The original design for the Internet required every organization to ask for and receive one or more registered IP network numbers. The people administering the program ensured that none of the IP network numbers was used by multiple companies or organizations. As long as every organization used only IP addresses inside their own registered network numbers, then IP addresses would never be duplicated and IP routing could work well.
Connecting to the Internet using only a registered network number or several registered network numbers worked very well for a while. In the early and mid-1990s, it became apparent that the Internet was growing so fast that all IP network numbers would be assigned by the mid-1990s. Concern arose that all the network numbers would be assigned, so some organizations would not be capable of connecting to the Internet. It would have been the equivalent of calling the local phone company to ask for a new phone line to be installed and being told that the company ran out of numbers; you would have to wait until someone didn't want a phone number any more!
This last section covers several features that together have allowed the Internet to grow, without letting us run out of IP addresses. Network Address Translation (NAT), along with a feature called private addressing, allows organizations to use unregistered IP network numbers internally and still communicate well with the Internet. Classless Interdomain Routing (CIDR) allows Internet service providers (ISPs) to reduce the wasting of IP addresses, by assigning a company a subset of a network number instead of the entire network. CIDR also reduces the size of Internet routing tables, allowing the Internet to grow. Also, a new version of IP, IP Version 6, uses much larger addresses, 128-bit long addresses, which allow for (hopefully) enough IP addresses so that we will never possibly run out again.
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