Broadband as supplied in the uk over a standard telephone needs a broadband filter to split the broadband signal from the regular telephone signal.
Where the line is connected to one telephone and one broadband modem or router, things are easy enough, the filter has a phone plug that goes into the phone socket in the wall and a phone socket for the telephone. Another socket on the filter is for the modem or router. The plugs are different shapes and can only be plugged in one way round.
Things can be a little more difficult when there are more than one phone on the line.
Any phone must be behind a filter unit. Any filters must be connected directly into the phone line as it emerges from the wall - this could be on the end of an extension lead, but it is better if it is plugged directly into the wall socket. An extension could be used to carry the phone line to one or more phones behind the filter.
When working out how many filters you might need, count any existing devices that plug into the phone line as phones - these include, phones, fax machines and satellite video systems. Computers that have modems will no longer need to be connected unless they are to be used to send and receive faxes via the phone line - if so, they will also need to be connected via a filter.
If you have things sharing the phone line via a 'T' adaptor in the wall, then plug a filter into the wall and the 'T' adaptor behind it. In this way, one filter should be able to service several phones or devices. Extra filters will only be required for phones plugged into wall sockets in other rooms.
You can only connect one Broadband Modem or Router to a phone line. To share the Broadband, you should use a wired or wireless Router and network the computers. It is possible to network computers and 'share' a Broadband or Dialup Modem, but with a Broadband connection, it is much better to use a Router and let it share the Broadband connection.