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a. Everyone : Any host capable of contacting your system is allowed access. This option should be used with care, especially if your the system is connected to the Internet.
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f. IPv6 Address and Prefix: Any host in the specified subnet is allowed access.
Example of my /etc/exports file for your the reference. I used option Option (e) was used to lock down my the share to only 2 specific IP CIDR ranges on my the network. So all the systems within that CIDR range will be able to mount and access the NFS share and no one else.
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