Configuring IPv6 Networks

At the beginning, IANA gave requestors an entire class A network space thereby granting requestors 16.7 million addressesmany more than necessary. Realizing their error, they began to assign class B networksagain, providing far too many addresses for the average requestor. As the Internet grew, it quickly became clear that allocating class A and class B networks to every requestor did not make sense. Even their later action of assigning class C banks of addresses still squandered address space, as most companies didn't require 254 IP addresses. Since IANA could not revoke currently allocated address space, it became necessary to deal with the remaining space in a way that made sense. One of these ways was through the use of Classless Inter-Domain Routing CIDR

IPv4 space is becoming scarcer by the day. By 2005, some estimates place the number of worldwide Internet users at over one billion. Given the fact that many of those users will have a cellular phone, a home computer, and possibly a computer at work, the available IP address space becomes critically tight. China has recently requested IP addresses for each of their students, for a total of nearly 300 million addresses. Requests such as these, which cannot be filled, demonstrate this shortage. When IANA initially began allotting address space, the Internet was a small and little- known research network. There was very little demand for addresses and class A address space was freely allocated. However, as the size and importance of the Internet started to grow, the number of available addresses diminished, making obtaining a new IP difficult and much more expensive. NAT and CIDR are two separate responses to this scarcity. NAT is an individual solution allowing one site to funnel its users through a single IP address. CIDR allows for a more efficient division of network address block. Both solutions, however, have limitations.

CIDR allows network blocks to be allocated outside of the well-defined class A/B/C ranges. In an effort to get more mileage from existing class C network blocks, CIDR allows administrators to divide their address space into smaller units, which can then be allocated as individual networks. This made it easier to give IPs to more people because space could be allocated by need, rather than by predefined size-of-space. For example, a provider with a class C subnet could choose to divide this network into 32 individual networks, and would use the network addresses and subnet masks to delineate the boundaries. A sample CIDR notation looks like this:

In this example, the /29 denotes the subnet mask, which means that the first 29 bits of the address are the subnet. It could also be noted as, which gives this network a total of six usable addresses.
While CIDR does deal with the problem in a quick and easy way, it doesn't actually create more IP addresses, and it does have some additional disadvantages. First, its efficiency is compromised since each allocated network requires a broadcast IP and a network address IP. So if a provider breaks a class C block into 32 separate networks, a total of 64 individual IPs are wasted on network and broadcast IPs. Second, complicated CIDR networks are more prone to configuration errors. A router with an improper subnet mask can cause an outage for small networks it serves.