What is Ethereum Sepolia Chain ID?

The Ethereum Sepolia Chain ID is 11155111 (or 0xaa36a7 in hexadecimal). Every EVM-compatible network has a unique Chain ID that distinguishes it from other networks and allows dApps and RPC nodes to connect to it. This Chain ID is crucial when connecting wallets (like MetaMask) or dApps to the Sepolia testnet for development and testing purposes.

How to add Ethereum Sepolia RPC URL to MetaMask?

First, you need to get the RPC URL endpoint, either by running your own node or using a provider, such as GetBlock. Then, you need to open the MetaMask network settings, enable test networks, and add the Ethereum Sepolia network there.

Why does MetaMask say that Chain ID already exists?

It means that this chain is already present in your MetaMask, probably with another RPC endpoint. In this case, you need to check the Networks menu while enabling test networks and see whether Ethereum Sepolia is present. If yes, you can either delete it and add ait gain with the GetBlock endpoint, or click Edit and change the endpoint there.