diff --git a/CHANGELOG.md b/CHANGELOG.md
index 60bc04b..f9f7998 100755
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 
 - New method `matches($ip, $mask)` to perform wildcard mask matching common in network Access Control Lists and OSPF dynamic routing [#51](https://github.com/rlanvin/php-ip/pull/51)
 - IPv4Block: Allow to specify the prefix also as an old-style netmask [#53](https://github.com/rlanvin/php-ip/pull/53)
+- New method `IPv4::isNetmask()` tests whether the IPv4 address is a valid subnet mask. [#55](https://github.com/rlanvin/php-ip/pull/55)
 
 ## [2.0.0] - 2019-09-01
 
diff --git a/src/IPv4.php b/src/IPv4.php
index 6acb33a..99df9aa 100644
--- a/src/IPv4.php
+++ b/src/IPv4.php
@@ -50,6 +50,11 @@ class IPv4 extends IP
      */
     protected static $link_local_block = '169.254.0.0/16';
 
+    /**
+     * @var bool
+     */
+    protected $is_netmask;
+
     /**
      * {@inheritdoc}
      */
@@ -73,4 +78,26 @@ public function reversePointer(): string
 
         return implode('.', $octets).'.in-addr.arpa.';
     }
+
+    /**
+     * Return true if the IP address is a valid sub network mask, for instance: '255.255.252.0'.
+     *
+     * @return bool
+     */
+    public function isNetmask(): bool
+    {
+        if ($this->is_netmask !== null) {
+            return $this->is_netmask;
+        }
+
+        if (gmp_cmp($this->ip, 0) === 0) {
+            $this->is_netmask = true;
+        } else {
+            $y = $this->bit_negate()->plus(1);
+            $z = $this->bit_negate()->bit_and($y);
+            $this->is_netmask = gmp_cmp($z->ip, 0) === 0;
+        }
+
+        return $this->is_netmask;
+    }
 }
diff --git a/tests/IPv4Test.php b/tests/IPv4Test.php
index 1d39b55..616685c 100644
--- a/tests/IPv4Test.php
+++ b/tests/IPv4Test.php
@@ -219,6 +219,27 @@ public function getMatchesData(): array
         return $data;
     }
 
+    /**
+     * Data provider for testIsNetmask().
+     *
+     * @return array
+     */
+    public function netmaskProvider(): array
+    {
+        return [
+            //IP address        is valid netmask
+            ['0.0.0.0',         true],
+            ['64.0.0.0',        false],
+            ['128.0.0.0',       true],
+            ['127.0.0.0',       false],
+            ['255.255.0.0',     true],
+            ['255.0.255.252',   false],
+            ['255.255.255.0',   true],
+            ['255.255.255.224', true],
+            ['255.255.255.252', true],
+        ];
+    }
+
     /**
      * @dataProvider validAddresses
      */
@@ -517,4 +538,15 @@ public function testDefaultMaskValueMatchesEntireIpAddress()
         $this->assertTrue($ip->matches('172.16.3.5'));
         $this->assertFalse($ip->matches('172.16.3.4'));
     }
+
+    /**
+     * @dataProvider netmaskProvider
+     *
+     * @param $ip
+     * @param bool $expectation
+     */
+    public function testIsNetMask($ip, bool $expectation)
+    {
+        $this->assertEquals($expectation, IPv4::create($ip)->isNetmask());
+    }
 }