What Is Hard Fork?
Also known as: Hardfork, Backward-Incompatible Fork, Protocol Upgrade
In short
A hard fork is a change to a blockchain protocol that is not backward compatible with the previous rules; because old software considers blocks produced under the new rules invalid, all nodes must update, or the chain may split in two.
Hard forks are generally carried out for two reasons. The first is planned protocol upgrades: regular network upgrades such as Ethereum's London upgrade in 2021 or Dencun in 2024 are hard forks implemented with the support of the vast majority of the community. In this case, since nearly all nodes update their software, only a single chain remains.
The second is disagreements within the community. If one group supports a change while another insists on the old rules, two separate chains and two separate coins emerge; balances at the moment of the split are valid on both chains. The rollback decision taken after the DAO attack in 2016 led to the split of Ethereum and Ethereum Classic, and the block size debate in 2017 led to the split of Bitcoin and Bitcoin Cash.
In contentious hard forks, protection is needed against replay attacks, in which a transaction on one chain is replayed on the other. Exchanges decide, according to their own policies, whether to list the coin of the split-off chain and whether to distribute it to their users. For backward-compatible changes, the soft fork method is used.
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