Static IP for home network

HI,

Is it possible to give devices on a home network their own IP address ?
I have an Eircom Zxyel router that has my Sky box and an android set top box connected by ethernet cable.
I used my phone as a remote for XBMC on the Set top Box but every so often the IP address of the box changes so I have to change the setting on my phone.

Not a major issue, more of a nuisance but ideally I'd want the box to have a constant IP so other less savy people in the house don't have to worry about the settings changing.