"free" ip number when I remove device from database?

Access 2007. Problem is when I delete the device (which has foreign
key, ipID) from device table the IP number from IP
table (has ID, primary key) also deletes (relation 1:1). I want to
"free" ip number when I remove device from database so IP number is
not deleted and can be used later. How to accomplish this?

Form looks for "free" addresses as follows:

SELECT tblIP.ID, [ip] & "." & [ip2] & "." & [ip3] & "." & [ip4] AS
[IP-
number], tblDevice.ipID
FROM tblIP LEFT JOIN tblDevice ON tblIP.ID = tblDevice.ipID
ORDER BY tblIP.ip, tblIP.ip2, tblIP.ip3, tblIP.ip4
WHERE (((tblDevice.ipID) Is Null));