Brian is absolutely right and I concur 100%.  However, more to your question, Joe,  They might change addresses once in a while because they are inactive for a time and which ever box is acting as your dhcp server is putting that address back in to the pool of available IP addresses.  Normally, dhcp will see the mac address of your network interface card (nic) and see if your previous address is available and if it is, will issue it to that machine again.  The duration of the time an ip address will be assigned to any particular machine is determined by a programmable variable called the "license".  The administrator is the one who determines this duration.  Normally it's a matter of days.  Usually three days is the shortest.  The longest? Who knows.  It's up to the administrator.  But that's why they are changing on you.  Again, unless you have a small IP address pool available to you, which you do not if you are using a private ip range as stated in the RFC's that pertain to TCP/IP and dhcp, then just go into your network settings, set them to static, lean back and cruise.  Then they will never mysteriously change on you again.  They will remain the same for ever and ever and ever...etc...etc.

Tim