If the Ubuntu VM is the only publicly available webserver on your LAN, then you can portforward all incoming requests for TCP/80 on your firewall/router to 192.168.100.136.
You have a problem when you need more internal hosts to be publicly available webservers. You need to either get more public IP addresses, or set up the 192.168.100.136 webserver to proxy all requests to different webservers, depending on the Host header of the incoming request.
This is called a reverse-proxy, and is quite easy to setup with Apache. The downside is that the load on the 136 webserver will increase because it proxies all traffic. The upside is that it can serve as a cache as well, reducing the load on the other webservers. Speaking of which...
Another option, similar to the Apache reverse-proxy, is installing a load-balancer, such as Varnish, and portforward your incoming TCP/80 traffic to that load-balancer. In the load-balancer you can define rules to define which requests are handled by which internal servers. In effect this is a reverse-proxy as well, but load-balancers such as Varnish are more advanced in their caching methods and configuration.