Suppose that i have a web server behind a nat. can users in the public internet access this server?

The answer is no.

Explanation:

Machines connected a web server behind a  NAT are not visible from the outside, so the user of the public internet cant access this sever . or a public internet user will be able to access only if they know this private IP

Normally the users won't be able to access it. In order to access a server behind NAT, you need to use VPN with port forwarding, UPNP or VPN with dedicated IP.

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