If you read my page on TCP, IP addresses and subnetting,  you would have seen how it is working presently and how it has been. However, that is not the way of the future. The CIDR is already in effect and most newer routers supports it. In a few years it is likely to be the way IP addressing is handled. It is already becoming common for ISP and major providers to use this schem.

The old schem if you remember provided for Class A, B and C networks.

Address Class            # Network bits        # host bits         Decimal

Class A                             8                             24                 1-126

Class B                             16                           16                 128-191

Class C                             24                           8                   192-223                        

• Class A : There are 126 networks each with 16,777,214 hosts addresses

• Class B: There are 16384 networks each with 65534 host addresses 

• Class C: There are 2097152 networks each with 254 host addresses

Of course these could be subnetted on a local level.

Why Changing?? - This looks good

• We are running out of IP addresses and present use is inefficient
• We are running out of capacity in the Routing tables

The inefficiencies is mainly in the block assignments. You get a class C network and use only a hundred of those. That means there are suddenly 154 unused and unavailable addresses. On the bigger scale some investigations have indicated that while we are running out of address blocks to assign only a very small percentage of the total addresses are being used. (less than 10%).

With the expansion of smaller networks the route table additions are increasing rapidly. The capacity is being overtaxed. What good is an Internet address is you can't get where you want or some can't find you. Thus, a need is developing for modifying the way it is handled.

Subnetting is a way that could help and another is to have all the smaller networks use the privately assigned address space and use proxies. That way only one external address is being employed. Not exactly true since the NAT (network address translation) needs to provide a public address for each IP address in the internal system when in use.However, this would most certainly solve the problems.

The new schem that has been developed is called CIDR.

How does it work ?? How does it differ ?? How does it solve the problems ??

First how is it constructed and how does it work. The class system is being replaced with a prefix anywhere from 13 to 27 bits which serves as a generalized network prefix. Thus a new IP address might look like this 192.168.255.48/25.  What this means is that the first 25 bit in the address is used to identify the network while the remaining   7 bits is used to identify the host.

This would then allow the big blocks to be provided to the ISPs who would then rent them out on an as needed basis to the users. The allocation might be bigger or smaller blocks depending on needs. It would allow for a much more efficient assignment off the IP address space.

The routing tables and architecture could now be changed to solve that problem too.   In this set up a major provider like UUENET or Alternet or whoever would have their set address space.  Thus the major routing tables (global ) would only need to include these major providers. It would then be necessary for the major providers as they give out address space to include those in their routing tables. The subset providers or maybe your local ISP would then provide the routing for their set.

This would be a hierarchical set up similar in many ways to how the phone system is set up. It would mean that if you change providers you would be required to change IP addresses on your network. Otherwise the routing would not be there.

This approach is in use at present and is being encourage

More Information here:

PC Webopedia     CIDR   

CIDR INFO        Information on CIDR

Reference information:

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Last revised: June 07, 1999.