Canonical question about the public Ethereum testnets. What's the difference between Olympic, Morden and Ropsten?
What are their network IDs, which ones are still considered active?
Olympic has the network ID
0. It was the pre-release testnet launched in early 2015. Somethimes this network is referred to as Ethereum Version 0.9. It was discontinued in July 2015 when Ethereum officially launched and replaced by Morden.
Morden has the network ID
2. It was the first full release testnet launched parallel to Frontier public main network with ID
1 in July 2015, the so called Ethereum 1.0. It was later forked to receive the Homestead updates. It was recently discontinued due to a testnet-only consensus bug between parity and geth. Also, the difficulty bomb on Morden was already causing blocktimes greater than 30 seconds.
Ropsten has the network ID
3. It was launched in November 2016 and replaces the bugged Morden testnet, and currently runs the latest Homestead branded version of Ethereum.