I have Apache in my Linux VM on Windows .... While trying to map ip address to fqdn .I don't know which ip address to use in port forwarding (host /guest/localhost) , in /etc/host and a complete process of mapping. My IP address in ifconfog is 10.0.2.15

Wikipedia has some fairly good explanations of Network Address Translation and port-forwarding.
Please take a little time to look into LAN, WAN, Internet and other terms with IP version 4 in mind.

i have tried with almost everything from google and youtube. still not clear with things made me post it to this site

Just please read what NAT means, what port-forwarding is and what DHCP and DNS are. There's no need to remember the numbers and calculations given, unless you want a career in networking, but you need to understand what a LAN is, what a NAT does and why that is important as well as taking in as much as you can about IP addresses in general. If you really annot get to the bottom of it then there are courses to take and people to pay to learn but it really is not that complicated.

First of all, private address space, i.e. 10.0.0.0/8, have no fqdn associated with them. So, your are missing information that is fundamental to Linux administration, and which is very easy to find online.

To reinforce what 273 has noted, you must learn at the very least: DNS, NAT, DHCP, and private address space. You'll find it's very simple just using google.

Maybe like this [?]: https://www.linuxquestions.org/quest...8/#post5750940

Actually it is not a question, not a request and even it is not a problem - from my side. If you have any problem, question, issue please post it including all the related information. Otherwise I can hardly help you because most probably you have already tried what I'm going to tell you.

There may be a slight confusion here maybe. The VM has a network configured by the name/term/phrase of "NAT".
Should you want to access via internet...
You also need to use that term for the router or modem access to wan. What you need to do at router is translate the wan IP and port to the lan side. That is called NAT but really it's PAT as well. Port address. By the way, you also need to consider if you have a dynamic ip address and if your isp blocks ports.

I might be tempted to change the vm to bridged.

Two places of concern on NAT or bridged choice in vm exit. One is router and two is host. Might as well add in client linux. All need to be in line for both IP addresses and port open and translated as well as any firewall based on programs.