A testnet is a separate blockchain network that runs the same or nearly the same software as a live network but uses coins with no market value. Developers use it to try out wallets, exchanges and smart contracts, node operators use it to rehearse upgrades, and core developers use it to test changes to the protocol before switching them on where real money is at stake.
Its counterpart is mainnet, the production network where transactions are final and coins are worth something. Major chains keep one or more public testnets as of 2026: Bitcoin has testnet4 and signet, and Ethereum has Sepolia and Hoodi. Testnet coins are handed out free through a faucet and are, by design, not meant to be bought or sold.
Testnet in plain terms
Think of a flight simulator. Trainee pilots use exactly the same controls and procedures as in a real cockpit, and mistakes look just as dramatic on screen, but a crash costs nothing. Airlines also test new procedures in the simulator before asking real crews to fly them with passengers on board.
A testnet is the simulator for a blockchain. Everything works like the real network, but the coins are play money, so developers can break things, find bugs and fix them before anyone's savings are involved.
How testnets work
A public testnet has its own genesis block, its own chain of blocks and its own peer-to-peer network, kept separate from mainnet by a different network identifier and, on Ethereum, a different chain ID, such as 11155111 for Sepolia, so that transactions cannot be replayed between them. Addresses often use a different prefix; Bitcoin testnet addresses in the bech32 format begin with tb1 rather than bc1.
Because testnet coins are worthless, normal economic security does not apply. Proof-of-work testnets usually have low mining difficulty and special rules to keep blocks coming, and some test networks replace open block production with a fixed set of signers or validators. That makes them practical but not a guide to how secure mainnet is.
Bitcoin test networks
- Testnet3. Launched in 2012 and used for more than a decade. Its rule allowing minimum-difficulty blocks after 20 minutes without one was abused to produce floods of blocks, and its coins began to trade, which undermined its purpose.
- Testnet4. Defined in BIP 94 and supported from Bitcoin Core 28.0 in October 2024, with fixes for those problems. It remains permissionless mining.
- Signet. Defined in BIP 325 and added in Bitcoin Core 0.21.0 in 2021. Blocks must be signed by designated keys as well as meeting proof of work, giving a predictable, stable network; anyone can also launch a custom signet, which developers have used to trial proposed soft forks.
- Regtest. A private "regression test" mode in which a developer creates blocks on demand on their own machine.
Ethereum test networks
Ethereum has retired several public testnets as its technology changed. Ropsten and Rinkeby were deprecated after the 2022 move to proof of stake, and Goerli was deprecated after that. As of November 2026, ethereum.org recommends:
- Sepolia for application and smart contract testing. It has a permissioned validator set, which keeps it stable and its test ether easy to get.
- Hoodi for validator and protocol-upgrade testing. It was launched in March 2025 after the Holesky testnet ran into problems during the Pectra upgrade, and Holesky has been deprecated since September 2025.
Each scheduled hard fork runs on these networks first. Fusaka, for example, was activated on the public testnets in October 2025, with Hoodi last, before reaching mainnet on 3 December 2025. Those upgrades are specified as EIPs. Layer-2 networks and decentralized applications typically run their own test deployments on top of Sepolia.
Uses, limits and misuse
Testnets let teams check a full node upgrade, run integration tests for a crypto exchange deposit system, estimate gas fees, or rehearse a Lightning Network channel setup without risk. Auditors and bug bounty hunters also use them to reproduce issues safely.
Their limits are well known. Testnets carry little real traffic, have different fee markets and attract few attackers, so code that works on a testnet can still fail under mainnet load. Their coins sometimes acquire a grey-market price, especially when users try to farm a hoped-for airdrop by interacting with a new project's testnet, and scammers have sold "testnet tokens" to people who did not understand they were worthless. Anyone offered testnet coins for money should treat it as a red flag.