Index: wp-includes/comment-functions.php
===================================================================
--- wp-includes/comment-functions.php	(revision 4283)
+++ wp-includes/comment-functions.php	(working copy)
@@ -109,8 +109,6 @@
 	global $wpdb;
 	extract($commentdata);
 
-	$comment_user_domain = apply_filters('pre_comment_user_domain', gethostbyaddr($comment_author_IP) );
-
 	// Simple duplicate check
 	$dupe = "SELECT comment_ID FROM $wpdb->comments WHERE comment_post_ID = '$comment_post_ID' AND ( comment_author = '$comment_author' ";
 	if ( $comment_author_email )