~$ netmask -r 192.168.100.6/24
192.168.100.0-192.168.100.255 (256)

SYNOPSIS
       netmask [ options ] spec [ spec ... ]
DESCRIPTION
       This  program  accepts and produces a variety of common network address and netmask formats.  Not only can it convert address and
       netmask notations, but it will optimize the masks to generate the smallest list of rules.  This is very handy if you've ever con‐
       figured  a  firewall  or  router and some nasty network administrator before you decided that base 10 numbers were good places to
       start and end groups of machines.

(...)

DEFINITIONS
       A spec is an address specification, it can look like:

address             One address.
address1:address2   All addresses from address1 to address2.
address1:+address2  All addresses from address1 to address1+address2.
address/mask        A group starting at address spanning mask.

       An address is an internet network address, it can look like:
ftp.gnu.org   An internet hostname.
209.81.8.252  A standard dotted quad internet address notation.
100           A decimal number (100 in this case).
0100          An octal number preceded by "0" (64 in this case).
0x100         A hexadecimal number preceded by "0x" (256 in this case).

       A mask is a network mask, it can look like:
255.255.224.0  A dotted quad netmask (netmask will complain if it is not a valid netmask).
0.0.31.255     A Cisco style inverse netmask (with the same checks).
8              The number of bits set to one from the left (CIDR notation).
010            The number of bits set to one from the left in octal.
0x10           The number of bits set to one from the left in hexadecimal.

AUTHOR
       netmask was written by Robert Stone.  Some algorithm design and optimization was provided by Tom  Lear.   This  manual  page  was
       written by Robert Stone.