Goerli is a testnet for testing validating and staking. The Goerli network is open for users wanting to run a testnet validator. Stakers wanting to test protocol upgrades before they are deployed to mainnet should therefore use Goerli.

The DoS attack on Ropsten led to the birth of two new testnets, Kovan is one of them. This testnet was created by the Parity team in March 2017. The Kovan testnet uses the Proof-of-Authority consensus mechanism which abandons decentralization for security, by maintaining a small group of trusted signers and validators, who ensure the creation of new blocks in the network by staking their reputation.

Ropsten is the third testnet and follows the Proof of Work consensus mechanism. This testnet was named after a subway station in Stockholm, Sweden. It has a chain and network ID of 3. This testnet was launched in late 2016 due to multiple issues in the old testnet and finally replaced The Morden Testnet. Clients Support: The Ropsten test network supports all major Ethereum clients such as Geth, Parity (deprecated), Nethermind, Hyperledger Besu and even the Akula, the latest Ethereum client.

As the public Ethereum main network was launched back in July 2015, there was a need for a new Testnet which could go hand-in-hand with the mainnet. This was the basis for the developement of the Morden Testnet which eventually replaced the Olympic Testnet. This testnet was also based on PoW consensus mechanism. The Morden Testnet had a network ID of 2 and was the only Testnet for over a year.

Olympic Testnet was to reward people who try to test the limits of the Ethereum blockchain during the pre-release period, spamming the network with transactions and doing crazy things with the state, so that the developers could see how the network holds up under high levels of load. At the same time, application developers, data providers, exchanges, and users were encouraged to develop and deploy on the testnet and run nodes.