the invention will now be described in greater detail referring first to fig1 . the electronic module 1 consists of a printed circuit board 2 , a lower housing 3 and an upper housing 4 . a data distribution device , for example a hub or a switch is located on the printed circuit board 2 . a plurality of connections 5 , here four connections which are designed as universal module locations 6 , are also provided on the printed circuit board 2 . the four connections 5 are , in each case , a respective network port which is correspondingly connected to the data distribution device . the printed circuit board 2 has a respective power supply connection 7 , 8 on opposing sides and plug connectors 9 , 10 for the databus line on opposing sides . the plug connectors 9 , 10 are complementary , just as the connectors 7 , 8 are complementary . this is the case so that a plurality of electronic modules can be laterally butt mounted and connected to each other . the lower housing 3 has a recess 11 in its bottom . this recess 11 serves to receive a mounting rail such as a top hat rail , on which the electronic module 1 can be mounted . openings 12 , 13 are also provided in the side walls of the lower housing 3 . whereas the openings 12 serve to receive the power supply connections 7 , 8 , the openings 13 are suitable for receiving the plug connectors 9 , 10 for the databus . the upper housing 4 has , in its front region , four universal module locations 6 for each connection 5 . in the rear region , it is possible to provide various indicators 15 . the universal module locations 6 each have a coded key way 14 . fig2 shows a fully assembled electronic module 1 . the four universal module locations 6 with the coded key way 14 are arranged in the front region of the module 1 . the power supply connections 7 and the plug connector for the databus 9 are located on the side of the module 1 . in the rear region of the module 1 , indicators 15 are provided . the universal module locations 6 of the four connections 5 have a plurality of contacts 16 which are used depending on the design of the connection module . fig3 and 4 show two electronic modules 1 , 1 ′ butt mounted on a top hat rail 17 . they can be moved along the top hat rail to mate databus plug connectors 9 and the power supply connectors 7 on the adjacent modules . as shown in fig3 four connection modules 18 , 19 , 20 , 21 are already latched onto the left electronic module 1 and only one connection module 22 is latched onto the right electronic module 1 ′. three further connection modules 22 can still be placed as necessary from above on to the corresponding module locations 6 ′. the connection modules 18 , 19 , 21 differ from one another . the module 21 is an optical module which has an active component for converting the optical signal into an electric signal . the connection modules 18 - 20 are modules which have a modular jack plug connector and are suitable for receiving an rj45 plug connector . the module 18 differs from the modules 19 and 20 in that it additionally has a connection for feeding in the supply current . the modules 22 are identically constructed . they are rapid connection modules which have a flap lid 23 and an insulation displacement contact 24 for contacting the conductor of a cable . a cable strain relief device 25 is also provided . these modules can be designed for the connection of various cables . different physical designs of cables can be used , depending on the application , in each case . the system is suitable both for copper and for fiber optic cables depending upon the module selected . a plurality of connection modules may be connected to each electronic module . these connection modules can be attached to the connections of the electronic modules and mechanically fastened there to produce an electrical connection between an optical or any electrical cable . a plurality of electronic modules can be butt mounted to a system . it is therefore possible , to produce , for example star hubs with optionally 4 , 8 , 12 , etc . connections . each electronic module has a universal module location per connection . depending on the connection module used , it produces a connection to specific contacts of the module location . it is an advantage for differentiating between the function - specific connection modules to provide a coding of the connections or the module locations , and connection modules . it is also an advantage to provide the electronic modules with a screen . this can be effected by means of a coated plastic housing , by means of a metal housing or by means of screening plates . each electronic module can be contacted by means of a plurality of connection modules . the connection modules here are particularly advantageously capable of latching onto the electric module . the connection modules can be very different in design . purely passive connection modules are also possible which have , for example , an rj45 socket - outlet for receiving a corresponding plug which is connected to the cable . it is also possible to provide connection modules for directly clamping - on installation cables , for example insulation displacement contacts . there can be connection modules of this type for various cable types . an integrated screen cover and a strain relief device can also be provided in the process . however , active connection modules are also within the scope of the invention . an optical transceiver can be located therein with an mtrj connection to which an optical fiber can be attached . there can still be alternatives here in that multimode , single mode or also plastic fibres can be attached . an extension of the modular connection system is possible , for example with special modules . it is advantageous that the power supply can be butt mounted to the electronic modules in a special module . it is also advantageous to provide special modules with components with diagnostic functions or network management functions . butt mountable special modules such as isdn routers , couplers to other networks and so on can also be provided . it is also advantageous to provide special connection modules , such as media converters to special transmission media .