diff --git a/lib/ipaddr.rb b/lib/ipaddr.rb
index ad99782..be7b8c0 100644
--- a/lib/ipaddr.rb
+++ b/lib/ipaddr.rb
@@ -103,13 +103,13 @@ class InvalidPrefixError < InvalidAddressError; end
 
   # Creates a new ipaddr containing the given network byte ordered
   # string form of an IP address.
-  def IPAddr::new_ntoh(addr)
-    return IPAddr.new(IPAddr::ntop(addr))
+  def self.new_ntoh(addr)
+    return new(ntop(addr))
   end
 
   # Convert a network byte ordered string form of an IP address into
   # human readable form.
-  def IPAddr::ntop(addr)
+  def self.ntop(addr)
     case addr.size
     when 4
       s = addr.unpack('C4').join('.')