17 Discuss the various Address Schemes:

Public and Private IP Address Schemes: A network to which anyone can connect is known as a public network. The best example of this is the internet. A private network is network to which access is restricted. The best example of this type would be a corporate network or a school network. The addressing of a device on a public network has to done very carefully. It is mandatory for hosts communicating using TCP/IP to have unique addresses. The address is the logical network each host belongs to along with the host's address on the network. An internetwork private in nature with three logical networks and 100 nodes on each is not much to handle when it comes to addressing, but the considerations cannot be the same for a network of the scale of internet.

A unique address, known as a registered address for every device, has to be assigned, as it is assigned to a specific party. In case a duplication of the address occurs for devices, it is almost certain that none would be able to communicate. Hence it is to be done very carefully and is a controlled matter. Initially, it was the responsibility of IANA to assign these addresses, but it has gone ahead and delegated some of its responsibilities to other organizations. Primarily, the responsibility rests with three organizations:

- The American Registry for Internet Numbers (ARIN);

- The Asia Pacific Network Information Centre (APNIC); and

- Reseaux IP Europeens Network Coordination Centre (RIPE NCC).

A valid registered IP address has to be obtained from any of these organizations for connecting a system directly to the internet. Addresses are also obtainable from the Internet Service Provider.

Private Address Ranges: For private use address ranges have been set aside. This helps to prevent from later on complications in case an address is wrongly configured. These are called private ranges as these been specifically allocated fro private networks. The internet routers are configured in a manner that they ignore, when they come across packets with these addresses. It would actually be stopped right at the first router.

In RFC 1918, three private ranges are defined one each from each class, that is, Class A, B and C. Classes A and B offer more options for addressing than Class C.


Address Range

Default Subnet Mask

A -

B -

C -

Table 7: Private Address Range

Practical Uses of Public and Private IP Addressing: Generally, companies have a few registered IP addresses for Internet use and a large number of private IP addresses for private use. The figure given below illustrates with the help of a simple example:

External interface with registered IP address:

Figure 45: Example illustrating Public and Private Network Address Assignments.

The network shown in the above example provides internet access to the clients through a proxy server system. The registered IP addresses of the external interface of the proxy server will be available. The systems on the internal network will use one from the private range.