Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 72
Making the Address Efficient

The rapid expansion for connectivity and the exploding corporate infrastructure initially caused problems on the Internet. IP addresses were assigned sequentially to requesting organizations without regard to the requester’s location or method of Internet connection. What this means is that a requesting company simply called in for an IP address assignment and was assigned an IP address from a list of sequentially listed numbers. For example, a company in California could be assigned 150.1.0.0 and a company in Virginia would be assigned 151.1.0.0 and maybe 40 Class C addresses. Then a company in Texas could apply for 160.1.0.0 and 50 Class C addresses. They could then sign up for any ISP they desired with their newly assigned IP addresses. Very inefficient, but at the time, who knew? The routing system filled up with smaller IP addresses across multiple, long hops of routers, instead of large contiguous addresses. Supernetting, CIDR, and address aggregation provided address flexibility and efficiency to the ISP and the Internet. CIDR is very similar to VLSM. Today, blocks of addresses (as indicated toward the end of this section) are handed out to Internet Service Providers (ISPs) in blocks (or a range) through the Internet Registry (RFC 2050 fully explains this). For example, an ISP may be assigned the address block of 205.24.0.0/16, which allows the ISP to hand out addresses in the range of 205.24.0.0 through 205.24.255.255. In this way, the global routing tables only know that addresses 205.24.0.0 through 205.24.255.255 go in one direction to an ISP. All of these addresses are summarized into one routing table entry, which, using the old method, would have been 255 entries. The entry in the global routing tables would have been 205.24.0.0/16 instead of listing all 255 addresses—the global routing tables do not care about the individual network assignments.

The ISP subdivides this block to hand out individual addresses to its customers as Classful addresses, but how an ISP cuts up the addresses and assigns these blocks is affected using the protocols previously mentioned. One whole block would not be assigned to one company, but multiple companies.

A company requiring Internet connection calls its ISP, detailing its topology and requesting address space. The ISP (knowing it has to assign network numbers sparingly) will then assign the correct number and network range to its downstream customers. The range is then entered into the ISP’s routing table, perhaps as one address even though multiple Classes were given to the customer.