Chain ID

A chain ID is the number that identifies a particular EVM network, and it is part of the data a wallet signs. Because the signature covers the chain ID, a transaction signed for one network cannot be replayed on another that shares the same address format — the protection introduced with EIP-155. It matters at the interface level too: networks are added to a wallet by ID and RPC endpoint, so a site offering to add a network for you can attach a familiar-looking name to a chain ID and endpoint of its own choosing. Checking the ID against an independent source before adding a network is what catches that.