have a SMC 7904BRA2 router at home, and send documents / files to my iphone using "air sharing". I notice the IP address always changes i.e. one day its 192.168.1.103, and today its 105. Gets a little annoying.

Any way to fix the IP address ( good for controlling use of connections as well - i.e. auto shut off daughters connection when it is bed time )

Turn off DHCP in the router and set all IP's on your devices manually.

If the router is good enough you can set time of day access per IP or as a whole as well as block/filter connections to certain addresses.