Check for exact IP instead of leaving away quads

The old check did only properly work if the given CIDR was a multiple
of 8. In every other case an improper assumption was made.

For example, if first a range of 10.0.0.0/20 was added and later existance
of the range 10.0.16.0/20 was tested, the result would wrongfully be true.

Thereby, checking whether the given IP exists, is much more precise. Still,
converting the IP range to a list of single IPs and checking for their
existance would be best but rather complicated.