user4334: Is extranonce ever necessary to find a block, e.g., in cases where exhausting the regular nonce range fails to block? Is this the rationale behind extranonce?

user5757: Yes, it's almost always necessary.  The current Bitcoin difficulty is 48,807,487,245, which means (approximately) that if you go through all possible values for the regular 32-bit nonce, your probability of finding a valid proof of work is only 1 in 48,807,487,245.  So the vast majority of times, you will exhaust all possibilities for the regular nonce, at which point you can increment the extraNonce and continue.

4334: <p>Is extranonce ever necessary to find a block, e.g., in cases where exhausting the regular nonce range fails to block? Is this the rationale behind extranonce?</p>
 5757: <p>Yes, it's almost always necessary.  The current Bitcoin difficulty is 48,807,487,245, which means (approximately) that if you go through all possible values for the regular 32-bit nonce, your probability of finding a valid proof of work is only 1 in 48,807,487,245.  So the vast majority of times, you will exhaust all possibilities for the regular nonce, at which point you can increment the extraNonce and continue.</p>
