You don't have to do this, but I have an old spare rounter sitting around and I also have my main router.
I am gonna leave the PS3 connected to this router all the time(the spare one), but since this router has no internet access there is no harm. Whenever I want to transfer stuff from my laptop to the PS3 I will just let the laptop connect to the spare router, transfer the files and then connect back to my main router which actually has internet.
Again, this is a long way of doing things, but that's just me. The easier solution is to set a static IP to the PS3 with a fake gateway.