The posts on phpBB boards are already enumerated; the enumeration is just not visible. To the left of the word "Posted:" and the date, there is a little icon that looks like a piece of paper. If you drag your mouse cursor over that icon, and if your internet browser is set to display the web address you're about to link to, then you'll see the URL for that particular post at the bottom of your browser or wherever the URL is displayed. The last numbers will be the number of the post.
For instance, the number for the message that began this thread is #6216, and you can reach it via the URL
http://forum.enjoysudoku.com/viewtopic.php?p=6216#p6216 from anywhere.
You can also locate a particular enumerated topic by dragging your cursor over the name of the thread. This particular thread is #1111, so you can reach it by linking to
http://forum.enjoysudoku.com/viewtopic.php?t=1111Rather than referring to the topic or message by number, it works best to type the URL into the message, either as I did above or by using the BBCode for it, like so:
click to see earliest existing message on the board. That way people don't have to memorize the whole address just to get to the message. Call it netiquette, I guess.