Show only free IP addresses when selecting address to certain device?

There is one-to-one relationship between IP table and Device table.

IP table has ID (primary key) and four ip fields.
Device table has ipID field (foreign key) and other fields which are
not relevant to this matter.

The probles is that in the device form the ip dropdown now only shows
the reserved IPs, not the free IPs from IP table.

The query in dropdown is as follows:

SELECT tblIP.ID, [ip] & "." & [ip2] & "." & [ip3] & "." & [ip4] AS [IP-
number], tblDevice.ipID
FROM tblIP INNER JOIN tblDevice ON tblIP.ID = tblDevice.ipID
ORDER BY tblIP.ip, tblIP.ip2, tblIP.ip3, tblIP.ip4;

the result set shows only IP numbers that has assigned to device.

if I add condition
WHERE (((tblDevice.ipID) Is Null)), so I want a "negation" (show all
records in IP table that has not assigned to device (free), the result
set is empty)

Now what?

btw. Is it right to have IPs in four separate fields? Is there a
better way?

Sandroid