An address pool is a set of IP addresses assigned by IANA or an organisation tied to it. These address pools are assigned primarily in three classes, enumerated A, B and C. A class A network is actually never assigned in practice; there are only 254 such usable networks containing 256^3-2 available client IP addresses. Class B is somewhat smaller, having 65534 networks and the same amount of hosts. This is suitable for larger entities. Class C address pools are the most common and allow for 254 hosts. 256^3-2 class C networks could be assigned. Note that only one network is assigned, so that if an entity was assigned a class C address, they would be able to host 254 clients on one network. The other 256^3 or so prefixes would not be usable by them.
An additional two classes, D and E, have been assigned by IANA for "experimental use".
It is important to note that because of IP routing and NAT, each IP address can be used more than once.
Originally the address block is a parameter set of the underlying protocol, and the intended purpose of segments of the address space is described in an address architecture.
Address space is allocated to the RIRs in units of /8 address blocks, and the specific address blocks allocated to the RIR is an IANA decision.
Addresses are made available from the unallocated pool to meet demands for their use in networks, and are intended to be assigned for as long as the need condition continues.
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