I've never had a need to do any bit twiddling in bash... thanks for this 
solution,  It was very educational.

Brian Cluff

On 10/27/2011 11:43 AM, Dale Farnsworth wrote:
> Larry wrote:
>> I would like to apply a netmask to an arbitrary IP in a bash/dash script
>> (e.g. apply 255.255.255.0 to 173.10.3.155 to get 173.10.3.0).  Is there any
>> easy way to do that without taking the IP apart, doing 4 operations and
>> reassembling the results?
>
> I know you said without disassembly/reassembly, but here are some
> bash functions that apply the mask using disassembly/reassembly.
>
> ===================== begin =======================
> to_int()
> {
> 	# Convert from dotted-quad to integer
> 	local i=$1
> 	local OIFS="$IFS"
> 	IFS=.
> 	i=($i)
> 	IFS="$OIFS"
> 	echo $(((${i[0]}<<  24) + (${i[1]}<<  16) + (${i[2]}<<  8) + ${i[3]}))
> }
>
> to_ip()
> {
> 	# Covert from integer to dotted-quad
> 	local d=$1
> 	echo $(($d>>  24)).$(($d>>  16&  255)).$(($d>>  8&  255)).$(($d&255))
> }
>
> and_ip()
> {
> 	# And two ip addresses in dotted-quad format
> 	local ip1=$1
> 	local ip2=$2
> 	echo $(to_ip $(($(to_int $ip1)&  $(to_int $ip2))))
> }
> ===================== end =======================
>
> Just pass the ip and netmask to and_ip:
> Example:
>
> $ and_ip 11.22.33.44 255.255.255.0
> 11.22.33.0
>
> -Dale
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