How about nowhere. As far as I can tell, there is no such rule or provision.
Vancouver Canucks forward Raffi Torres will have no further discipline assessed following his hit on Blackhawks' defenceman Brent Seabrook during Game 3 of their series on Sunday night. There was no scheduled hearing with the league on Monday. The NHL's Hockey Operations department said that the rule did not violate Rule 48, which states in part that the area behind the net is designated as a "hitting area". Torres caught Seabrook with a shoulder to the head during a collision behind Chicago's net midway through the second period. The Vancouver player was given an interference penalty on the play.
The phrase "hitting area" doesn't (unless my Adobe Reader search function is faulty) appear anywhere in the rule book. The word hitting appears once, in the phrase "hitting the boards with a stick." Rule 48 mentions no special area on the ice where illegal hits to the head are okay.
Too bad the NHL didn't remember about the hitting area when it was suspending Jarret Stoll.
Here's all of Rule 48:
Rule 48 – Illegal Check to the Head
48.1 Illegal Check to the Head – A lateral or blind side hit to an opponent
where the head is targeted and/or the principle point of contact is not
48.2 Minor Penalty – There is no provision for a minor penalty for this rule.
48.3 Major Penalty – For a violation of this rule, a major penalty shall be
assessed (see 48.4).
48.4 Game Misconduct Penalty – An automatic game misconduct penalty
shall be assessed whenever a major penalty is assessed under this
48.5 Match Penalty – The Referee, at his discretion, may assess a match
penalty if, in his judgment, the player attempted to or deliberately
injured his opponent with an illegal check to the head.
48.6 Fines and Suspensions – Any player who incurs a total of two (2)
game misconducts under this rule, in either regular League or playoff
games, shall be suspended automatically for the next game his team
plays. For each subsequent game misconduct penalty the automatic
suspension shall be increased by one game.
If deemed appropriate, supplementary discipline can be applied by
the Commissioner at his discretion (refer to Rule 28).