= IPAddress

IPAddress is a Ruby library designed to make the use of IPv4 and IPv6
addresses simple, powerful and enjoyable. It provides a complete set of
methods to handle IP addresses for any need, from simple scripting to
full network design.

IPAddress is written with a full OO interface, and its code is easy to
read, maintain and extend. The documentation is full of examples, to
let you start being productive immediately.

This document provides a brief introduction to the library and
examples of typical usage. 

== Requirements

* Ruby 1.9.3 or later

Please refer to {Travis CI}[https://travis-ci.org/ipaddress2-gem/ipaddress2] for Build Tests on specific versions of Ruby.

{<img src="https://travis-ci.org/ipaddress2-gem/ipaddress2.svg?branch=master" alt="Build Status" />}[https://travis-ci.org/ipaddress2-gem/ipaddress2] {<img src="https://codeclimate.com/github/ipaddress2-gem/ipaddress2/badges/gpa.svg" />}[https://codeclimate.com/github/ipaddress2-gem/ipaddress2]

If you want to contribute, please refer to {Contributing.md}[https://github.com/ipaddress2-gem/ipaddress2/blob/master/CONTRIBUTING.md].


== Installation

Install the library using rubygems

  $ gem install ipaddress_2

You can then use it in your programs:
  
  require 'ipaddress_2'

== Documentation

The code is fully documented with RDoc. You can generate the
documentation with Rake:

  ipaddress$ rake rdoc

The latest documentation can be found online at 
{this address}[http://www.rubydoc.info/gems/ipaddress_2/0.9.0]
 
== IPv4

Class `IPAddress::IPv4` is used to handle IPv4 type addresses. IPAddress
is similar to other IP Addresses libraries, like Ruby's own
IPAddr. However it works slightly different, as we will see.

=== Create a new IPv4 address

The usual way to express an IP Address is using its dotted decimal
form, such as `172.16.10.1`, and a prefix, such as `24`, separated by a
slash. 

  172.16.10.1/24

To create a new IPv4 object, you can use IPv4 own class

  ip = IPAddress::IPv4.new "172.16.10.1/24"

or, in a easier way, using the IPAddress parse method

  ip = IPAddress.parse "172.16.10.1/24"

which accepts and parses any kind of IP (uint32, IPv4, IPV6 and 
IPv4 IPv6 Mapped addresses).

If you like syntactic sugar, you can use the wrapper method
`IPAddress()`, which is built around `IPAddress::parse`:

  ip = IPAddress "172.16.10.1/24"

You can specify an IPv4 address in any of two ways:

  IPAddress "172.16.10.1/24"
  IPAddress "172.16.10.1/255.255.255.0"
  
In this example, prefix `/24` and netmask `255.255.255.0` are the same and
you have the flexibility to use either one of them.

If you don't explicitly specify the prefix (or the subnet mask), 
IPAddress thinks you're dealing with host addresses and not with 
networks. Therefore, the default prefix will be `/32`, or
`255.255.255.255`. For example:

  # let's declare an host address
  host = IPAddress::IPv4.new "10.1.1.1"

  puts host.to_string
    #=> "10.1.1.1/32"
  
The new created object has prefix `/32`, which is the same 
as we created the following:

  host = IPAddress::IPv4.new "10.1.1.1/32"

You can also pass a `uint32` to obtain an `IPAddress::IPv4` object:

  # Create host object
  ip = IPAddress 167837953
  puts ip.to_string
    #=> "10.1.1.1/32"

=== Handling the IPv4 address

Once created, you can obtain the attributes for an IPv4 object:

  ip = IPAddress("172.16.10.1/24")

  ip.address
    #=> "172.16.10.1"
  ip.prefix
    #=> 24

In case you need to retrieve the netmask in IPv4 format, you can use
the `IPv4#netmask` method:

  ip.netmask
    #=> "255.255.255.0"

A special attribute, `IPv4#octets`, is available to get the four 
decimal octets from the IP address:

  ip.octets
    #=> [172,16,10,1]

Shortcut method `IPv4#[]`, provides access to a given octet whithin the
range:

  ip[1]
    #=> 16

If you need to print out the IPv4 address in a canonical form, you can
use `IPv4#to_string`:

  ip.to_string
    #=> "172.16.10.l/24"

=== Changing netmask
    
You can set a new prefix (netmask) after creating an IPv4 
object. For example:

  ip.prefix = 25

  ip.to_string
    #=> "172.16.10.l/25"
    
If you need to use a netmask in IPv4 format, you can achive so by
using the `IPv4#netmask=` method:

  ip.netmask = "255.255.255.252"

  ip.to_string
    #=> "172.16.10.1/30"

=== Working with networks, broadcasts and addresses

Some very important topics in dealing with IP addresses are the
concepts of +network+ and +broadcast+, as well as the addresses
included in a range.

When you specify an IPv4 address such as `172.16.10.1/24`, you are
actually handling two different information: 
 
* The IP address itself, "172.16.10.1"
* The subnet mask which indicates the network

The network number is the IP which has all zeroes in the host
portion. In our example, because the prefix is 24, we identify our
network number to have the last 8 (32-24) bits all zeroes.  Thus, IP
address `172.16.10.1/24` belongs to network `172.16.10.0/24`.

This is very important because, for instance, IP `172.16.10.1/16` is
very different to the previous one, belonging to the very different
network `172.16.0.0/16`.

==== Networks

With IPAddress it's very easy to calculate the network for an IP
address:

  ip = IPAddress "172.16.10.1/24"

  net = ip.network
    #=> #<IPAddress::IPv4:0xb7a5ab24 @octets=[172, 16, 10, 0], 
                                     @prefix=24,
                                     @address="172.16.10.0">
  net.to_string
    #=> "172.16.10.0/24"

Method IPv4#network creates a new IPv4 object from the network
number, calculated after the original object. We want to outline here
that the network address is a perfect legitimate IPv4 address, which
just happen to have all zeroes in the host portion. 

You can use method `IPv4#network?` to check whether an IP address is a
network or not:

  ip1 = IPAddress "172.16.10.1/24"
  ip2 = IPAddress "172.16.10.4/30"

  ip1.network?
    #=> false
  ip2.network?
    #=> true

==== Broadcast

The broadcast address is the contrary than the network number: where
the network number has all zeroes in the host portion, the broadcast
address has all one's. For example, ip `172.16.10.1/24` has broadcast
`172.16.10.255/24`, where ip `172.16.10.1/16` has broadcast
`172.16.255.255/16`.

Method `IPv4#broadcast` has the same behavior as is `#network`
counterpart: it creates a new IPv4 object to handle the broadcast
address:
 
  ip = IPAddress "172.16.10.1/24"

  bcast = ip.broadcast
    #=> #<IPAddress::IPv4:0xb7a406fc @octets=[172, 16, 10, 255],
                                     @prefix=24, 
                                     @address="172.16.10.255">
  bcast.to_string
    #=> "172.16.10.255/24"

==== Addresses, ranges and iterators

So we see that the netmask essentially specifies a range for IP
addresses that are included in a network: all the addresses between
the network number and the broadcast. IPAddress has many methods to
iterate between those addresses. Let's start with `IPv4#each`, which
iterates over all addresses in a range

  ip = IPAddress "172.16.10.1/24"

  ip.each do |addr|
    puts addr
  end

It is important to note that it doesn't matter if the original IP is a
host IP or a network number (or a broadcast address): the #each method
only considers the range that the original IP specifies. 

If you only want to iterate over hosts IP, use the `IPv4#each_host`
method:

  ip = IPAddress "172.16.10.1/24"

  ip.each_host do |host|
    puts host
  end

Methods `IPv4#first` and `IPv4#last` return a new object containing
respectively the first and the last host address in the range

  ip = IPAddress "172.16.10.100/24"

  ip.first.to_string
    #=> "172.16.10.1/24"

  ip.last.to_string
    #=> "172.16.10.254/24"

Checking if an address is loopback is easy with the `IPv4#loopback?`
method:

  ip = IPAddress "127.0.0.1"

  ip.loopback?
    #=> true

Checking if an address is in the multicast range can be done using the `IPv4#multicast?`
method:

  ip = IPAddress "224.0.0.1/32"

  ip.multicast?
    #=> true

The ability to generate a range also exists by using the `IPv4#to()` method. This allows you to create a subnet agnostic range based off a fixed amount.

  ip = IPAddress "172.16.10.100/24"
  ip.to('172.16.10.110')
    #=> ["172.16.10.100", ..., "172.16.10.110"]

=== IP special formats    

The IPAddress library provides a complete set of methods to access an
IPv4 address in special formats, such as binary, 32 bits unsigned int,
data and hexadecimal.

Let's take the following IPv4 as an example:

  ip = IPAddress "172.16.10.1/24"

  ip.address
    #=> "172.16.10.1"

The first thing to highlight here is that all these conversion methods
only take into consideration the address portion of an IPv4 object and
not the prefix (netmask).

So, to express the address in binary format, use the `IPv4#bits` method:

  ip.bits
    #=> "10101100000100000000101000000001"

To calculate the 32 bits unsigned int format of the ip address, use
the `IPv4#to_u32` method

  ip.to_u32
    #=> 2886732289

This method is the equivalent of the Unix call `pton()`, expressing an
IP address in the so called +network byte order+ notation. However, if
you want to transmit your IP over a network socket, you might need to
transform it in data format using the `IPv4#data` method:

  ip.data
    #=> "\254\020\n\001"

Also, you can transform an IPv4 address into a format which is
suitable to use in IPv4-IPv6 mapped addresses:

  ip.to_ipv6
    #=> "ac10:0a01"

Finally, much like `IPv4#to_ipv6` you can use to `IPv4#to_h` method to return a non-semicolon delineated string (useful with pcap/byte level usage):

  ip.to_h
    #=> "ac100a01"

=== Classful networks

IPAddress allows you to create and manipulate objects using the old 
and deprecated (but apparently still popular) classful networks concept.

Classful networks and addresses don't have a prefix: their subnet mask
is univocally identified by their address, and therefore divided in classes.
As per RFC 791, these classes are:

* Class A, from 0.0.0.0 to 127.255.255.255
* Class B, from 128.0.0.0 to 191.255.255.255
* Class C, from 192.0.0.0 to 255.255.255.255

Since classful networks here are only considered to calculate the default
prefix number, classes D and E are not considered. 

To create a classful IP and prefix from an IP address, use the 
IPv4::parse_classful method:

  # classful ip 
  ip = IPAddress::IPv4::parse_classful "10.1.1.1"

  ip.prefix
    #=> 8

The method automatically created a new IPv4 object and assigned it
the correct prefix.

You can easily check which CLASSFUL network an IPv4 object belongs:

  ip = IPAddress("10.0.0.1/24")
  ip.a?
    #=> true
  
  ip = IPAddress("172.16.10.1/24")
  ip.b?
    #=> true
  
  ip = IPAddress("192.168.1.1/30")
  ip.c?
    #=> true

Remember that these methods are only checking the address portion of an IP, and are
independent from its prefix, as classful networks have no concept of prefix.

For more information on CLASSFUL networks visit the 
{Wikipedia page}[http://en.wikipedia.org/wiki/Classful_network]

=== Network design with IPAddress

IPAddress includes a lot of useful methods to manipulate IPv4 and IPv6
networks and do some basic network design. 

==== Subnetting

The process of subnetting is the division of a network into smaller
(in terms of hosts capacity) networks, called subnets, so that they
all share a common root, which is the starting network. 

For example, if you have network "172.16.10.0/24", we can subnet it
into 4 smaller subnets. The new prefix will be /26, because 4 is 2^2
and therefore we add 2 bits to the network prefix (24+2=26).

Subnetting is easy with IPAddress. You actually have two options:

* IPv4#subnet: specify a new prefix
* IPv4#split: tell IPAddress how many subnets you want to create.

Let's examine `IPv4#subnet` first. Say you have network "172.16.10.0/24"
and you want to subnet it into /26 networks. With IPAddress it's very 
easy:

  network = IPAddress "172.16.10.0/24"

  subnets = network.subnet(26)

  subnets.map{|i| i.to_string}
    #=> ["172.16.10.0/26", 
         "172.16.10.64/26", 
         "172.16.10.128/26", 
         "172.16.10.192/26"]

As you can see, an Array has been created, containing 4 new IPv4 objects
representing the new subnets. 

Another way to create subnets is to tell IPAddress how many subnets you'd 
like to have, and letting the library calculate the new prefix for you.

Let's see how it works, using `IPv4#split` method. Say you want 4 new subnets:

  network = IPAddress("172.16.10.0/24")

  subnets = network.split(4)

  subnets.map{|i| i.to_string}
    #=> ["172.16.10.0/26", 
         "172.16.10.64/26", 
         "172.16.10.128/26", 
         "172.16.10.192/26"]

Hey, that's the same result as before! This actually makes sense, as the
two operations are complementary. When you use `IPv4#subnet` with the new
prefix, IPAddress will always create a number of subnets that is a power 
of two. This is equivalent to use IPv4#split with a power of 2.

Where `IPv4#split` really shines is with the so called "uneven subnetting".
You are not limited to split a network into a power-of-two numbers of
subnets: IPAddress lets you create any number of subnets, and it will
try to organize the new created network in the best possible way, making
an efficient allocation of the space.

An example here is worth a thousand words. Let's use the same network
as the previous examples:

  network = IPAddress("172.16.10.0/24")

How do we split this network into 3 subnets? Very easy:

  subnets = network.split(3)

  subnets.map{|i| i.to_string}
    #=> ["172.16.10.0/26",
         "172.16.10.64/26",
         "172.16.10.128/25"]

As you can see, IPAddress tried to perform a good allocation by filling up
all the address space from the original network. There is no point in splitting
a network into 3 subnets like `172.16.10.0/26`, `172.16.10.64/26` and 
`172.16.10.128/26`, as you would end up having `172.16.10.192/26` wasted (plus,
I suppose I wouldn't need a Ruby library to perform un-efficient IP 
allocation, as I do that myself very well ;) ).

We can go even further and split into 11 subnets:

  network.split(11)
    #=> ["172.16.10.0/28", "172.16.10.16/28", "172.16.10.32/28",
         "172.16.10.48/28", "172.16.10.64/28", "172.16.10.80/28",
         "172.16.10.96/28", "172.16.10.112/28", "172.16.10.128/27",
         "172.16.10.160/27", "172.16.10.192/26"]

As you can see, most of the networks are `/28`, with a few `/27` and one
`/26` to fill up the remaining space.

==== Summarization

Summarization (or aggregation) is the process when two or more
networks are taken together to check if a supernet, including
all and only these networks, exists. If it exists then this supernet 
is called the summarized (or aggregated) network.
It is very important to understand that summarization can only
occur if there are no holes in the aggregated network, or, in
other words, if the given networks fill completely the address space
of the supernet. So the two rules are:

1) The aggregate network must contain +all+ the IP addresses of the
original networks;   

2) The aggregate network must contain +only+ the IP addresses of the
original networks; 

A few examples will help clarify the above. Let's consider for
instance the following two networks:

  ip1 = IPAddress("172.16.10.0/24")
  ip2 = IPAddress("172.16.11.0/24")

These two networks can be expressed using only one IP address
network if we change the prefix. Let Ruby do the work:

  IPAddress::IPv4::summarize(ip1,ip2).map(&:to_string)
    #=> "172.16.10.0/23"

We note how the network `172.16.10.0/23` includes all the
addresses specified in the above networks, and (more important) includes
ONLY those addresses.

If we summarized +ip1+ and +ip2+ with the following network:

  "172.16.0.0/16"

we would have satisfied rule #1 above, but not rule #2. So

  "172.16.0.0/16"

is not an aggregate network for +ip1+ and +ip2+.

If it's not possible to compute a single aggregated network for
all the original networks, the method returns an array with all the
aggregate networks found. For example, the following four networks can be
aggregated in a single `/22`:

  ip1 = IPAddress("10.0.0.1/24")
  ip2 = IPAddress("10.0.1.1/24")
  ip3 = IPAddress("10.0.2.1/24")
  ip4 = IPAddress("10.0.3.1/24")

  IPAddress::IPv4::summarize(ip1,ip2,ip3,ip4).map{|i| i.to_string}
    #=> ["10.0.0.0/22"]

But the following networks can't be summarized in a single
network:

  ip1 = IPAddress("10.0.1.1/24")
  ip2 = IPAddress("10.0.2.1/24")
  ip3 = IPAddress("10.0.3.1/24")
  ip4 = IPAddress("10.0.4.1/24")

  IPAddress::IPv4::summarize(ip1,ip2,ip3,ip4).map{|i| i.to_string}
    #=> ["10.0.1.0/24","10.0.2.0/23","10.0.4.0/24"]

In this case, the two summarizables networks have been aggregated into
a single `/23`, while the other two networks have been left untouched.

==== Supernetting

Supernetting is a different operation than aggregation, as it only
works on a single network and returns a new single IPv4 object,
representing the supernet.

Supernetting is similar to subnetting, except that you getting as a
result a network with a smaller prefix (bigger host space). For
example, given the network

  ip = IPAddress("172.16.10.0/24")

you can supernet it with a new /23 prefix

  ip.supernet(23).to_string
    #=> "172.16.10.0/23"

However if you supernet it with a `/22` prefix, the network address will
change: 

  ip.supernet(22).to_string
    #=> "172.16.8.0/22"

This is because `172.16.10.0/22` is not a network anymore, but an host
address.

== IPv6

IPAddress is not only fantastic for IPv4 addresses, it's also great to
handle IPv6 addresses family! Let's discover together how to use it in
our projects.

=== IPv6 addresses

IPv6 addresses are 128 bits long, in contrast with IPv4 addresses
which are only 32 bits long. An IPv6 address is generally written as
eight groups of four hexadecimal digits, each group representing 16
bits or two octet. For example, the following is a valid IPv6
address: 

  2001:0db8:0000:0000:0008:0800:200c:417a

Letters in an IPv6 address are usually written downcase, as per
RFC. You can create a new IPv6 object using uppercase letters, but
they will be converted.

==== Compression

Since IPv6 addresses are very long to write, there are some
simplifications and compressions that you can use to shorten them. 

* Leading zeroes: all the leading zeroes within a group can be
  omitted: "0008" would become "8"
  
* A string of consecutive zeroes can be replaced by the string
  "::". This can be only applied once.

Using compression, the IPv6 address written above can be shorten into
the following, equivalent, address

  2001:db8::8:800:200c:417a

This short version is often used in human representation.

==== Network Mask

As we used to do with IPv4 addresses, an IPv6 address can be written
using the prefix notation to specify the subnet mask:

  2001:db8::8:800:200c:417a/64

The /64 part means that the first 64 bits of the address are
representing the network portion, and the last 64 bits are the host
portion. 

=== Using IPAddress with IPv6 addresses

All the IPv6 representations we've just seen are perfectly fine when
you want to create a new IPv6 address:

  ip6 = IPAddress "2001:0db8:0000:0000:0008:0800:200C:417A"
  
  ip6 = IPAddress "2001:db8:0:0:8:800:200C:417A"
 
  ip6 = IPAddress "2001:db8:8:800:200C:417A"

All three are giving out the same IPv6 object. The default subnet mask
for an IPv6 is 128, as IPv6 addresses don't have classes like IPv4
addresses. If you want a different mask, you can go ahead and explicit
it:

  ip6 = IPAddress "2001:db8::8:800:200c:417a/64"

Access the address portion and the prefix by using the respective
methods:

  ip6 = IPAddress "2001:db8::8:800:200c:417a/64"

  ip6.address
    #=> "2001:0db8:0000:0000:0008:0800:200c:417a"

  ip6.prefix
    #=> 64

A compressed version of the IPv6 address can be obtained with the
`IPv6#compressed` method:

  ip6 = IPAddress "2001:0db8:0000:0000:0008:200c:417a:00ab/64"

  ip6.compressed
    #=> "2001:db8::8:800:200c:417a"

=== Handling the IPv6 address

Accessing the groups that form an IPv6 address is very easy with the
`IPv6#groups` method:

  ip6 = IPAddress "2001:db8::8:800:200c:417a/64"
 
  ip6.groups
    #=> [8193, 3512, 0, 0, 8, 2048, 8204, 16762]

As with IPv4 addresses, each individual group can be accessed using
the `IPv6#[]` shortcut method:

  ip6[0]
    #=> 8193
  ip6[1]
    #=> 3512
  ip6[2]
    #=> 0
  ip6[3]
    #=> 0

Note that each 16 bits group is expressed in its decimal form. You can
also obtain the groups into hexadecimal format using the `IPv6#hexs`
method:

  ip6.hexs
    #=> => ["2001", "0db8", "0000", "0000", "0008", "0800", "200c", "417a"]

A few other methods are available to transform an IPv6 address into
decimal representation, with `IPv6.to_i`

  ip6.to_i
    #=> 42540766411282592856906245548098208122

or to hexadecimal representation

  ip6.to_hex
    #=> "20010db80000000000080800200c417a"

To print out an IPv6 address in human readable form, use the `IPv6#to_s`, `IPv6#to_string`
and `IPv6#to_string_uncompressed` methods

  ip6 = IPAddress "2001:db8::8:800:200c:417a/64"
 
  ip6.to_string
    #=> "2001:db8::8:800:200c:417a/96"

  ip6.to_string_uncompressed
    #=> "2001:0db8:0000:0000:0008:0800:200c:417a/96"

As you can see, `IPv6.to_string` prints out the compressed form, while
`IPv6.to_string_uncompressed` uses the expanded version.

==== Compressing and uncompressing

If you have a string representing an IPv6 address, you can easily
compress it and uncompress it using the two class methods IPv6::expand
and IPv6::compress.

For example, let's say you have the following uncompressed IPv6
address:

  ip6str = "2001:0DB8:0000:CD30:0000:0000:0000:0000"

Here is the compressed version:

  IPAddress::IPv6.compress ip6str
    #=> "2001:db8:0:cd30::"

The other way works as well:

  ip6str = "2001:db8:0:cd30::"

  IPAddress::IPv6.expand ip6str
    #=> "2001:0DB8:0000:CD30:0000:0000:0000:0000"

These methods can be used when you don't want to create a new object
just for expanding or compressing an address (although a new object is
actually created internally).
 
=== New IPv6 address from other formats

You can create a new IPv6 address from different formats than just a
string representing the colon-hex groups.

For instance, if you have a data stream, you can use `IPv6::parse_data`,
like in the following example:

  data = " \001\r\270\000\000\000\000\000\b\b\000 \fAz"

  ip6 = IPAddress::IPv6::parse_data data 
  ip6.prefix = 64

  ip6.to_string
    #=> "2001:db8::8:800:200c:417a/64"

A new IPv6 address can also be created from an unsigned 128 bits
integer:

  u128 = 42540766411282592856906245548098208122

  ip6 = IPAddress::IPv6::parse_u128 u128
  ip6.prefix = 64

  ip6.to_string
    #=>"2001:db8::8:800:200c:417a/64"	

Finally, a new IPv6 address can be created from an hex string:

  hex = "20010db80000000000080800200c417a"   

  ip6 = IPAddress::IPv6::parse_hex hex
  ip6.prefix = 64

  ip6.to_string
    #=> "2001:db8::8:800:200c:417a/64"

=== Special IPv6 addresses

Some IPv6 have a special meaning and are expressed in a special form,
quite different than an usual IPv6 address. IPAddress has built-in
support for unspecified, loopback and mapped IPv6 addresses.

==== Unspecified address

The address with all zero bits is called the +unspecified+ address
(corresponding to `0.0.0.0` in IPv4). It should be something like this:

  0000:0000:0000:0000:0000:0000:0000:0000

but, with the use of compression, it is usually written as just two
colons:

  ::

or, specifying the netmask:

  ::/128

With IPAddress, create a new unspecified IPv6 address using its own
subclass:

  ip = IPAddress::IPv6::Unspecified.new

  ip.to_string
    #=> "::/128"

You can easily check if an IPv6 object is an unspecified address by
using the IPv6#unspecified? method

  ip.unspecified?
    #=> true

An unspecified IPv6 address can also be created with the wrapper
method, like we've seen before

  ip = IPAddress "::"

  ip.unspecified?
    #=> true

This address must never be assigned to an interface and is to be used
only in software before the application has learned its host's source
address appropriate for a pending connection. Routers must not forward
packets with the unspecified address.

==== Loopback address

The loopback address is a unicast localhost address. If an
application in a host sends packets to this address, the IPv6 stack
will loop these packets back on the same virtual interface.

Loopback addresses are expressed in the following form:

  ::1

or, with their appropriate prefix,

  ::1/128

As for the unspecified addresses, IPv6 loopbacks can be created with
IPAddress calling their own class:

  ip = IPAddress::IPv6::Loopback.new

  ip.to_string
    #=> "::1/128"

or by using the wrapper:

  ip = IPAddress "::1"

  ip.to_string
    #=> "::1/128"

Checking if an address is loopback is easy with the `IPv6#loopback?`
method:

  ip.loopback?
    #=> true
  
The IPv6 loopback address corresponds to `127.0.0.1` in IPv4.

==== Mapped address

It is usually identified as a IPv4 mapped IPv6 address, a particular
IPv6 address which aids the transition from IPv4 to IPv6. The
structure of the address is 

  ::ffff:w.y.x.z

where `w.x.y.z` is a normal IPv4 address. For example, the following is
a mapped IPv6 address:

  ::ffff:192.168.100.1

IPAddress is very powerful in handling mapped IPv6 addresses, as the
IPv4 portion is stored internally as a normal IPv4 object. Let's have
a look at some examples. To create a new mapped address, just use the
class builder itself

  ip6 = IPAddress::IPv6::Mapped.new "::ffff:172.16.10.1/128"

or just use the wrapper method

  ip6 = IPAddress "::ffff:172.16.10.1/128"

Let's check it's really a mapped address:

  ip6.mapped?
    #=> true

  ip6.to_string
    #=> "::ffff:172.16.10.1/128"

Now with the `#ipv4` attribute, we can easily access the IPv4 portion
of the mapped IPv6 address:

  ip6.ipv4.address
    #=> "172.16.10.1"

Internally, the IPv4 address is stored as two 16 bits
groups. Therefore all the usual methods for an IPv6 address are
working perfectly fine:

  ip6.to_hex
    #=> "00000000000000000000ffffac100a01"

  ip6.address
    #=> "0000:0000:0000:0000:0000:ffff:ac10:0a01"

A mapped IPv6 can also be created just by specify the address in the
following format:
  
  ip6 = IPAddress "::172.16.10.1"

That is, two colons and the IPv4 address. However, as by RFC, the `ffff`
group will be automatically added at the beginning

  ip6.to_string
    => "::ffff:172.16.10.1/128"
 
making it a mapped IPv6 compatible address.

== Why not using IPAddr?

IPAddr is the IP addresses library that comes with Ruby standard
lib. We found this library, although very well written, not very
suitable for all our needs, and not very flexible. 

Some quick examples of things you can't do with IPAddr:

* store both the address and the prefix information
* quickly find the broadcast address of a network
* iterate over hosts 
* perform subnetting or network aggregation

Many methods and procedures are so old that they have been 
declared deprecated by the IETF, and some others have bugs in their 
implementation.  

Moreover, IPAddress is more robust and is already around 50% faster than IPAddr,
in addition to provide an organic API with logical separation and OO structure.

We hope that IPAddress will address all these issues and meet all your
needs in network programming.


== Thanks to
 
Thanks to Luca Russo (vargolo) and Simone Carletti 
(weppos) for all the support and technical review. Thanks to Marco Beri, 
Bryan T. Richardson, Nicolas Fevrier, jdpace, Daniele Alessandri, jrdioko, 
Ghislain Charrier, Pawel Krzesniak, Mark Sullivan, Leif Gensert, 
Erik Ahlström, Peter Vandenberk and Steve Rawlinson for their support, 
feedback and bug reports.
  
== Copyright

Copyright (c) 2009-2015 Marco Ceresa and Mike Mackintosh. See LICENSE for details.