[X4U] IP address on local network

[alexandre]

hi

i have three macs on my local network, connected via airport locally  
and to the internet via dsl.

on the mini (used as a media server) i have installed plex (www.plexapp.com 
  - great software!) to replace front row, telling it to stream music  
from my imac. the problem is that the imac local ip address keeps  
changing (apparently). it went for being 10.0.1.3 to 10.0.1.6… so plex  
can't find my music anymore.

is there any way within the airport setup utility to prevent this and  
to keep attributing fix ip addresses on my local network?

thanks,

alexandre
17" 2.33ghz ic2d macbook pro / 2.0gb / 160gb / X.5.6
24" 2.33ghz ic2d imac / 3.0gb / 1tb / X.5.6
2.0ghz ic2d mac mini / 4.0gb / 500gb / X.5.6
iphone 3G / white / 16gb
…and a few iPods.