I'm running Wireshark in Windows inside a virtual machine. Wireshark keeps reporting network activity, even though I don't have anything running in there that's accessing the network.
Windows constantly broadcasts NetBIOS node information, just as OSX constantly broadcasts mDNS (Bonjour) information.
If you have an ethernet network and any hosts running IP, then you will see occasional ARP (Address Resolution Protocol) messages, even if you aren't actively using the network.
ARP is the protocol that hosts use to figure out which IP addresses match to ethernet addresses (MAC).
Now host 1 knows that any packet which has 192.168.1.2 as its destination can be sent in an ethernet packet that has aa:bb:cc:dd:ee:ff as its destination.