As far as I know, the default test network is "Ropsten" and you can connect to ropsten with following command.Since it is default, when you connect to testnet its "Ropsten".
geth --testnet
I would like to connect "Kovan" / "Rinkeby" test networks. What are the required parameters connecting to these test networks?
For rinkeby, just type geth --rinkeby from command prompt and you're in
For Kovan:
parity --chain kovan
it won't be possible to connect to Rinkeby with parity, only with geth.
Rinkeby with geth:
geth --rinkeby
You have to use either the --chain or --networkid options, depending on your client.
The Kovan network is only available when using the Parity client.
From the Parity --help output:
--chain CHAIN Specify the blockchain type. CHAIN may be either a JSON chain specification file or olympic, frontier, homestead, mainnet, morden, ropsten, classic, expanse, testnet, kovan or dev (default: homestead).
The Rinkeby network is only available when using the Geth client.
From the Geth --help output:
--networkid value Network identifier (integer, 1=Frontier, 2=Morden (disused), 3=Ropsten, 4=Rinkeby) (default: 1)
geth --rinkeby --syncmode "fast" --rpc --rpcapi db,eth,net,web3,personal --cache=1024 --rpcport 8545 --rpcaddr 127.0.0.1 --rpccorsdomain "*"
