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tftv256's description is correct for Linux (if you wish, you can replace "allowedhost" by "@allowednetgroup"), but the format for HP-UX is very different:

/path -access=allowedhost1:allowedhost2,otheroptions

Two useful other options are "ro" (export the directory read-only) and "root=host1:host2:..." which grants root on certain other machines full access to the exported directory, rather than treating root as an anonymous user.