mac computers bounced off line, PC's are ok

we have 4 computers, 2 macs and 2 pcs. all were working perfectly this morning and suddenly the 2 macs were bounced off line.

here are the steps i have taken...

1. rebooted router/modem and restarted all machines

2. renewed lease in tcp/ip settings

i did receive this message at one point on one of the macs- "another device on the network is using your computer’s ip address"

this may or may not be related but our network printer stopped working also.

please help!