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Harry talked to Sirius twice (from Gryffindor common room) and Lupin once (from Umbridge's office) using The Floo Network in The Order of the Phoenix while they were at Grimmauld Place. So that means 12 Grimmauld Place was connected to The Floo Network.

What happens if someone uses The Floo Network and says "12 Grimmauld Place" after putting powder in their fire? Will they reach the kitchen of Grimmauld Place?

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    More importantly, Harry uses the Floo Network from the Gryffindor common room. What if someone uses the Floo Network and says "Hogwarts, Gryffindor common room"? Will they reach Hogwarts despite all the magical protections defending the castle?
    – Stef
    Jun 26 at 10:49
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    @Stef Entering Hogwarts via FN isn't that easy as - "The fireplaces of Hogwarts are not generally connected, although there have been occasions when one or more has been tampered with, often without the staff’s knowledge."(wizardingworld.com/writing-by-jk-rowling/the-floo-network)
    – CCCC
    Jun 26 at 11:21
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    Harry, Ron and Ginny returned to McGonnagal's office from Burrow using floo powder in Book 6, chapter 17. That connection was a one-off connection arranged by the Ministry to return students quickly and safely to the school. Otherwise that connection was closed or maybe only intra-school. But if Sirius travelled from Grimmauld place to Hogwarts then that connection was open as Ministry won't open/close the connection there. Jun 26 at 11:27
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TLDR : The odds are that the Fidelius charm would serve its purpose depending on how exactly it works or what happens technically that it is used to safe-guard a secret.

Full explanation-

Now, in my understanding, there are 2 cases that may arise here:

For ease of understanding, I'll be calling all those (combined) from whom the secret is to be kept as X.

  1. Case 1: Secret is - A location’s address which means that the entire place is to remain hidden from X
  2. Case 2: Secret is - The presence of a particular person(s) at a location from X and NOT the entire location's address or existence per se

As explained by Filius Flitwick in PoA

"As long as the Secret Keeper refused to speak, You-Know-Who could search the village where Lily and James were staying for years and never find them, not even if he had his nose pressed against their sitting room window!"

Now other than this, IIRC, there is no other canonical explanation of what exactly is happening in a Fidelius charm. IMO, there are 2 options:

  1. Option 1: The charm works because X is simply not able to see some or all part of the objects included in the secret even when standing right in front of them
  2. Option 2: Under the charm's effect, X's brain isn't able to process all the information correctly even when standing right in front of the secret and looking at everything. Because of this X is unable to act accordingly which eventually also serves the main purpose of keeping the secret.

Combining the 2 cases (of what the secret is) and 2 options (of how the charm works), 4 possibilities arise (Now, I'm assuming that if X is taking the exact place's name while using the Floo-network without being aware of its intended secrecy from itself, then it is doing so, either by chance or it knows the location per se but is unaware or unsure of it being used for hiding sth that X seeks and just to try its luck, plans to go there using the Floo network to search what it is looking for) -

Possibility 1: Location's address is a secret and the charm makes it invisible to X

Floo-network effect: Either, the charm stops X's entry making it getting lost in the network unable to reach the hidden place or if the charm only keeps the outer facade invisible then X's "long shot in the dark" works and X makes a serendipitous discovery of a weakness in the Fidelius charm and puts it to its use.

Possibility 2: Location's address is a secret and the charm affects X's brain to know why it wants to visit this place

Floo-network effect: X gets inside the location and is not able to comprehend why it is there, gets confused and walks out.

Possibility 3: Location's address isn't a secret per se but the presence of someone in particular is and the charm makes that person invisible to X

Floo-network effect: X gets inside the location and is not able to see the person it is looking for, disappointed and failed X comes out.

Possibility 4: Location's address isn't a secret per se but the presence of someone in particular is and the charm affects X's brain to know why it is searching for that person.

Floo-network effect: X gets inside the location, sees that person and is not able to process all the information pieces (either because it forgets its purpose or forgets the face of the person it is looking for) and comes out confused.

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    "Under the charm's effect, X's brain isn't able to process all the information correctly" I don't like this interpretation. The charm is on the house, not on X. Presumably X can have magical defenses up to avoid being charmed or having their brain scrambled with. But the charm will still protect the house. Just like a cloak of invisibility is making its owner invisible rather than blinding observers, I think the Fidelius Charm is affecting the house and not affecting observers.
    – Stef
    Jun 27 at 7:43
  • @Stef I agree that it is a much less likelier explanation for how the charm would work given that we don't see any other charm in the universe that affects the observer instead of the object it's put on. And that's why I presented it as one of the 2 probabilities, both of which occured to me while reading Flitwick's description of the charm. And yes, you made a good point about X's magical defences which makes the 'brain-affecting' theory even less likely as in this case the charm's success will be dependent on the observer's unpreparedness making it a weak charm.
    – CCCC
    Jun 27 at 19:21
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In Chapter Twenty-Two of Order of the Phoenix we find the following passage:

“Your father has been injured in the course of his work for the Order of the Phoenix,” said Dumbledore before Harry could speak. “He has been taken to St. Mungo’s Hospital for Magical Maladies and Injuries. I am sending you back to Sirius’s house, which is much more convenient for the hospital than the Burrow. You will meet your mother there.”

“How’re we going?” asked Fred, looking shaken. “Floo powder?”

“No,” said Dumbledore, “Floo powder is not safe at the moment, the Network is being watched. You will be taking a Portkey.”

The problem described here is that the Ministry is watching the Floo Network; the implication is that it is otherwise possible to use the Floo Network to travel to Grimmauld Place.

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    Yeah, but all the participants of that conversation already knew the Secret. They could see and access Grimauld Place. The question here is what would happen if someone who doesn't know the Secret attempts to travel there via Floo Network.
    – Roberto
    Jun 27 at 15:24
  • @Roberto I don’t see that the question specifies someone who doesn’t know the secret.
    – Alex
    Jun 27 at 15:26
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    It's implicit since he already provide examples of Lupin and Sirius using the Floo Network in Grimauld Place.
    – Roberto
    Jun 27 at 16:03

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