OK, here we go. I am making assumptions (which I shall state). Please clarify in these areas by EDITING YOUR QUESTION rather than commenting.
A1- The website works internally.
A2- You want the website to work across the internet.
A3- The domain "domainname.com" already exists on the internet.
First, you need to identify an external IP address for the extranet, and you need to setup NAT on your firewall to router traffic from that external IP address to the actual server. How this is done depends on the firewall, but is a standard firewall function. Look in the firewall documentation, on their site, or search here for NAT and the firewall.
Then, you want to add a CNAME record to the internal DNS to map "extranet.domainname.com" to "extranet" (or "extranet.domainname.local"). This should make the FQDN work internally.
Finally, you need to add an "A record" to the "domainname.com" internet DNS zone mapping "extranet.domainname.com" to the external IP address created above. Whoever administers the "domainname.com" domain can do this. This should (after a delay of a few hours) make the external access work properly.
Hope this helps.