add more detials to parsing errors

types/ip.go

@@ -23,10 +23,10 @@ func (s *Subnet) UnmarshalJSON(in []byte) error {
 
 	ip, ipnet, err := net.ParseCIDR(string(in))
 	if err != nil {
-		return fmt.Errorf("not a valid subnet")
+		return fmt.Errorf("not a valid subnet: %#v", err)
 	}
 	if !ipnet.IP.Equal(ip) {
-		return fmt.Errorf("provided IP is not a subnet")
+		return fmt.Errorf("provided IP '%s' is not network address '%s' of declared subnet", ip.String(), ipnet.IP.String())
 	}
 	*s = Subnet(*ipnet)
 	return nil
@@ -40,6 +40,17 @@ func (s *Subnet) String() string {
 	return (*net.IPNet)(s).String()
 }
 
+func (i IP) Is4() bool {
+	if a := net.IP(i).To4(); a == nil {
+		return false
+	}
+	return true
+}
+
+func (i IP) Is6() bool {
+	return !i.Is4()
+}
+
 // Value implements the database Value interface.
 //
 // This function is needed so that a subnet can be inserted into