feat: add a function to format IPs in CIDR notation

This is frequently used in Talos.

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>

net.go

@@ -57,6 +57,13 @@ func FormatAddress(addr string) string {
 	return addr
 }
 
+// FormatCIDR formats IP from the network as CIDR notation.
+func FormatCIDR(ip net.IP, network net.IPNet) string {
+	ones, _ := network.Mask.Size()
+
+	return fmt.Sprintf("%s/%d", ip, ones)
+}
+
 // AddressContainsPort checks to see if the supplied address contains both an address and a port.
 // This will not catch every possible permutation, but it is a best-effort routine suitable for prechecking human-interactive parameters.
 func AddressContainsPort(addr string) bool {

net_test.go

@@ -43,6 +43,17 @@ func TestFormatAddress(t *testing.T) {
 	assert.Equal(t, talosnet.FormatAddress("alpha.beta.gamma.com"), "alpha.beta.gamma.com")
 }
 
+func TestFormatCIDR(t *testing.T) {
+	ip4 := net.ParseIP("192.168.1.1")
+	_, cidr4, _ := net.ParseCIDR("192.168.0.0/16") //nolint: errcheck
+
+	ip6 := net.ParseIP("2001:db8::1")
+	_, cidr6, _ := net.ParseCIDR("2001:db8::/32") //nolint: errcheck
+
+	assert.Equal(t, talosnet.FormatCIDR(ip4, *cidr4), "192.168.1.1/16")
+	assert.Equal(t, talosnet.FormatCIDR(ip6, *cidr6), "2001:db8::1/32")
+}
+
 //nolint: scopelint
 func TestNthIPInNetwork(t *testing.T) {
 	type args struct {