Library /sys$common/syshlp/tcpip$ucp_help.hlb SET, ROUTE, Qualifiers, /MASK  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /MASK=mask_length

    Optional. Default: None.

    Defines the Classless Inter-Domain Routing (CIDR) mask length.
    (The mask length is sometimes referred to as the prefix length.)

    CIDR is a method of associating blocks of Internet addresses
    through the use of a mask. With CIDR, a route is a combination of
    the IP address and a value describing the length of the leftmost
    contiguous set of bits.