BGP

Appendix C. Notes on the IPv4 Address Space

Up until 1993, address space was assigned using classful mechanisms: everybody needing 254 addresses or less got a Class C network, everybody needing 16384 addresses or less got a Class B network, and everyone needing more than 16384 got a Class A network. About half the Class A and two thirds of the Class B space were assigned this way. Use of the Class C space was limited at this time: only 192.0.0.0/8 and 193.0.0.0/8 were in use to some degree at that moment. Then, after the introduction of Classless Inter-Domain Routing, most new allocations to ISPs and assignments to end users were taken from Class C space, which consequently started to run out quickly around the turn of the millennium. Today, most new address space is allocated as classless blocks from Class A space. During the mid-1990s, most allocations were /19 or larger blocks, but this was changed to /20 later. Thus, prefixes longer than /20 anywhere other than in 192.0.0.0/7 are almost always the result of poor aggregation, which may or may not be for good reasons. Table C-1 shows an overview of the IPv4 address space.

Table C-1. Overview of the IPv4 address space

Address range	Class
0.0.0.0/1	A, classful networks with mask 255.0.0.0 (/8)
128.0.0.0/2	B, classful networks with mask 255.255.0.0 (/16)
192.0.0.0/3	C, classful networks with mask 255.255.255.0 (/24)
224.0.0.0/4	Class D range, meant for multicast use
240.0.0.0/4	Class E range, reserved for future applications

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