What is testnet?

A copy of a blockchain used for testing, where coins have no value and are handed out free by a faucet.

Developers use testnets to try contracts before deploying them to the live network, which is called mainnet. Testnet tokens are sometimes sold to newcomers as though they were the real asset.

That last sentence is the reason this term belongs in a buyer's glossary. Testnet coins are free, unlimited, and worthless, and every so often somebody sells them to someone who did not know.

How it works

A testnet runs the same software and the same rules as the live chain, with a separate genesis block and a separate set of participants. Because it is a different network, its coins are a different asset entirely. They are not convertible, not scarce, and not tradable for anything.

Coins are distributed by a faucet, a service that sends a small amount to any address that asks. Faucets are free precisely because the coins are meaningless outside the test environment.

Networks distinguish themselves by chain id, a number included in every transaction so that a transaction signed for one network cannot be replayed on another. That is what stops a testnet transaction from being valid on mainnet, and it is why wallets ask which network you are on.

Ethereum's public test network for application developers is Sepolia (source: the Ethereum Foundation's developer documentation). Bitcoin runs its own testnet plus signet, a variant with controlled block production. Most chains also support a local devnet that a developer runs entirely on their own machine.

Test networks are deprecated and replaced periodically, which is normal and has no bearing on the live chain.

Example

Illustrative. A newcomer is told to send $200 for a discounted allocation of a token, and receives a balance that shows up correctly in their wallet with the right name and symbol. The wallet is pointed at a test network. The balance is real in the sense that it exists on that network, and it is worth nothing, because anyone can request more from a faucet for free. Switching the wallet back to the main network shows a balance of zero. Nothing was hacked and nothing can be recovered.

Why it matters when you buy

Real assets are bought on venues that publish prices and settle in your currency, not from individuals offering allocations. The buy pages show which exchanges sell an asset where you live, the coin pages show what is actually tracked, and the guide on spotting a crypto scam covers this pattern and its relatives.

Related terms

• faucet — where testnet coins come from
• chain id — what keeps networks separate
• mainnet — the live network testnets imitate
• devnet — a private network for development
• block explorer — how to check which chain a transaction is on
• rug pull — the wider category of worthless-token schemes

Questions

Can testnet coins be sold?

Not for anything of value. They are free from a faucet in unlimited quantity, so there is no market and no price. Anyone offering to buy or sell them is running something else.

How do I tell which network I am on?

Wallets display the network name, and a block explorer for the live chain will not show a testnet transaction at all. The chain identifier is the underlying distinction. See chain id.

Do testnets ever become mainnets?

No. A test network is deprecated and replaced when it has served its purpose. Its balances do not carry over anywhere.