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.







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