Lion can't see other iOS and OSX devices on the network Context

At my house I have one network, and have my iOS 5 iPad and Lion Mac connected to it.

At my house, these two devices can see each other on the network.

At my girlfriend's house, she has a Snow Leopard Mac, and an Apple Tv 2 with the latest software updates. Both devices are connected to her network.

The Apple Tv 2 is very clunky with home sharing and can't always see the Snow Leopard Mac. However her Mac can always see the Apple Tv and AirPlay is available in iTunes.

The Issue

The Issue is that when I take my iPad and Lion mac to hers, I can connect to the network just fine, but it's like my iPad and Mac are on a different network or "silo" because I can't see any other devices.

The main problem is that I can't AirPlay to her apple tv from my mac nor my ipad. That's the main issue. But also my mac can't see my ipad, and vice verse, and even with file sharing enabled on both macs (lion and snow leopard) I can't see her mac in finder, and she can't see mine.

Any ideas?

thanks

Update

Just did a reboot on my girlfriend's mac, and now she can't see the Apple Tv either...?? Just did a "find all devices ping" using


  ping 192.168.xxx.255 (where 255 is the subnet mask value)


The results were that my mac only returned my ip, and her mac only returned her ip...?

whats going on?
Turns out I had to just had to reboot the router... That fixed everything.

Still, the question remains, why?