diff --git a/pkg/apis/kops/validation/helpers_test.go b/pkg/apis/kops/validation/helpers_test.go
new file mode 100644
index 0000000000..c1a9130ebb
--- /dev/null
+++ b/pkg/apis/kops/validation/helpers_test.go
@@ -0,0 +1,75 @@
+/*
+Copyright 2016 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package validation
+
+import (
+	"net"
+	"testing"
+)
+
+func Test_isSubnet(t *testing.T) {
+	grid := []struct {
+		L        string
+		R        string
+		IsSubnet bool
+	}{
+		{
+			L:        "192.168.1.0/24",
+			R:        "192.168.0.0/24",
+			IsSubnet: false,
+		},
+		{
+			L:        "192.168.0.0/16",
+			R:        "192.168.0.0/24",
+			IsSubnet: true,
+		},
+		{
+			L:        "192.168.0.0/24",
+			R:        "192.168.0.0/16",
+			IsSubnet: false,
+		},
+		{
+			L:        "192.168.0.0/16",
+			R:        "192.168.0.0/16",
+			IsSubnet: true, // Not a strict subnet
+		},
+		{
+			L:        "192.168.0.1/16",
+			R:        "192.168.0.0/24",
+			IsSubnet: true,
+		},
+		{
+			L:        "0.0.0.0/0",
+			R:        "101.0.1.0/32",
+			IsSubnet: true,
+		},
+	}
+	for _, g := range grid {
+		_, l, err := net.ParseCIDR(g.L)
+		if err != nil {
+			t.Fatalf("error parsing %q: %v", g.L, err)
+		}
+		_, r, err := net.ParseCIDR(g.R)
+		if err != nil {
+			t.Fatalf("error parsing %q: %v", g.R, err)
+		}
+		actual := isSubnet(l, r)
+		if actual != g.IsSubnet {
+			t.Errorf("isSubnet(%q, %q) = %v, expected %v", g.L, g.R, actual, g.IsSubnet)
+		}
+	}
+}