CREST Practitioner Security Analyst (CPSA) Practice

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In classful IP addressing, how many networks are available in Class A?

16,384
128
2,097,152
128 Networks

The correct answer is: 128 Networks

In classful IP addressing, Class A is defined with a significant portion of its address space reserved for the network portion. Class A addresses range from 0.0.0.0 to 127.255.255.255. The first octet of a Class A address determines the network, and bits are allocated as follows: the first bit is fixed as '0,' which means that the other seven bits can be utilized for network identification. Given that the first octet has 7 bits available for addressing (the 0 in the first bit is not used for network identification), we can calculate the number of possible networks in Class A by taking 2 raised to the power of the available bits. This means: 2^7 = 128. Thus, there are 128 possible unique networks in Class A address space. These networks can host a vast number of devices due to the large amount of host address space available in each network (over 16 million hosts per Class A network), making Class A suitable for organizations needing a significant number of IP addresses.