diff --git a/README.md b/README.md
index 237b161..2f9f1a3 100644
--- a/README.md
+++ b/README.md
@@ -494,25 +494,56 @@ Use function variable when you need to test a package level function:
 func GetIPNetDeviceByName(ifaceName string) (v4IPNet *net.IPNet, v6IPNet *net.IPNet, link *net.Interface, err error) {
 	link, err = net.InterfaceByName(ifaceName) // External dependency makes it hard to test directly.
 	...
+	addrList, err := link.Addrs() // Calling original method of link makes it hard to test with custom output.
+	...
 }
 
 // Declare a package level function variable.
-var interfaceByName = net.InterfaceByName
+var(
+	interfaceByName = net.InterfaceByName
+	netInterfaceAddrs = (*net.Interface).Addrs
+)
 
 func GetIPNetDeviceByName(ifaceName string) (v4IPNet *net.IPNet, v6IPNet *net.IPNet, link *net.Interface, err error) {
-	link, err = interfaceByName(ifaceName)  // Use the variable for actual call. 
+	// Use the variables for actual call.
+	link, err = interfaceByName(ifaceName)
 	...
+	addrList, err := netInterfaceAddrs(link)
+	...
+}
+
+func mockNetInterfaceByName(testNetInterface *net.Interface, err error) func() {
+	// Mock the variable for test.
+	originalNetInterfaceByName := netInterfaceByName
+	netInterfaceByName = func(name string) (*net.Interface, error) {
+		return testNetInterface, err
+	}
+	return func() {
+		netInterfaceByName = originalNetInterfaceByName
+	}
+}
+
+func mockNetInterfaceAddrs(testNetInterfaceAddrs []net.Addr, err error) func() {
+	// Mock the method for test.
+	originalNetInterfaceAddrs := netInterfaceAddrs
+	netInterfaceAddrs = func(i *net.Interface) ([]net.Addr, error) {
+		return testNetInterfaceAddrs, err
+	}
+	return func() {
+		netInterfaceAddrs = originalNetInterfaceAddrs
+	}
 }
 
 func TestGetIPNetDeviceByName(t *testing.T) {
 	tests := []struct {
-		name          string
-		interfaceName string
-		interface     *net.Interface
-		wantV4IPNet   *net.IPNet
-		wantV6IPNet   *net.IPNet
-		wantLink      *net.Interface
-		wantErr       error
+		name           string
+		interfaceName  string
+		interface      *net.Interface
+		interfaceAddrs []net.Addr
+		wantV4IPNet    *net.IPNet
+		wantV6IPNet    *net.IPNet
+		wantLink       *net.Interface
+		wantErr        error
 	}{
 		{name: "case 1", ...},
 		{name: "case 2", ...},
@@ -521,13 +552,9 @@ func TestGetIPNetDeviceByName(t *testing.T) {
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
-			// Mock the variable for test.
-			interfaceByName = func(name string) (*net.Interface, error) {
-				return tc.interface
-			}
-			defer func() {
-				interfaceByName = net.InterfaceByName
-			}()
+			// Set mocks, defer the returned functions for reset.
+			defer mockNetInterfaceByName(tc.interface, nil)()
+			defer mockNetInterfaceAddrs(tc.interfaceAddrs, nil)()
 			gotV4IPNet, gotV6IPNet, gotLink, gotErr := GetIPNetDeviceByName(tc.name)
 			assert.Equal(t, tc.wantV4IPNet, gotV4IPNet)
 			assert.Equal(t, tc.wantV6IPNet, gotV6IPNet)