#!/usr/bin/perl -w
use strict;

my $first;
my $ip;

# use the address from @ARGV if it exists or request from STDIN
if (@ARGV) {
    $ip = shift @ARGV;        
} else { 
    print "Enter an ip address: ";
    chomp( my $ip = <STDIN> );
}


# split the ip address into subnets and store in an array
my @subnets = split(/\./, $ip);
# ensure the ip address is valid. 
sanitize(@subnets);


for (0..$#subnets) {
    # take each subnet and generate the hex out of the subnet divided 
+by 16 
    # then the remainder
    my $pair = hexize((int($subnets[$_]/16)),($subnets[$_]%16));
    print $pair;
}
print "\n";

sub sanitize {
    # takes a subnet and makes sure that its not over 255,
    # greater than 0 and that there are 4 subnets. 
    foreach $_ (@_) {
        if ($_ > 255) {
            print "illegal ip address\n";
            exit;
        } elsif ($_ < 0) {
            print "illegal ip address\n";
            exit;
        } elsif ($#_ != 3) {
            print "illegal ip address\n";
            exit;
        }
    }
}
sub hexize {
    # Takes two values 
    # 1: the subnet divided by 16 
    # 2: the subnet divided by 16's remainder
    # Then it applies the right letter if applicable
    my @unit;
    foreach $_ (@_) {
        if ($_ == 10) {
            push @unit,"A";
        } elsif ($_ == 11) {
                    push @unit,"B";
            } elsif ($_ == 12) {
                    push @unit,"C";
            } elsif ($_ == 13) {
                    push @unit,"D";
            } elsif ($_ == 14) {
                    push @unit,"E";
            } elsif ($_ == 15) {
                    push @unit,"F";
            } else {
            push @unit,$_;
        }
    }
    return "$unit[0]$unit[1]";
}
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