The block header nonce is only 32 bits — about 4.29 billion values. Modern ASICs exhaust that entire range in well under a second, so miners must vary other header fields to keep hashing. The most common trick is the extranonce, a counter embedded in the coinbase transaction's input script.

Changing the coinbase changes the Merkle root, which changes the header, which resets the nonce search space. This gives miners effectively unlimited attempts.

Nonce: 32-bit field inside the 80-byte header
Extranonce: bytes in coinbase scriptSig, set by pool or miner
Timestamp / version: additional rolling fields if needed
Merkle root: recomputed whenever coinbase or tx set changes

Field	Size	Rolled by
Nonce	4 bytes	ASIC (fastest)
Extranonce2	4–8 bytes	Miner / pool software
Timestamp	4 bytes	Pool (seconds)
Coinbase / Merkle	32 bytes	Pool (per job)