Content









4.2
IP
Addressing



4.2.7
The
IPv4 address crisis





The designers of the early Internet could
not have foreseen the explosive growth that the Internet has
experienced. They naturally believed that the number of addresses
required to identify interconnected networks and host systems would be
sufficient for many years to come. They assigned addresses to
companies and organizations as requests were made. By the mid-1980s,
however, it became clear that unless some action was taken, the number
of unused addresses would diminish rapidly and the Internet would face
a crisis that would be the result of its own success.
The total number of addresses that are available for use is called
the address space. Think about what might happen if the telephone
companies used up all the available phone numbers (that is, their
address space) for identifying telephones, faxes, and other devices
that allow communication over the phone system. No new phones could be
connected to the system, and that would stop its expansion. The
Internet was actually faced with this situation, when it appeared that
growth would be limited or even stopped because the Internet address
space could become exhausted.
In response to this problem, Internet engineers developed a set of
techniques to make more efficient use of the Internet address space.
Among these techniques was the "subnetting" of networks. Figure
shows an
example of a network address that has been subnetted. Subnetting is
the process of splitting a network portion of an IP address, which
allows an administrator to partition or divide a network without
having to use a new address for each network partition. By using
subnetting techniques, the designers of networks could divide them,
using the same network number, but each partition would be given its
own subnet number. Remote systems could still reach the network by
sending packets to the destination network address. Once the packets
arrived at the boundary of the destination network they could be
forwarded into the appropriate subnet within the network. This
technique has been very successful in conserving the number of major
network addresses and allowing for continued growth of the Internet.

The basic idea is to take the IP address, which is divided into a
network portion and a host portion, and then to divide it further by
adding a third part, the subnet number. The result is an address that
has the form network number, subnet number, and host number. As
discussed earlier, the IP address is divided into the network portion
and the host portion by knowing its class. When a subnet number is
added in between the network and host portions, how is the part that
identifies the subnet identified?
To answer this question, it is important to understand the function
of another number that was invented to go along with subnetting,
called the subnet mask. Like the IP address itself, this new number is
also written in dotted-decimal notation, as four octets that represent
32 bits. In the mask number, the "1" values are placed if the
corresponding bit in the IP address belongs to the network or subnet
part of the address. The "0" values are placed in the mask where the
corresponding bit in the IP address is part of the host portion. So,
if the class of the IP address is known and the subnet mask is known,
it can then be divided into network-subnet-host. It takes some
practice to thoroughly understand the process.
Where the class of an IP address determines the size of the network
part, the size of the subnet part can vary. The information needed to
determine the size of the subnet part is contained in the mask. Write
the mask in binary as 1s and 0s. This takes practice converting
decimal numbers to binary. Next, continue to proceed as follows:

Identify the class of the address.
Eliminate any "1" bits from the mask that correspond to the
network part of the address.
The remaining "1" bits in the mask indicate the bits in the
address that are the subnet part of the address.

Private IP Addresses
When dealing with IP addresses, corporate networks, and home
networks, it is important to know the difference between private IP
addressing and public IP addressing. Internet Protocol version 4
(IPv4) and the amount of these available public IP addresses is
quickly diminishing. The reason for this is that there is a limit to
the amount of IP addresses that IPv4 can provide. To help reserve the
amount of public IP addresses that are available, the concept of
private IP addressing is used. The address ranges of the reserved
private IP address are shown in Figure
. What this
means is that a corporation for example may have only a few IP address
that are public or that are known. All of the IP address that the
company uses within their network are contained within their network
and are therefore considered private. They are considered private
because they are only known to the company administrator and not known
to the public. Figure

illustrates an example of this process showing how the private IP
network addresses are used within the WAN.
Network Address Translation (NAT)
The concept of public and private IP addressing is further
explained through the use of Network Address Translation (NAT). NAT
enables companies to keep their private addresses secure and not known
to the public. NAT is enabled on a router or a gateway device, which
translates all of the incoming and outgoing traffic through the known,
or public IP addresses. Figure

illustrates how the IP address structure might be displayed when using
NAT. The Internal IP address is different and kept private from the
external public address that is exposed to others through the
Internet. The public IP addresses are what allow people with the
company to access information and networks outside of the LAN by
connecting to other public IP addresses. NAT also provides security by
hiding the IP addresses of clients and servers within the company
network. Assigning the public IP address to the NAT device does this.
If an attempt is made to gain access to the network, they are directed
to the NAT device and then are usually stopped by a firewall in place
on the same system or device that NAT is configured on.
IPv6
Internet Protocol version 6 (IPv6) is the next generation protocol
designed to replace the current version of the Internet Protocol,
IPv4. The following is an example of how IPv6 Address will be
numbered.
IPv6 addresses are written in hexadecimal:
1080:0000:0000:0000:
0008:0800:200C:417A

Leading zeros in each 16-bit value can be omitted.
So, the same address can be expressed as the following:


1080:0:0:0:8:800:200C:417A
Because IPv6 addresses, especially in the early implementation
phase, may contain consecutive 16-bit values of zero, one such string
of 0s per address can be omitted and replaced by a double colon. As a
result, this address can be shortened:


1080::8:800:200C:417A
Most of the Internet now uses IPv4, which is now nearly twenty
years old. IPv4 has been remarkably resilient in spite of its age, but
it is beginning to have problems. Most importantly, there is a growing
shortage of IPv4 addresses, which are needed when new systems are
added to the Internet.
IPv6 fixes a number of problems in IPv4, such as the limited number
of available IPv4 addresses. It will also add many improvements to
IPv4 in routing and in various network configuration tasks. IPv6 is
expected to gradually replace IPv4, with the two coexisting for a
number of years during a transition period.
Software is available that supports IPv6. This software is only
available in the latest releases like Windows XP and some of the
latest versions of Linux for example. Many common Internet
applications already work with IPv6, and more are being adapted.