IP addresses are unique, there are some address domains that are private.
10.0.0.0 to 10.255.255.255 and 192.168.0.0 to 192.168.255..255
are private domains. MILLIONS of people use these numbers and are NEVER seen by the public.
If you are on the same network, meaning the same building or floor or room, then the IP address you are typing will work. You are connected through the SAME router.
IF you are not in the same room or floor or building, you need an IP address that is UNIQUE. You pay for this.
OR you tell your router to send requests about Filemaker to the you Filemaker Computer (www.portforward.com)