What is the story behind Bitcoin test networks?

External question — Bitcoin Stack Exchange Author: Pol Espinasa Original post: https://bitcoin.stackexchange.com/questions/130806 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. What differences did each one propose in comparison to the previous network? What was the reason for each testnet to be introduced? When was each test network introduced? The question includes from Testnet 1 to 4 (or even 5) and signet(s). There are two similar questions 36252 and 9975 , but they are old and outdated. I am looking for a detailed historical description and motivation about each test network introduction.

External answer — Bitcoin Stack Exchange Author: Murch Original post: https://bitcoin.stackexchange.com/a/130820 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. The two linked topics covered Testnet1 and Testnet2. From what I gather: Testnet1 (launched 2010) essentially became the first altcoin when people began trading it. It was abandoned by resetting the ledger to create Testnet2 in February 2011. Testnet2 also started to get traded for money and its difficulty got too high to be useful for testing. In February 2012, Testnet2 was hardforked to introduce the 20-minute exception, which requires that a block with a timestamp at least 20 minutes greater than the preceding block must be mined at difficulty 1. Both Testnet1 and Testnet2 preceded Namecoin which was launched in April 2011. Testnet3 (PR#686) Testnet3 was started in April 2012. It also incorporated the 20-minute exception just introduced two months prior to Testnet2. This was supposed to make the testnet sufficiently "broken" to stave off monetization and allow any users to mine blocks if the difficulty got run up. Otherwise blocks were required to adhere to the difficulty as calculated per the usual rules. Unfortunately, when this exception is used on the last block of a difficulty period, it causes the difficulty reset to base its calculation on the minimum difficulty which almost completely resets the difficulty. This led to “block storms”: after a reset to almost minimal difficulty, blocks would be mined extremely rapidly until difficulty reset several times to a higher level. Later, attackers exploited the block storm vulnerability to keep the difficulty programmatically low for an extended period throughout which a block storm persisted. The block storms caused Testnet3’s blockchain to proceed much more quickly than intended and exhausted the block subsidy before its time: today, Testnet3 has just under 5 million blocks which should usually take about 95 years instead of 15 years, and after its 23rd halving, the subsidy is 596 sats. Additionally, market places started again to monetize Testnet3 coins, which on one hand made it more straightforward to get some, but on the other hand disincentivized returning them to faucets and faucets from handing them out. Testnet4 (BIP94) Testnet4 was launched in May 2024. It kept the 20-minute exception, but based the difficulty calculation on the previous period’s first block instead of the last block. Because the first block is exempt from the 20-minute exception, this prevents block storm attacks. However, some users started systematically post-dating their blocks timestamps 20 minutes past the prior block to quickly mine the new subsidy. The popularity of using the 20-minute exception reached an extent that causes a large count of competing chaintips at most heights and frequent chain reorganizations. Some of the same parties monetizing Testnet3 immediately started trying to monopolize and monetize Testnet4 coins. Screenshot via https://x.com/0xB10C/status/1876642278439346339 Thanks for corrections by Pol Espinasa (block storm vulnerability) and Antoine Poinsot (introduction of 20-minute exception in Testnet2).

External answer — Bitcoin Stack Exchange Author: Antoine Poinsot Original post: https://bitcoin.stackexchange.com/a/130822 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I believe the idea of testnet was first floated by Gavin Andresen, who also contributed the patch . Mining difficulty soon reached 300, and Gavin started a discussion about how to deal with the situation. Eventually, contributor Davout's solution to simply reset testnet with a new genesis block was adopted. Confirmation times on testnet2 became too unreliable and Gavin started a new discussion about another reset and, more importantly, how to sustain a test network in the long run. In this thread, Gavin described a minimal rule to reset difficulty. In February 2012 this (backward incompatible) rule was applied to testnet2 . Testnet2 was then reset into testnet3 a couple months later, as Gavin wanted to add interesting test cases to the testnet chain. Testnet3 inherited the difficulty reset rule, which enabled regular block storms during its 13 years of operation. This was one of the reasons why Jameson Lopp proposed a new reset in 2024. This discussion would lead to testnet4, specified in BIP 94 by Fabian Jahr. Testnet4 came with different difficulty-reset rules, which immediately started being abused too . As of June 2026, a testnet5 has been proposed that does away with tweaks to the difficulty adjustment logic entirely.