I’m not sure why you didn’t find a few things “wrong” with your mod_rewrite among the review threads, though.
First, the addon domain has its own directory structure - generally at the main domain’s {addondomain} subdirectory. What that means to you is that any request to the addon domain will (okay, SHOULD - unless you parked it on top of the main domain) bypass the main domain’s .htaccess file.
Options +FollowSymLinks
# Your host should already have this in the httpd.conf - if mod_rewrite is enabled.
# Check on mod_rewrite being enabled by using the simple test in my signature
RewriteEngine on
# :tup: That ensures that mod_rewrite is NOT in comment mode
RewriteCond %{HTTP_HOST} ^(www\\.)?myaddondomain\\.net$ [NC]
# :tup: checking on the addon domain name - but that restricts the
# following from acting on the maindomain/subdomain-directory request
RewriteRule ([0-9]+)/[COLOR="Gray"](.*)/[/COLOR]$ index.php?page_id=$1 [COLOR="Red"][L][/COLOR]
# Why bother capturing the "garbage" after the first /
# (unless you will accept 0// or ...)?
# What? No Last flag! Horrors! :rolleyes:
# I KNOW your PHP won't work if you don't terminate a statement -
# so don't do that to mod_rewrite!
Okay, MINOR nags about your code as, in general, it appears that you know what you’re doing (with the exception of the :kaioken: EVERYTHING :kaioken: atom - (.*) which IS intended to match no/all garbage so it’s only purpose is to force two /'s after your page_id digits).