How a testnet works
A testnet runs the same software as the main network, with its own validators and chain ID. Because the tokens are free, anyone can experiment.[1]
Sepolia's validator set is permissioned and run by client and testing teams, and ethereum.org recommends it as the default testnet for application development. Hoodi has an open validator set so stakers can rehearse network upgrades.[1]
Separate chain IDs keep test and real transactions apart. Under EIP-155 the chain ID is part of what gets signed, so a transaction signed for a testnet is not valid on Ethereum mainnet, which is chain ID 1.[2]
Common testnets
Sepolia is the main testnet for app developers on Ethereum; Hoodi is used for validator and staking tests. L2s run their own, such as Arbitrum Sepolia and Optimistic Sepolia.[1]
Holesky was deprecated as of September 2025, and ethereum.org points infrastructure providers to Hoodi instead. Older testnets listed in EIP-155, Ropsten, Rinkeby and Goerli, used chain IDs 3, 4 and 5.[1]
Ephemery resets fully on a 28-day cycle and has the smallest node requirements, under 5 GB on average. It suits short tests where nothing needs to persist.[1]
L2 networks run their own Sepolia-based testnets. ethereum.org lists Arbitrum Sepolia, Optimistic Sepolia and Starknet Sepolia, each with its own faucets and block explorers.[1]
Hoodi has a large state, takes longer to sync and needs more storage to run a node. Ephemery is the lighter choice when a node only needs to run for a few weeks.[1]
Risks and scams
ethereum.org says testnet ETH is supposed to have no real value, and most people get it free from faucets. There is no reason to pay for it.[1]
If someone sold you testnet tokens or a "conversion" service, report it at ReportFraud.ftc.gov or CFTC.gov/complaint, both listed by the FTC for crypto fraud.[5]
Example: testing a contract before mainnet
Illustrative example: a developer adds Sepolia to MetaMask through Networks, then Add a custom network, and fills in the network's details. They request test ETH from a faucet, deploy the contract and run each function.[3]
Once the flows work, they deploy the same code to mainnet. The test ETH stays behind on Sepolia with no value. ethereum.org advises against reusing mainnet accounts on testnets or the other way around, for security reasons.[1]
Wallets that follow BIP-44 can keep test coins on a separate path: SLIP-0044 reserves coin type 1 for testnets of all coins, so a test account can sit at m/44'/1'/0'/0/0.[6]
For local work before any public testnet, a Geth private chain defaults to chain ID 1337. A contract deployed there never touches a public network.[2]
How to check which network you are on
Look at the network name in the wallet and confirm the chain ID. An app can read it with the eth_chainId call. If you expect mainnet and see anything other than 1, stop before signing.[2]
The block explorer is a second check. Sepolia, Hoodi and each L2 testnet have their own explorers, so a transaction hash that exists only on a testnet explorer never moved real ETH.[1]
In MetaMask, custom networks are saved per device, so a testnet you added on a laptop will not appear on your phone until you add it there too.[3]
Common mistakes
Paying for test ETH. ethereum.org says testnet ETH is supposed to have no real value and most people get it free from faucets. A seller who promises it will turn into real ETH fits the FTC warning that only scammers guarantee returns.[5]
Testing a validator on Sepolia. Sepolia uses a permissioned validator set; staking tests belong on Hoodi, which is open for users who want to run a testnet validator.[1]
Skipping the testnet. ethereum.org suggests testing the contract code you write on a testnet before deploying to mainnet.[1]
The Bottom Line
A testnet is a safe place to practice. Get tokens from a faucet, not from a seller.
Educational content, not financial advice.