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Bitcoin can be run on a public testnet (by using a different port than the real Bitcoin network). This is very useful for development, because you can test software handling Bitcoin without handling actual value.
What is the situation for Bitcoin alternatives? Is there a testnet for most (or all) altcoins? Which altcoins have such a testnet feature?
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Most mine-able coins are essentially clones of the bitcoin code and therefore have testnet. Many alt-coins do not have very active testnets so finding another peer on this network can take time, but given enough patience, your alt-coin testnet node will find another connection.
If you are impatient you can google for "testnet addnode" and include the coin you are looking for; this will (hopefully) find forum posts of people who have setup/found the IP address of a testnet in your desired coin, adding this to your configuration file will make your client attempt to connect to this IP on startup. If you need help with a particular coin, let me know :)
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Talking about alternatives, CryptoNote based coins do not have a testnet mode but you can effectively create a testnet with a few simple steps.
https://github.com/zone117x/node-cryptonote-pool#setting-up-testnet
However there's one drawback to this method. You'll be able to set up your own testnode but the connection with other peers is done manually.
If you haven't heard of CryptoNote, it's a platform allowing for creation of absolutely anonymous cryptocurrencies. The best example of which is Bytecoin (BCN). This coin was launched in 2012 and is the first cryptocurrency to have used CN protocol as a core technology.
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Ethereum has multiple testnets. The general testnet running at the moment is called Ropsten, while the Rinkeby and Kovan testnets are used to test different versions of a new consensus algorithm.
Bytecoin was actually launched in 2014, but backdated to hide an 82% premine. Today, the most widely used CryptoNote blockchain is Monero, which does have a public testnet mode. – ProkhorZ – 2017-09-22T18:54:46.967