Question: Who is considered to be ignorant according to Buddhism? What does the term ignorance cover ? When does one realize, one is ignorant ?

Can one know nothing of ignorance and still become enlightened?

Answer: 
*"**Who is considered to be ignorant according to Buddhism? What does the term ignorance cover ?**"*

In the Sammaditthi Sutta: The Discourse on Right View:

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> "And what is ignorance, what is the origin of ignorance, what is the cessation of ignorance, what is the way leading to the cessation of ignorance?
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> Not knowing about dukkha, not knowing about the origin of dukkha, not knowing about the cessation of dukkha, not knowing about the way leading to the cessation of dukkha — this is called ignorance.
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*"**When does one realize, one is ignorant ?**"*

I guess, when one realizes one does not understand / is unskillful in the above.

*"**Can one know nothing of ignorance and still become enlightened?**"*

As far as enlightenment is understood as nirvana (which is understood as cessation of dukkha), and ignorance is understood as the above, no. That would be the same as asking if someone who knows nothing of arithmetic to be able to sum numbers: his sum would denounce the very knowledge of arithmetic.

In the same way, someone skillful and wise on origin and cessation of dukkha, someone who put an end on dukkha necessarily is someone who is not an ignorant on the origin and cessation of dukkha.
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> Can one know nothing of ignorance and still become enlightened?
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This is a very big epistemological question.

Ignorance is the state of being unaware, or uninformed. 

Enlightenment is the state of being informed. 

Knowing ignorance or knowing nothing of ignorance are posited opposites of non-existent states. Probably the answer is no.

However; take the Rumsfeldian view of knowledge - there are known knowns, and known unknowns. We know that we know things, and we know there are things we don't know - these are the states of knowledge everyone understands. There could also be unknown knowns and unknown unknowns. I have yet to imagine a scenario for unknown unknowns - that's like proving the existence of a negative / non-existent thing. Perhaps unknown unknown knowns.... ugh. 

Stay with me- for the case of unknown knowns - there may be things we would know if we were made aware of the possibility they could be known. A case that comes to mind is say you have knowledge in one science that people in another field of science don't realize exists. Phlogiston perhaps or the ether. All I need to do is tell you the knowledge you are seeking exists in another field of science. I don't say where and I don't say what.

Now you are in a state of knowing you are ignorant about that knowledge, and you may become enlightened.