IPv4 subnetting reference

Certain IPv4 address space regions are specially allocated and reserved for special uses such as loopback interfaces, Private networks (RFC 1918), and state-less autoconfiguration (Zeroconf, RFC 3927) of interfaces. Such addresses may be used without registration or allocation from Regional Internet Registries (RIRs). However, these address ranges must not be routed into the public Internet infrastructure.

The netmask is a bitmask that can be used to separate the bits of the network identifier from the bits of the host identifier. It is often written in the same notation used to denote IP addresses.

Not all sizes of prefix announcement may be routable on the public Internet: see routing, peering.

"While the 127.0.0.0/8 network is a Class A network, it is designated for loopback and cannot be assigned to a network."

"Class D multicasting"

"Class E reserved", but also see [http://tools.ietf.org/html/draft-fuller-240space] .

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".

ee also

* Subnetwork
* Classless Inter-Domain Routing (CIDR)