That information is stored in the database and you can use the following data explorer query to retrieve the real user behind a post. Using IP addresses to find the user is not accurate.
-- [params]
-- int :topic_id
-- int :post_number
SELECT u.id AS user_id, p.id AS post_id
FROM users u
LEFT JOIN anonymous_users ua ON ua.master_user_id = u.id
LEFT JOIN posts p ON ua.user_id = p.user_id
WHERE p.topic_id = :topic_id AND p.post_number = :post_number