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Full Discussion: How to change ip addressing format from CIDR notation to netmask and vice versa?
Top Forums Shell Programming and Scripting How to change ip addressing format from CIDR notation to netmask and vice versa? Post 302927657 by Corona688 on Thursday 4th of December 2014 10:48:12 AM
Old 12-04-2014
What's your shell? What's your system?

CIDR to netmask is straightforward if tedious -- just binary math. Shells aren't generally that good at it but modern BASH has large enough numbers to handle this.

Code:
#!/bin/bash

while IFS="/" read IP S
do
        M=$(( 0xffffffff ^ ((1 << (32-S)) -1) ))
        echo "$IP netmask $(( (M>>24) & 0xff )).$(( (M>>16) & 0xff )).$(( (M>>8) & 0xff )).$(( M & 0xff ))"
done

If you have an older bash, it may generate negative values due to overflowing its 32-bit integers.
This User Gave Thanks to Corona688 For This Post:
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LEARN ABOUT CENTOS

ipcalc

IPCALC(1)						      General Commands Manual							 IPCALC(1)

NAME

       ipcalc - perform simple manipulation of IP addresses

SYNOPSIS

       ipcalc [OPTION]... <IP address>[/prefix] [netmask]

DESCRIPTION

       ipcalc provides a simple way to calculate IP information for a host.  The various options specify what information ipcalc should display on
       standard out. Multiple options may be specified.  An IP address to operate on must always be specified.	Most  operations  also	require  a
       netmask or a CIDR prefix as well.

OPTIONS

       -c, --check
	      Validate the IP address under the specified family.  If no address family is specified, IPv4 is assumed.

       -4, --ipv4
	      Specify IPv4 address family (default).

       -6, --ipv6
	      Specify IPv6 address family.

       -b, --broadcast
	      Display the broadcast address for the given IP address and netmask.

       -h, --hostname
	      Display the hostname for the given IP address.

       -m, --netmask
	      Calculate  the netmask for the given IP address. It assumes that the IP address is in a complete class A, B, or C network. Many net-
	      works do not use the default netmasks, in which case an inappropriate value will be returned.

       -p, --prefix
	      Show the prefix for the given mask/IP address.

       -n, --network
	      Display the network address for the given IP address and netmask.

       -s, --silent
	      Don't ever display error messages.

AUTHORS

       Erik Troan <ewt@redhat.com>
       Preston Brown <pbrown@redhat.com>

       IPv6 supported wedged in by David Cantrell <dcantrell@redhat.com>

REPORTING BUGS

       Report bugs at http://bugzilla.redhat.com/

COPYRIGHT

       Copyright (C) 1997-2008 Red Hat, Inc.
       This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICU-
       LAR PURPOSE.

Red Hat, Inc.							   April 30 2001							 IPCALC(1)
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