A Network card for as long as I can remmber either has a RJ45 Socket (pretty Standard), BNC or one that looks like a Many port plug as the piccy:
Now it just so happens a RJ11 Socket might to some look like a Network Socket:
These are normally on the Laptop / PC for 56K Modems, ADSL etc..
I have a feeling you are trying to plug in a Network Lead into a RJ11 or indeed the Other way around !!