IPv4 Subnetting Reference

Certain chunks of IPv4 address space are specially allocated by RFCs for special uses such as loopback (RFC 1643), Private networks (RFC 1918), and Zeroconf (RFC 3927) usage, and are not available for allocation by Regional Internet Registries (RIRs).

The 127.0.0.1 network is left out because it is designated for loopback and cannot be assigned to a network
Class D multicasting
Class E reserved

Note that in common usage, the "host all zeros" address is reserved for referring to the whole network, while the "host all ones" address is reserved as a broadcast address; this reduces the number of hosts available by 2, explaining the reference to /31 as "Useless Network".

CIDR

Classless Inter-Domain Routing (CIDR, pronounced "cider") was introduced in 1993 and is the latest refinement to the way IP addresses are interpreted. It replaced the previous generation of IP address syntax, classful networks. Specifically, rather than allocating address blocks on eight-bit (i.e., octet) boundaries forcing 8, 16, or 24-bit prefixes, it used the technique of variable-length subnet masking (VLSM) to allow allocation on arbitrary-length prefixes. CIDR encompasses:

• The VLSM technique of specifying arbitrary length prefix boundaries. A CIDR-compliant address is written with a suffix indicating the number of bits in the prefix length, such as 192.168.0.0/16. This permits more efficient use of increasingly scarce IPv4 addresses.
• The aggregation of multiple contiguous prefixes into supernets, and, wherever possible in the Internet, advertising aggregates, thus reducing the number of entries in the global routing table. Aggregation hides multiple levels of subnetting from the Internet routing table, and reverses the process of "subnetting a subnet" with VLSM.
• The administrative process of allocating address blocks to organizations based on their actual and short-term projected need, rather than the very large or very small blocks required by classful addressing schemes.

IPv6 utilizes the CIDR convention of indicating prefix length with a suffix, but the longer address field of IPv6 made it unnecessary to practice great economy in allocating the minimum amount of address space an organization could justify. The concept of class was never used in IPv6.

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