Classful vs. Classless Addressing in IPv4: Understanding IP Address Allocation
Compare and contrast classful and classless (CIDR) addressing in IPv4. This guide explains the limitations of classful addressing, the advantages of CIDR's variable-length subnet masks, and how CIDR notation improves IP address allocation efficiency.
Classful vs. Classless Addressing in IPv4
Introduction to IPv4 Addressing
Every device on an IP network needs a unique IP (Internet Protocol) address for communication. IPv4 (Internet Protocol version 4) uses 32-bit addresses. These addresses are hierarchical, meaning they are divided into two parts: a network portion (identifying the network) and a host portion (identifying the specific device on that network).
Classful Addressing
Early IPv4 addressing used a system called classful addressing. This divided the 32-bit address into five classes (A, B, C, D, E), each with a fixed-length network prefix. The class was determined by the starting bits of the IP address.
Classful Address Classes:
| Class | Address Range | Network Bits | Host Bits | Number of Networks | Hosts per Network |
|---|---|---|---|---|---|
| A | 0.0.0.0 - 127.255.255.255 | 8 | 24 | 128 | 16,777,214 |
| B | 128.0.0.0 - 191.255.255.255 | 16 | 16 | 16,384 | 65,534 |
| C | 192.0.0.0 - 223.255.255.255 | 24 | 8 | 2,097,152 | 254 |
| D (Multicast) | 224.0.0.0 - 239.255.255.255 | - | - | - | - |
| E (Reserved) | 240.0.0.0 - 255.255.255.255 | - | - | - | - |
Classless Addressing (CIDR)
Classful addressing proved inefficient, leading to significant IP address wastage. Classless addressing (CIDR - Classless Inter-Domain Routing) was developed to address this. CIDR allows for variable-length subnet masks, providing more flexibility in allocating IP addresses and improving efficiency.
CIDR Notation:
CIDR uses slash notation (e.g., 192.168.1.0/24). The number after the slash indicates the number of bits used for the network portion of the address. The remaining bits are for the host portion.
Example:
The address 167.199.170.82/27 has 2(32-27) = 32 addresses. The first address is found by setting the last 5 bits to 0, and the last address is found by setting them to 1.
Classful vs. Classless Addressing: A Comparison
| Feature | Classful Addressing | Classless Addressing (CIDR) |
|---|---|---|
| Network Size | Fixed | Variable |
| Prefix Length | Fixed (8, 16, 24) | Variable (0-32) |
| Efficiency | Inefficient (address wastage) | More efficient |
| Subnetting | Less flexible | Highly flexible |
| Current Use | Obsolete | Current standard |
Conclusion
Classless addressing (CIDR) has replaced classful addressing as the standard for IPv4 because it's more flexible and efficient. CIDR's variable-length subnet masking allows for better utilization of IP addresses and simplifies network planning and management.