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: 
everyone who is subject to samsara or the round of rebirths is ignorant, because

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> **From ignorance as a requisite condition** come fabrications.  
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>  From fabrications as a requisite condition comes consciousness.  
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>  From consciousness as a requisite condition comes name-&-form. 
>  From name-&-form as a requisite condition come the six sense media.  
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>  From the six sense media as a requisite condition comes contact.  
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>  Fromc ontact as a requisite condition comes feeling.  
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>  From feeling as a requisite condition comes craving.  
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>  From craving as a requisite condition comes clinging.  
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>  From clinging as a requisite condition comes becoming.  
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>  From becoming as a requisite condition comes birth.  
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>  From birth as a requisite condition, then aging & death, sorrow, lamentation, pain, distress, & despair come into play.
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[Paticcasamuppadavibhanga sutta ( SN 12.2)](http://www.accesstoinsight.org/tipitaka/sn/sn12/sn12.002.than.html)

i think ignorance covers nescience of the phenomenon of suffering and the way of escape from it

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> ***When does one realize, one is ignorant ?***
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perhaps once one realizes that they suffer while having no clue how to avoid that

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> ***Can one know nothing of ignorance and still become enlightened?***
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i find this an interesting question, my guess is 'NO', but i'm not sure i can explain why i think so and can't remember suttas which explain this point.  

that's as far as awakening is concerned

as for nibbana i'm a bit more certain, just as suffering is effected by a chain of causes, quoted above, so is nibbana, for example

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> What is impermanent is suffering. What is suffering is nonself. What
>  is nonself should be seen as it really is with correct wisdom thus:
>  ‘This is not mine, this I am not, this is not my self.’  
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>  When one sees this thus as it really is with correct wisdom, the mind becomes dispassionate and is liberated from the taints by nonclinging.  
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>  By being liberated, it is steady;  
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>  by being steady, it is content;  
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>  by being content, he is not agitated.  
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>  Being unagitated, he personally attains Nibbāna.
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[Anicca sutta (SN 22.45)](https://suttacentral.net/en/sn22.45)

an aspirant needs to go through a series of stages of mind development to arrive at that destination and those cannot be bypassed

the first step in curing an illness is its acknowledgement
> 
> 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.