diff --git a/google/resource_container_cluster.go b/google/resource_container_cluster.go
index 6f6581abaa2..d9b21af3cde 100644
--- a/google/resource_container_cluster.go
+++ b/google/resource_container_cluster.go
@@ -936,7 +936,6 @@ func flattenMasterAuthorizedNetworksConfig(c *container.MasterAuthorizedNetworks
 				"display_name": v.DisplayName,
 			})
 		}
-		// now does this adhere to the schema?
 		result["cidr_blocks"] = cidrBlocks
 	}
 	return []map[string]interface{}{result}
diff --git a/google/resource_container_cluster_test.go b/google/resource_container_cluster_test.go
index 38dab58ebd1..5201b9b8121 100644
--- a/google/resource_container_cluster_test.go
+++ b/google/resource_container_cluster_test.go
@@ -137,8 +137,8 @@ func TestAccContainerCluster_withMasterAuthorizedNetworksConfig(t *testing.T) {
 				Config: testAccContainerCluster_withMasterAuthorizedNetworksConfig(clusterName, []string{"8.8.8.8/32"}),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckContainerCluster("google_container_cluster.with_master_authorized_networks"),
-					resource.TestCheckNoResourceAttr("google_container_cluster.with_master_authorized_networks",
-						"master_authorized_networks_config.0.cidr_blocks"),
+					resource.TestCheckResourceAttr("google_container_cluster.with_master_authorized_networks",
+						"master_authorized_networks_config.0.cidr_blocks.#", "1"),
 				),
 			},
 		},
@@ -943,7 +943,7 @@ resource "google_container_cluster" "with_master_authorized_networks" {
 
 	master_authorized_networks_config {
 		%s
-    }
+	}
 }`, clusterName, cidrBlocks)
 }
 
diff --git a/google/validation_test.go b/google/validation_test.go
index e76b954e022..4b9a5c91236 100644
--- a/google/validation_test.go
+++ b/google/validation_test.go
@@ -28,12 +28,45 @@ func TestValidateGCPName(t *testing.T) {
 	}
 }
 
+func TestValidateRFC1918Network(t *testing.T) {
+	x := []RFC1918NetworkTestCase{
+		// No errors
+		{TestName: "valid 10.x", CIDR: "10.0.0.0/8", MinPrefix: 0, MaxPrefix: 32},
+		{TestName: "valid 172.x", CIDR: "172.16.0.0/16", MinPrefix: 0, MaxPrefix: 32},
+		{TestName: "valid 192.x", CIDR: "192.168.0.0/32", MinPrefix: 0, MaxPrefix: 32},
+		{TestName: "valid, bounded 10.x CIDR", CIDR: "10.0.0.0/8", MinPrefix: 8, MaxPrefix: 32},
+		{TestName: "valid, bounded 172.x CIDR", CIDR: "172.16.0.0/16", MinPrefix: 12, MaxPrefix: 32},
+		{TestName: "valid, bounded 192.x CIDR", CIDR: "192.168.0.0/32", MinPrefix: 16, MaxPrefix: 32},
+
+		// With errors
+		{TestName: "empty CIDR", CIDR: "", MinPrefix: 0, MaxPrefix: 32, ExpectError: true},
+		{TestName: "missing mask", CIDR: "10.0.0.0", MinPrefix: 0, MaxPrefix: 32, ExpectError: true},
+		{TestName: "invalid CIDR", CIDR: "10.1.0.0/8", MinPrefix: 0, MaxPrefix: 32, ExpectError: true},
+		{TestName: "valid 10.x CIDR with lower bound violation", CIDR: "10.0.0.0/8", MinPrefix: 16, MaxPrefix: 32, ExpectError: true},
+		{TestName: "valid 10.x CIDR with upper bound violation", CIDR: "10.0.0.0/24", MinPrefix: 8, MaxPrefix: 16, ExpectError: true},
+		{TestName: "valid public CIDR", CIDR: "8.8.8.8/32", MinPrefix: 0, MaxPrefix: 32, ExpectError: true},
+	}
+
+	es := testRFC1918Networks(x)
+	if len(es) > 0 {
+		t.Errorf("Failed to validate RFC1918 Networks: %v", es)
+	}
+}
+
 type GCPNameTestCase struct {
 	TestName    string
 	Value       string
 	ExpectError bool
 }
 
+type RFC1918NetworkTestCase struct {
+	TestName    string
+	CIDR        string
+	MinPrefix   int
+	MaxPrefix   int
+	ExpectError bool
+}
+
 func testGCPNames(cases []GCPNameTestCase) []error {
 	es := make([]error, 0)
 	for _, c := range cases {
@@ -55,3 +88,26 @@ func testGCPName(testCase GCPNameTestCase) []error {
 
 	return es
 }
+
+func testRFC1918Networks(cases []RFC1918NetworkTestCase) []error {
+	es := make([]error, 0)
+	for _, c := range cases {
+		es = append(es, testRFC1918Network(c)...)
+	}
+
+	return es
+}
+
+func testRFC1918Network(testCase RFC1918NetworkTestCase) []error {
+	f := validateRFC1918Network(testCase.MinPrefix, testCase.MaxPrefix)
+	_, es := f(testCase.CIDR, testCase.TestName)
+	if testCase.ExpectError {
+		if len(es) > 0 {
+			return nil
+		}
+		return []error{fmt.Errorf("Didn't see expected error in case \"%s\" with CIDR=\"%s\" MinPrefix=%v MaxPrefix=%v",
+			testCase.TestName, testCase.CIDR, testCase.MinPrefix, testCase.MaxPrefix)}
+	}
+
+	return es
+}