Managing NFS and NIS, 2nd Edition - Mike Eisler [14]
The problem of only two million class C networks was mitigated by the introduction of dynamically assigned IP addresses, and by the introduction of policies that tended to assign IP network numbers only to Internet Service Providers (ISPs), or to organizations that effectively acted as their own ISP, which would then use the free market to efficiently reallocate the IP addresses dynamically or statically to their customers. Thus most Intenet users get assigned a single IP address, and the ISP is assigned the corresponding network number.
The second reason was routing scalability. When the Internet was orders of magnitude smaller then it is today, most address assignments were for class A or B and so routing between networks was straightforward. The routers simply looked at the network number, and sent it to a router responsible for that route. With the explosion of the Internet, and with most of that growth in class C network numbers, each network's router might have to maintain tables of hundreds of thousands of routes. As the Internet grew rapidly, keeping these tables up to date was difficult.
This situation was not sustainable, and so the concept of "classless addressing" was introduced. With the exception of grandfathered address assignments, each IP address, regardless of whether it's class A, B, or C, would not have an implicit network number part and host number part. Instead the network part would be designated explicitly via a suffix of the form: "/XX", where XX is the number of bits of the IP address that refer to the network. Those organizations that needed more than the 254 hosts that a class C address would provide, would instead be assigned consecutive class C addresses. For example, an ISP that was assigned 192.1.2 and 192.1.3 could have a classless network number of 192.1.3.0/23. Any router on a network other than 192.1.2 or 192.1.3 that wanted to send to either network number would instead route to a single router associated with the classless network number 192.1.3.0/23 (i.e., any IP address that had its first 23 bits equal to 1100 0000 0000 0001 0000 001).
With this new scheme, larger organizations get more consecutive class C network numbers. Within their local networks ("Intranets"), they can either use traditional class-based routing or classless routing that further subdivides the local network address space that can be used. The largest organizations may find that class-based routing doesn't scale, and so classless routing is the best approach.
Virtual interfaces
In Section 1.3.2, we noted that a host could have multiple IP addresses assigned to it if it had multiple physical network interfaces. It is possible for a physical network segment to support more than one IP network number. For example, a segment might have 128.0.0.0/16 and 192.4.5.6/24. Some hosts on that segment might want to directly address hosts with either network number. Some operating systems, such as Solaris, will let you define multiple virtual or logical interfaces for a physical network interface. On most Unix systems, the ifconfig command is used to set up interfaces. See your vendor's ifconfig manual page for more details.
IP Version 6
Until now we have been discussing IPv4 addresses that are four octets long. The discussion in Section 1.3.4 showed a clever way to extend the life of the 32 bit IPv4 address space. However, it was recognized long ago, even before the introduction of the World Wide Web, that the IPv4 address space was under pressure. IP Version 6 (IPv6) has been defined to solve the address space limitations by increasing the address length to 128 bit addresses. At the time of this writing, while most installed systems either do not support it or do not use it, most marketed systems support IPv6. Since it seems inevitable that you'll encounter some IPv6 networks in the next few years,