Ethereum has three test network for developer to develop and to do testing, namely ROPSTEN, KOVAN and RINKEBY.

Ropsten

PoW testnet

Network id: 3

Block time: sub-30 seconds

Commands:

geth --testnet or geth --networkid 3

or parity --chain ropsten

Explorer https://ropsten.etherscan.io/

Github https://github.com/ethereum/ropsten

History:

Started in November 2016. Named after a subway station in Stockholm.

Was DoS attacked in February 2017 which made synching slow and made clients consume a lot of disk space.

Was revived in March 2017 and became usable again.

Pros:

Best reproduces the current production environment , i.e. system and network conditions on the live Ethereum mainnet, because it’s PoW net.

, i.e. system and network conditions on the live Ethereum mainnet, because it’s PoW net. Can be used with both geth and parity.

Ether can be mined . Or requested from a faucet:

. Or requested from a faucet: https://faucet.metamask.io/

http://faucet.ropsten.be:3001

https://faucet.bitfwd.xyz/

What public test-networks and faucets exist?

Cons:

Not immune to spam attacks. Because of this it’s less stable.

Kovan

PoA testnet started by the Parity team

started by the Parity team Network id: 42

Block time: 4 seconds

Command: parity --chain kovan

Explorer https://kovan.etherscan.io/

Github https://github.com/kovan-testnet/proposal

History:

Started in March 2017. Named after a subway station in Singapore.

Pros:

Immune to spam attacks (as Ether supply is controlled by trusted parties)

Cons:

Not supported in geth .

. Doesn’t fully reproduce the current production environment as it uses PoA.

Ether can’t be mined. It has to be requested from the faucet.

Rinkeby

PoA testnet started by the Ethereum team. Uses Clique PoA consensus protocol.

started by the Ethereum team. Uses Clique PoA consensus protocol. Network id: 4

Block time: 15 seconds

Command: geth --rinkeby or geth --networkid 4

or Explorer https://rinkeby.etherscan.io/

Github https://github.com/ethereum/EIPs/issues/225

Website https://www.rinkeby.io

History:

Started in April 2017. Named after a metro station in Stockholm.

Pros:

Immune to spam attacks (as Ether supply is controlled by trusted parties)

Cons:

Supported by geth only

Doesn’t fully reproduce the current production environment as it uses PoA.

Ether can’t be mined . It has to be requested from a faucet:

. It has to be requested from a faucet: https://faucet.rinkeby.io/

Görli

Work on the testnet began at the ETHBerlin hackathon in early September, when Parity’s Afri Scohedon banded together with Chainsafe Systems in a bid to create a “next-generation public Ethereum test network”.

PoA testnet

Explorer https://goerli.etherscan.io/

Github https://github.com/goerli/testnet

Website https://www.rinkeby.io

Faucet: https://goerli-faucet.slock.it/

Other:

0 : Olympic, Ethereum public pre-release testnet

: Olympic, Ethereum public pre-release testnet 1 : Frontier, Homestead, Metropolis, the Ethereum public main network

: Frontier, Homestead, Metropolis, the Ethereum public main network 1 : Classic, the (un)forked public Ethereum Classic main network, chain ID 61

: Classic, the (un)forked public Ethereum Classic main network, chain ID 1 : Expanse, an alternative Ethereum implementation, chain ID 2

: Expanse, an alternative Ethereum implementation, chain ID 2 : Morden, the public Ethereum testnet, now Ethereum Classic testnet

: Morden, the public Ethereum testnet, now Ethereum Classic testnet 3 : Ropsten, the public cross-client Ethereum testnet

: Ropsten, the public cross-client Ethereum testnet 4 : Rinkeby, the public Geth PoA testnet

: Rinkeby, the public Geth PoA testnet 8 : Ubiq, the public Gubiq main network with flux difficulty chain ID 8

: Ubiq, the public Gubiq main network with flux difficulty chain ID 42 : Kovan, the public Parity PoA testnet

: Kovan, the public Parity PoA testnet 77 : Sokol, the public POA Network testnet

: Sokol, the public POA Network testnet 99 : Core, the public POA Network main network

: Core, the public POA Network main network 100 : xDai, the public MakerDAO/POA Network main network

: xDai, the public MakerDAO/POA Network main network 401697 : Tobalaba, the public Energy Web Foundation testnet

: Tobalaba, the public Energy Web Foundation testnet 7762959 : Musicoin, the music blockchain

: Musicoin, the music blockchain 61717561 : Aquachain, ASIC resistant chain

: Aquachain, ASIC resistant chain [Other] : Could indicate that your connected to a local development test network.

Source : https://ethereum.stackexchange.com/questions/27048/comparison-of-the-different-testnets/30072#30072

https://ethereum.stackexchange.com/questions/17051/how-to-select-a-network-id-or-is-there-a-list-of-network-ids/17101#17101