diff --git a/stdlib/ipaddr/0/ipaddr.rbs b/stdlib/ipaddr/0/ipaddr.rbs
index 600c53524..49f606431 100644
--- a/stdlib/ipaddr/0/ipaddr.rbs
+++ b/stdlib/ipaddr/0/ipaddr.rbs
@@ -276,6 +276,14 @@ class IPAddr
   #
   def native: () -> IPAddr
 
+  # <!--
+  #   rdoc-file=lib/ipaddr.rb
+  #   - netmask()
+  # -->
+  # Returns the netmask in string format e.g. 255.255.0.0
+  #
+  def netmask: () -> String
+
   # <!--
   #   rdoc-file=lib/ipaddr.rb
   #   - prefix()