I am trying to connect to my headless RaspberryPi using SSH via putty but have some networking problems, although it used to work sometimes.

The Pi is running Raspbian and is connected to my router with an ethernet cable. In '/boot/cmdline.txt' I have added:

ip=[RandomIP] 

But when I ping that IP with a laptop (connected with W-Lan, so it should be the same network) to check it, it doesn't respond and with putty I of course get a timeout error.

Unfortunately my Router has no DHCP so I can't check if the IP really is the one I've set, but I actually think so.

I have already set up Raspbian again several times, but didn't get any further. Also I tried connecting it directly to my laptop - Again, first it worked, but after reinstalling Raspbian (for other reasons) I had the same problem.