Eruvin 59B

ואין לה אלא פתח אחד ⁠— מערבין את כולה.

and it has only one entrance, as it is surrounded by a wall or enclosed by houses on all sides, one may establish an eiruv for all of it.

מאן תנא דמיערבא רשות הרבים? אמר רב הונא בריה דרב יהושע: רבי יהודה היא, דתניא: יתר על כן אמר רבי יהודה: מי שיש לו שני בתים בשני צידי רשות הרבים ⁠— עושה לחי מכאן ולחי מכאן, או קורה מכאן וקורה מכאן, ונושא ונותן באמצע. אמרו לו: אין מערבין רשות הרבים בכך.

The Talmud raises a question concerning this baraita: Who is the tanna who holds that an eiruv may be established for a public domain in this manner? Rav Huna, son of Rav Yehoshua, said: It is R' Yehuda, as it was taught in a baraita: Furthermore, R' Yehuda said: One who has two houses opposite each other on two sides of the public domain, if he chooses, he may create a private domain for himself in the public domain. He may place a 10-handbreadth high post from here, on one side, and an additional post from there, the other side. This creates symbolic walls that provide the public domain with the legal status of a private domain. Or, one may place a beam extending from here, one end of the house, and a beam from there, the other end of the house, thereby creating symbolic partitions across the width of the street. In that way, one is permitted to carry objects and place them in the area between the symbolic partitions, as he would in a private domain. The Rabbis said to him: One may not establish an eiruv in the public domain in that way.

אמר מר: ואין מערבין אותה לחצאין. אמר רב פפא: לא אמרו אלא לארכה, אבל לרחבה ⁠— מערבין.

The Master said in the baraita quoted above: And one may not establish an eiruv for half the city. Rav Pappa said: They said this only in a case where one wishes to divide the city according to its length. Generally, a city had a public domain that ran straight across it, from the entrance on one side of the city to the entrance on its other side. The baraita rules that it is prohibited to establish an eiruv separately for the residents of each side of the public domain. But if one wants to divide the city according to its width, he may establish an eiruv for half the city. This distinction is made because in the first case the public domain that runs between the two halves is used by the residents of both halves, and therefore it joins the two into a single unit; in the second case, the residents of each half use only the half of the public domain located on their side and not the half of the public domain located on the other side.