Remember, The White Council expelled the Necromancer from Dol Guldur during the Hobbit.
Right, but The One Ring is in Bilbo's possession during The Hobbit and Sauron's physical form was defeated in the Second Age at The Last Alliance. So How can the Necromancer (Sauron in physical form) be expelled when his Spirit is bound to the One Ring that Bilbo finds in Gollum's Cave? The One Ring was with Gollum for five hundred years! That means Sauron hasn't been able to take physical form and yet during the events of the Hobbit he is The Necromancer?
Sauron's Timeline:
Gorthaur/Sauron destroyed in Numenor -First Age
Annatar/Sauron forges rings of power and is destroyed -Second Age
Necromancer/Sauron at Dol Guldur -Third Age
Sauron/The One Ring -Third Age
Sauron/The Lidless Eye (Baradur) -Third Age
The first two make sense, Sauron is destroyed by Eru Illuvatar (God) when Numenor is destroyed. Then Sauron returns as Annatar to forge the rings of power and returns in physical form and faces Isildur at the Plains of Gorgaroth. Isildur slays Sauron by cutting the One Ring from his hand. Sauron is destroyed again, although we know that his Spirit is bound to the One Ring. Isildur is slain by orcs and the One Ring is lost. Sméagol and Deagol find The One Ring. Gollum keeps The One Ring for 500 years.
[Here is where it gets confusing]
The Necromancer starts his sorcery at Dol Guldur while Bilbo, Thorin & Company are on their adventure. The White Council expels The Necromancer (Sauron) after Bilbo has already found The One Ring. Then after the end of the Hobbit, The One Ring is given to Frodo. Sauron is not seen in physical form, he is the Lidless Eye and is destroyed completely when Gollum falls into Orodruin (Mount Doom's) magma.
The discrepancy is that Sauron is the Necromancer who faces the White Council while Bilbo carries his Spirit in form of the One Ring. Saruman claims that "Sauron cannot yet take physical form, but his spirit has not lost It's potency." (LOTR FOTR).