Not too long ago, I have been having a strange problem with my Apple TV. The problem was not the device itself, but it's the connection to it from my iPad and Macbook Air. Sometimes AirPlay didn't work when the computers did not detect Apple TV even though it was on, and on the same LAN subnet. What I did was to switch off and on the WIFI radio on the computers to find ATV again.
I figured it might have been problem with the UDP broadcast that did not reach my computers. Before I purchased my ATV, I have been running with the 192.168.0.0/25 network. This means the broadcast address is 192.168.0.127. Thinking along this line, I verified that all devices have been set with subnet mask of 255.255.255.128 - on the computers, the network access storage server, Apple TV, router settings. All seemed OK but the problem still persisted.
Believing the broadcast message still didn't reach all my computers, I decided to configure the subnet mask to 255.255.255.0, CIDR 192.168.0.0/24. So starting with setting my router's LAN IP address, its DHCP server, and setting the router LAN IP address as one of the DNS servers in the DHCP configuration. Then I renewed the IP address on all the devices, starting with my NAS storage (this is using fixed IP actually), ATV, and finally the client devices.
I have been running in this setup a couple of days now. This seems to have solve the problem. Now my devices can find the ATV all the time, and AirPlay-ed all my music and video to TV.
Anyway, I will still need to monitor this for a long period before I can call this a 'problem solved!'