To be clear, I'm not looking for someone to design an entire forum here, rather, I need some help determining how to relate a couple of tables, and whether or not these tables need to exist in the first place.

Basics of a forum

Topics

topic_id // Unique topic ID, AI, PK
topic_name // name of the topic
... etc

• Topics
• Posts in these topics (which is what this is here, a post
• Replies to posts, which is what the responses below are (your responses that is)

Users

user_id // Unique ID of the user, PK, AI
user_name
user_pass
user_email
user_date

Posts

post_id // unique ID of post - PK, AI
post_content
... etc

I also want to include the ability to like / dislike a post, and for that reason I created a table post_ranking which looks as follows

id // Auto increment ID    
post_id  // Foreign key, refers to post.post_id
user_id  // Foreign key, refers to user.user_id
vote_up  // Whether or not the post was voted up (0 for down, 1 for up)
rank_date // date the ranking occured

The problem I've just run into is that I cannot figure out how to incorporate the replies into this table. The only option I've thought of so far would be to create a second table called reply_ranking - but wouldn't this be somewhat unorganized?

So, my question:

Should I create two separate tables for rankings (post_ranking , reply_ranking) or is there another way that I should be arranging the tables above which would avoid this problem in the first place?