ID and endpoint before approving a network switch.
Example
When a user adds Base to MetaMask, the wallet stores chain ID 8453; transactions signed afterwards include that number and would be rejected by Ethereum mainnet nodes.