Add IP version check

net/ip.go

@@ -210,6 +210,9 @@ func (n Network) Contains(nn NetworkNumber) bool {
 
 // Contains returns true if Network covers o, false otherwise
 func (n Network) Covers(o Network) bool {
+	if len(n.Number) != len(o.Number) {
+		return false
+	}
 	nMaskSize, _ := n.IPNet.Mask.Size()
 	oMaskSize, _ := o.IPNet.Mask.Size()
 	return n.Contains(o.Number) && nMaskSize <= oMaskSize

net/ip_test.go

@@ -424,6 +424,8 @@ func TestNetworkCovers(t *testing.T) {
 		{"10.0.0.0/16", "10.0.0.0/15", false, "prefix false"},
 		{"10.0.0.0/15", "10.0.0.0/16", true, "prefix true"},
 		{"10.0.0.0/15", "10.0.0.0/15", true, "same"},
+		{"10::0/15", "10.0.0.0/15", false, "ip version mismatch"},
+		{"10::0/15", "10::0/16", true, "ipv6"},
 	}
 
 	for _, tc := range cases {