There are apple apps that will only work on WiFi. Airplay, Itunes share, and facetime are a few off the top of my head.
DHCP (Distributed Dynamic Host Configuration Protocol)
It is a method of sending information from one IP address to another. A simpler description is, a method used by your router to "talk to your computers". It doesn't matter if you use WiFI or Ethernet, both use DHCP. Normally, this setting is left to the router to assign. You can place your own in, providing you know the range of numbers you can use.
This is territory that is best left to ones that are familiar to setting up a router. There is a strong possibility of setting the wrong setting and you will have no network. Fortunately it is not dependant on the OS so if you have someone in the know, wether they are MAC or PC doesn’t matter.
This is getting way off of track concerning your problem. Did you try turning off your WiFi and see if the problem persists. Interference from something as simple as a microwave can affect your single (under certain circumstances!) You can turn it on and off without it affecting anything. So if you choose to facetime, turn on WiFi and be ready to smile.
Danarchy commented: I agree about Spotlight not being a very thorough finder. +6