Add a subnet checker

subnet.py

@@ -0,0 +1,17 @@
+import ipaddress
+
+def zone_is_subset(re2o_reverse_zone, network):
+    """Check if all the subnets are inside another network
+
+    Examples
+    --------
+    An Example to check if the function works as intended
+
+    >>> net1 = ipaddress.IPv4Network("10.11.0.0/16")
+    >>> cidrs = ['10.11.1.0/24', '10.11.2.0/23', '10.11.4.0/22']
+    >>> zone = {'cidrs': cidrs}
+    >>> zone_is_subset(zone, net1)
+    True
+    """
+    return all(ipaddress.ip_network(n).subnet_of(network) for n in
+            zone["cidrs"])