The hosts that will have access to the exported directory.
Warning: when exporting with krb5, the choice of the clients is made in the configuration of Kerberos5.
Six ways of specifying which hosts are allowed to mount the directory are available:

Everyone
  Any host capable of contacting your system is allowed access. This option should be used with care, especially if your system is connected to the Internet.
Host(s)
  This can either be a single IP address, a single hostname, or a regular expression like `*.foo.com`. In the last case, any host whose reverse DNS lookup by IP address matches the expression will be allowed access.
WebNFS clients
  Allows clients using Sun's WebNFS protocol to access this directory.
NIS Netgroup
  Allows all members of the specified NIS netgroup to access this export.
IPv4 Network and Netmask
  Any host in the specified subnet is allowed access. For example, if the network was `10.1.2.0` and the netmask `255.255.255.0`, all hosts with IP addresses from `10.1.2.0` to `10.1.2.255` would be allowed.
IPv6 Address and Prefix
  Any host in the specified subnet is allowed access.