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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 MINIX

ifconfig

IFCONFIG(8)						      System Manager's Manual						       IFCONFIG(8)

NAME

       ifconfig - configure a TCP/IP device

SYNOPSIS

       ifconfig [-I ip-device] [-h ipaddr] [-n netmask] [-iv]

DESCRIPTION

       Ifconfig  initializes a TCP/IP device setting the IP address and/or netmask.  It will report the address and netmask set.  This command may
       be used if the system has not been configured properly yet.  It is only used at boot time to set a fixed address for  a	system	without  a
       physical ethernet.  Normally the inet task will find it out by itself from the RARP server.

OPTIONS

       -h     The decimal TCP/IP address to set.

       -n     The netmask to set.

       -i     Don't set the IP address or netmask if already set.  This way ifconfig cannot interfere if the numbers have been found out by RARP.

       -v     Report IP address and netmask.  This is the default action if there are no other options.

SEE ALSO

       hostaddr(1), rarpd(8), set_net_default(8), boot(8).

AUTHOR

       Kees J. Bot (kjb@cs.vu.nl)

																       IFCONFIG(8)
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