I think that we are really going to need more information than this. My first guess is that you are about to connect a network line to a modem's phone line, which would be a bad idea, but I can't really tell.
First, please describe your "Before" case better. Do you have
o A phone line from the wall with an RJ11 plate
o A modem with an RJ11 socket on the input side
o What is the output connection from the modem, and what do you connect it to? Directly to the computer? To a router or hub?
Second, what part of the above does the new RJ45 connection replace? Is it your wall outlet? The connection on your computer? The input to the modem? The output from the modem?
A picture or three would help if it's easy for you to post...