Understanding IPv4 Address Space and Classes

The Internet Protocol version 4 (IPv4) address space consists of a total of $2^{32}$ unique addresses, because an IPv4 address is 32 bits long ($2^{32} \approx 4.3 \text{ billion}$ addresses). This vast address space is divided into different classes to manage the allocation and routing of IP addresses efficiently. The primary classes are Class A, Class B, and Class C, which are used for unicast addressing, along with Class D for multicast and Class E reserved for experimental purposes.

Identifying IPv4 Address Classes

Each IPv4 address class is identified by the leading bits of the address. This structure helps determine the network and host portions of the address.

Note: The table shows the theoretical distribution based on leading bits. Some ranges within Class A (like 0.0.0.0/8 and 127.0.0.0/8) are reserved, but the calculation for total class space is based on the bit pattern.

Calculating the Percentage of IPv4 Address Space for Class C

Class C addresses are defined by the first three bits being 110. Since an IPv4 address is 32 bits long, having the first 3 bits fixed leaves $32 - 3 = 29$ bits for the rest of the address. Therefore, the total number of unique addresses within the Class C range is $2^{29}$.

The total IPv4 address space is $2^{32}$ addresses.

To find the percentage of the total IPv4 address space that Class C addresses consume, we calculate the ratio of the number of Class C addresses to the total number of IPv4 addresses and multiply by 100%:

Percentage of Class C space $$ = \frac{\text{Number of Class C Addresses}}{\text{Total IPv4 Addresses}} \times 100\% $$

$$ = \frac{2^{29}}{2^{32}} \times 100\% $$

Using the rules of exponents ($a^m / a^n = a^{m-n}$):

$$ = 2^{29 - 32} \times 100\% $$

$$ = 2^{-3} \times 100\% $$

$$ = \frac{1}{2^3} \times 100\% $$

$$ = \frac{1}{8} \times 100\% $$

$$ = 0.125 \times 100\% $$

$$ = 12.5\% $$

Thus, Class C addresses consume 12.5% of the total IPv4 address space.

Revision Table: IPv4 Classes Summary

Additional Information: IPv4 Address Space and Subnetting

The class-based addressing system (Class A, B, C) was the original way to allocate IPv4 addresses. However, it led to inefficient use of the IPv4 address space. For example, assigning a Class B network (with over 65,000 host addresses) to an organization that only needed a few hundred addresses was wasteful.

• Classful Addressing: The traditional system dividing IP addresses into fixed-size classes.
• Classless Inter-Domain Routing (CIDR): Introduced to overcome the limitations of classful addressing. CIDR uses a variable-length subnet mask, denoted by a slash followed by the prefix length (e.g., 192.168.1.0/24), allowing for more flexible and efficient allocation of IPv4 address blocks. CIDR essentially ignores the old class boundaries.
• Private IP Addresses: Certain blocks within each class (specifically Class A, B, and C) are reserved for private networks and are not routable on the public internet. This helps conserve the public IPv4 address space. Examples include 10.0.0.0/8 (Class A), 172.16.0.0/12 (Class B), and 192.168.0.0/16 (Class C).
• IPv6: Due to the exhaustion of the IPv4 address space, IPv6 was developed. IPv6 addresses are 128 bits long, providing a vastly larger address space ($2^{128}$).

Understanding the breakdown of the IPv4 address space by class is fundamental for grasping the history of IP addressing and the concepts that led to CIDR and IPv6.