There might be a CPAN module to do this, but in case there isn't:

#!/usr/bin/perl -w

use strict;
use Socket;

sub MaskbitsToNetmask
{
        my ($bits) = @_;

        return 0xFFFFFFFF-((1<<(32-$bits))-1);
}

sub MaskToRange
{
        my ($range) = @_;

        my ($ip,$mask) = split (/\//, $range);
        $ip = CompactIP($ip);

        return ($ip, $ip | (1<<(32-$mask) - 1));
}

sub ExpandIP ($)
{
        return inet_ntoa (pack ("N", @_));
}

sub CompactIP ($)
{
        return unpack ("N", inet_aton (@_));
}

sub IsInRange
{
        my ($ip, $range) = @_;

        $ip = CompactIP($ip);
        my ($range_low, $range_hi) = MaskToRange ($range);

        return (($range_low <= $ip) && ($ip <= $range_hi));
}
[download]

Here's some examples:

print IsInRange ("192.168.2.42","192.168.0.0/16"),"\n";  # 1
print IsInRange ("192.168.2.42","192.168.0.0/24"),"\n";  # undef
print IsInRange ("192.168.2.42","192.168.2.0/24"),"\n";  # 1
print IsInRange ("24.123.234.1","24.0.0.0/8"),"\n";      # 1
[download]

Of course, this is just off the top, so YMMV.

Some notes:

• The numbers are converted to flat integers instead of the "dotted quad" notation used conventionally. This way, they can be compared numerically.
• The "netmask" generation routine could be optimized, even to the point of Perl Golf. This one contains the essentials.
• You must specify your entire network. Normally, you can get away with '24/8', but inet_ntoa gives you grief if you do that, interpreting it as '0.0.0.24' instead of '24.0.0.0'.

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