The Class A IPv4 address was designed to support extremely large networks. Because the need for very large-scale networks was perceived to be minimal, an architecture was developed that maximized the possible number of host addresses, but severely limited the number of possible Class A networks that could be defined.
A Class A IP address uses only the first octet to indicate the network address. The remaining three octets enumerate host addresses. The first bit of a Class A address is always a 0. This mathematically limits the possible range of the Class A address to less than or equal to 127, which is the sum of 64 + 32 + 16 + 8 + 4 + 2 + 1. The left-most bit’s decimal value of 128 is absent from this equation. Thus, there can only ever be 127 possible Class A IP networks.
The last 24 bits (that is, three dotted-decimal numbers) of a Class A address represent possible host addresses. The range of possible Class A network addresses or host range is from 1.0.0.0 to 126.255.255.255.
Notice that only the first octet bears a network address number. The remaining three are used to create unique host addresses within each network number. As such, they are set to zeros when describing the range of network numbers.
Each Class A address can support 16,777,214 unique host addresses. This value is calculated by multiplying 2 to the 24th power and then subtracting 2. Subtracting 2 is necessary because IP reserved the all zeros address for identifying the network and the all ones address for broadcasting within that network. The proportion of network to host octets is presented in the following table.
NB:
Technically, 127.0.0.0 is also a Class A network address, but it is reserved for loopback testing and cannot be assigned to a network. In retrospect, this design is poor because it wastes an entire class A address consisting of a large number of IP addresses.
Class A is generally used for very large networks such as the original ARPANET. Most of the class A addresses are allocated and it is very difficult, if not impossible, to get a class A network address assignment from the InterNIC.
0 comments:
Post a Comment