diff --git a/lib/ipaddress/ipv6.rb b/lib/ipaddress/ipv6.rb
index 1601a01..dc006b3 100644
--- a/lib/ipaddress/ipv6.rb
+++ b/lib/ipaddress/ipv6.rb
@@ -384,7 +384,7 @@ def size
     #     #=> false
     #
     def include?(oth)
-      @prefix <= oth.prefix and network_u128 == self.class.new(oth.address+"/#@prefix").network_u128
+      @prefix <= oth.prefix and network_u128 == (oth.to_u128 & @prefix.to_u128)
     end
 
     #