Library /sys$common/syshlp/nclhelp.hlb set, x25_access, reachable_address, mapping  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Default: X.121         Value: Manual or X.121

    Mechanism by which the destination DTE address is to be derived.

    manual  The destination DTE address is supplied manually by means
            of the destination characteristic.

    X.121   The destination DTE address is derived by an algorithm from
            an NSAP address in X.121 format. X.121 address mapping may
            only be specified with address prefixes of 36, 37, 49, 52
            and 53.

    Example:

    set x25 access reachable address x121 mapping X . 121