Definition
The extranonce is additional search space that lives inside a block's coinbase transaction, used by miners once the 32-bit nonce in the block header has been completely exhausted. With only about 4.3 billion header-nonce values, a modern ASIC burns through the entire range in a fraction of a second, so it needs another field to vary in order to keep generating fresh candidate block hashes. Changing any byte of the coinbase changes the Merkle root, which in turn produces an entirely new header to hash — and the extranonce is the field reserved for exactly that purpose.
Extranonce1 and extranonce2 in pooled mining
Under the Stratum protocol the extranonce is split in two. The pool assigns each connection a unique extranonce1, guaranteeing that no two miners ever search the same space and submit duplicate work. The miner is then free to roll extranonce2 locally across its full range. Together they give each device an enormous private search space layered on top of the header nonce, which is what keeps a fleet of identical machines from colliding.
Why it matters
The extranonce is the quiet reason modern terahash-scale hardware never runs out of work between header updates. When extranonce2 is also exhausted, the miner rolls the timestamp or requests fresh work, but in practice the extranonce space is deep enough to last until the next job arrives.
The extranonce sits inside the coinbase transaction and feeds the target search every miner performs against the block header.
In Simple Terms
The extranonce is additional search space that lives inside a block’s coinbase transaction, used by miners once the 32-bit nonce in the block header has…