Modular system for connecting an external communication network to a user network of a building, comprising: - at least one connection cable (51, 52) of said external communication network or connected to the external communication network, said cable (51, 52) comprising a plurality of optical fibres; - a plurality of fibre optic connection elements provided with a connector at a terminal end thereof; - a connection cable of said user network, said cable comprising a plurality of optical fibres associated to respective users; at least one user module comprising a box-shaped body comprising: - at least one passage opening for said connection cable of said user network; - a plurality of adapters associated to respective optical fibres of said connection cable of the user network; at least one operator module comprising a box-shaped body comprising: - at least one inlet opening for said at least one connection cable (51, 52) of said external communication network or connected to the external communication network; and - at least one outlet opening (60, 61) of said plurality of fibre optic connection elements;; said plurality of fibre optic connection elements being connected to the external communication network and coming out from said operator module; wherein said box-shaped bodies (11, 12) of said at least one user module and said at least one operator module are structurally independent from each other; wherein said external communication network is operatively associated to the user network by means of a connection of the connectors of the fibre optic connection elements with respective selected adapters of said plurality of adapters of the user module; and wherein said plurality of adapters are mounted in a plurality of adjacent vertical arrays in a separating wall arranged between respective lower and upper walls of the box-shaped body of the at least one user module. - at least one connection cable (51, 52) of said external communication network or connected to the external communication network, said cable (51, 52) comprising a plurality of optical fibres; - a plurality of fibre optic connection elements provided with a connector at a terminal end thereof; - a connection cable of said user network, said cable comprising a plurality of optical fibres associated to respective users; at least one user module comprising a box-shaped body comprising: - at least one passage opening for said connection cable of said user network; - a plurality of adapters associated to respective optical fibres of said connection cable of the user network; at least one operator module comprising a box-shaped body comprising: - at least one inlet opening for said at least one connection cable (51, 52) of said external communication network or connected to the external communication network; and - at least one outlet opening (60, 61) of said plurality of fibre optic connection elements;; said plurality of fibre optic connection elements being connected to the external communication network and coming out from said operator module; wherein said box-shaped bodies (11, 12) of said at least one user module and said at least one operator module are structurally independent from each other; wherein said external communication network is operatively associated to the user network by means of a connection of the connectors of the fibre optic connection elements with respective selected adapters of said plurality of adapters of the user module; and wherein said plurality of adapters are mounted in a plurality of adjacent vertical arrays in a separating wall arranged between respective lower and upper walls of the box-shaped body of the at least one user module. - at least one passage opening for said connection cable of said user network; - a plurality of adapters associated to respective optical fibres of said connection cable of the user network; - at least one inlet opening for said at least one connection cable (51, 52) of said external communication network or connected to the external communication network; and - at least one outlet opening (60, 61) of said plurality of fibre optic connection elements;;