Should look something like this:
SELECT dbo.tblComputers.Computername, dbo.tblNetwork.IPAddress, dbo.tblNetwork.IPSubnet
FROM dbo.tblComputers INNER JOIN
dbo.tblNetwork ON dbo.tblComputers.Computername = dbo.tblNetwork.Computername
WHERE (dbo.tblNetwork.IPAddress <> '') AND (dbo.tblNetwork.IPAddress <> '0.0.0.0')