Continuing shortly with a few words about IP adresses
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Written by Robert on April 12, 2008 – 12:59 pm
The original ARPANET of the late 1960s and early 1970s had a simple addressing scheme for the computers on its wide area network – a one-byte number sufficed as an identifier. In the mid- to late 1970s, local area networks were introduced, exploiting the then new technologies of Token Rings and Ethernet broadcasting. At the same time, interest in the ARPANET grew, with more organizations (universities, government laboratories and some commercial concerns) seeking to connect computers. It became obvious that an expanded version of the ARPANET would be required, and that this network was going to have to deal with organizations that had groups of locally networked computers rather than a single mainframe 1960s-style machine. A changed addressing scheme was required – one that would allow many more machines on the network and which would in some way make allowance for the existence of sub-networks – like the private, local networks that had evolved.
These considerations led to the addressing system adopted for the communications protocols that were devised for the revised ARPANET. The new Internet Protocol system was to be a ‘network of networks’, or an inter-net. A machine’s address was to be composed of a network part and a host part. Three classes of networks were envisaged; in addition, the scheme provided some limited support for multicasting of data and for other future extensions. The different classes of network varied in size. All combinations of network identifier and machine identifier fitted into a 32-bit number; the different classes of network used the bits in different ways. The network class for an address can be determined by examining the first few bits of its address (up to 4 bits). Really, an address is just a 32-bit binary pattern, but such patterns are unsuited for human use. Thus a convention was established where an address was represented a sequence of four decimal numbers, each in the range 0–255, with each number representing one byte of the address. This lead to the now familiar ‘dotted decimal’ form for IP addresses – e.g. 207.68.172.253 (this is one of Microsoft’s computers).
The class A addresses used the first 8 bits of the 32-bit address to identify a network, and the remaining 24 bits of the address were for a machine identifier. The class A group were those where this leading byte represented a number in the inclusive range 1–126; so there were to be at most one hundred and twenty six such networks, each with potentially sixteen million computers. A few of these class A addresses were allocated in the early days of the Internet to organizations such as IBM (which got network 9), AT&T (12), and US defense organizations (MILNET, 26). These class A addresses are distinguished by having a zero-bit as the first bit in the address.
The class B addresses used 16 bits for a network identifier and 16 bits for a machine identifier; this allowed for up to sixty five thousand machines on a network. The first byte in the address for a class B network could have a value in the range 128–191 (decimal values); the second byte could have any value from 0–255. (Class B addresses can be recognized by the first two address bits being 10-binary.) There were something like sixteen thousand such network addresses available. Amongst those allocated in the early days of the Internet were 128.6 which went to Rutgers University, 128.29 for Mitre corporation, 128.232 for the University of Cambridge’s Computer Laboratory, and 130.198 for Alcatel.
The class C addresses used 24 bits for the network identifier and only 8 bits for the computer identifier. Class C addresses have a first byte with a value in the range 192–223 (the first three bits are 110-binary). While there were a couple of million such network addresses possible, each of these networks could have at most 254 machines (the machine addresses 0 and 255 are reserved for things like broadcast messages on the network).
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