A testnet is an alternative blockchain network that serves as a sandbox environment for developers to test their code before rolling out new updates to the mainnet. Zoom on testnets and their importance with the example of the Ethereum blockchain.
What is a Testnet?
A testnet, short for “test network”, is a version almost identical to an active blockchain (mainnet) but only dedicated to carrying out tests.
Indeed, such a network aims to test new functionalities of a blockchain without cost or risk since the underlying cryptocurrency of this network has no real economic value.
Thus, a testnet uses its own cryptocurrency, a copy of the original asset, but has no value. Using cryptocurrency from a testnet has no impact on the public blockchain since a testnet is completely isolated from the original chain: it is impossible to transfer cryptocurrencies from a testnet to a mainnet and vice versa.
The code of a blockchain can therefore be tested with the testnet without affecting the active network that hosts the smart contracts and decentralized applications. In addition, on a testnet, any user can retrieve the underlying cryptocurrency at no additional cost, again for testing purposes.
Generally, it is enough to click on a button from a dedicated interface to recover a random amount of this cryptocurrency. This is one of the reasons why testnet cryptocurrencies are worthless.
A testnet is, therefore, an environment that allows new updates to be tested before deploying them on the public blockchain. The developers generally carry out batteries of tests there to detect possible errors in the code, bugs and, in general, to check that everything is working correctly.
A product should always be thoroughly tested before it goes into production to certify that it works in any industry. The cryptocurrency industry is no exception, and the testnet is the medium through which testing is done.
An update directly deployed on a public blockchain, without having tested it on a testnet, could have dramatic consequences on millions of users in the event that there are errors in the code; this is the responsibility of the developers to ensure that this does not happen through the use of testnets beforehand.
Finally, once the update has been thoroughly tested on a testnet, it can finally be implemented on the mainnet, usually not before a vote by network participants that approves the update.
The Ropsten Testnet of the Ethereum Blockchain
A concrete example of a testnet is Ropsten, a testnet of the Ethereum blockchain. Launched in November 2016, it is the oldest Ethereum testnet, but there are others like the Sepolia or Goerli testnet. A blockchain does not necessarily have only one test environment.
The Ropsten testnet is used by Ethereum developers to test protocol updates before deploying them to the Ethereum mainnet.
This testnet is nearly identical to the public Ethereum blockchain, and since it is a testnet, developers do not have to worry about the transaction implications and costs of testing applications.
Ropsten thus allows developers to access a test environment of Ethereum in which errors and failures have only very negligible consequences.
As of this writing, the Ropsten testnet is being used to test the migration of the Ethereum blockchain from a Proof of Work consensus to a Proof of Stake consensus. This famous and highly anticipated update, also called The Merge, presents a real technical challenge.
The new version of Ethereum will reduce the energy consumption of the network and facilitate the participation of users in securing the network, which will consequently increase the level of decentralization. Still, the migration from a PoS consensus to a PoW consensus is a very complex operation.
As Ethereum is currently the main network used for decentralized finance and non-fungible tokens, the slightest error that is not detected before the migration could prove fatal for the cryptocurrency ecosystem as a whole.
This is how the Ropsten testnet allows developers to perform a whole battery of absolutely necessary tests before deploying this migration on the public Ethereum blockchain.
The Complement of the Testnet, the Mainnet
A testnet has no interest without its complement, the mainnet. All scans conducted on a blockchain testnet are for the purpose of deploying the upgrade to the mainnet.
The latter is a network on which real transactions take place and on which native cryptocurrency has real economic value. A mainnet is the main and functional blockchain of an ecosystem.
Thus, the tests carried out on the testnet make it possible to get rid of errors, bugs and then deploy a functional update without bugs on the mainnet.
One of the best-known mainnets in the cryptocurrency industry is the Ethereum mainnet, which has been updated since 2015 by hard forks such as the Byzantium (2017), Constantinople (2019), Istanbul (2019) and London forks (2021).
All of these Ethereum protocol upgrades were first tested on one of Ethereum’s testnets to be operational and bug-free.