qwurx [shmem] ~ > perl - 172.24.43.226/27
$m=pack B32,pop=~'/'x$';printf"$` net %vd bc %vd mask %vd\n",($z=eval$
+`)&$m,$z|~$m,$m
__END__
172.24.43.226 net 172.24.43.224 bc 172.24.43.255 mask 255.255.255.224
[download]

"valid prefix" is indeed the network address - 172.24.43.224.

<update>

Skimming Net::IP I didn't find a function which returns network and broadcast addresses given an arbitrary IP address in CIDR notation. I'd do the following for IP v4 addresses

sub get_boundaries {
    my ($ip, $bits) = split /\//, $_[0];
    my $b_ip        = pack "C4", split /\./, $ip;
    my $b_mask      = pack "B32", 1 x $bits;
    my $b_net       = $b_ip  &  $b_mask;
    my $b_bcast     = $b_ip | ~ $b_mask;
    unpack("N", $b_net), unpack("N",  $b_bcast);
}
[download]

which returns the network and broadcast addresses as 32 bit integers, being the lower and upper limit of a range to test other IP addresses against (after having converted them to integers, of course).

sub in_range {
    my ($cidr, $ip)  = @_;
    my ($low, $high) = get_boundaries($cidr);
    my $i_ip         = unpack "N", pack "C4", split /\./, $ip;
    $i_ip > $low && $i_ip < $high;
}
print "yup\n" if in_range("172.24.43.226/27", "172.24.43.236");
[download]

</update>

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