Hard Fork of a Blockchain

A permanent split in a blockchain’s history, creating two separate chains due to incompatible consensus rule changes.

What is Hard Fork of a Blockchain?

A hard fork is a significant update to a blockchain’s protocol that introduces new consensus rules not backward-compatible with the existing chain, resulting in a permanent divergence into two distinct blockchains. When a hard fork occurs, nodes running the older version of the software cannot validate blocks created by nodes following the new rules, leading to a split where one chain follows the old rules and another follows the new rules. This differs from a soft fork, which is backward-compatible, or other types of forks like software or Git forks, which pertain to codebases rather than blockchain consensus.

In Ethereum, hard forks are often planned to introduce major upgrades or fix critical issues. For example, the Ethereum Merge in September 2022 was a hard fork transitioning the network from proof-of-work to proof-of-stake, though it was carefully coordinated to avoid a split. However, contentious hard forks can lead to new networks, as seen with Ethereum Classic (ETC) in 2016, when a hard fork to reverse the DAO hack caused a community split, with some nodes continuing on the original chain (ETC) and others adopting the new chain (ETH). Hard forks require broad coordination among miners, validators, and users to ensure network stability.

Hard forks can introduce new features, improve security, or resolve disputes but risk community division if consensus isn’t reached. They are a key mechanism for evolving blockchain protocols, as seen in Ethereum’s upgrades like Byzantium (2017) or London (2021), which introduced EIP-1559 for fee market changes. Unlike soft forks, hard forks demand that all nodes upgrade to avoid operating on a separate, potentially less secure chain.