How do I manually assign IP addresses to each of my machines so they won’t change when they reboot?

The reason I want to do this is because I use BOINC Tasks to manage my systems; unfortunately, when my systems reboot they tend to change IP addresses. Each time they change, I have to change the information in BOINC Tasks to match. This is becoming a pain since I have to change all of the remote_hosts.cfg on each machine as well.