From your post it appears you are behind a One-to-Many NAT[Network Address Translation]
What you want can only be done at the ISP's end at the gateway or router.They have to manually configure port forwarding port 80 inbound to your internal IP.
Unless & untill they do this at their end there is no possibility you could host a http site for the public.It will be only confined to the hosts within the internal LAN.