class c ip address

Classful vs. Classless IP Addressing: A Deep Dive into Networking Evolution

Classful vs. Classless IP Addressing: A Deep Dive into Networking Evolution

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IP addressing is the backbone of modern networking, enabling devices to communicate across local networks and the internet. Every device—whether a smartphone, server, or IoT sensor—requires a unique IP address to send and receive data.

However, the original IP addressing system (classful addressing) struggled to scale with the internet’s explosive growth. This led to inefficiencies like IPv4 address exhaustion and rigid network structures. The solution? Classless Inter-Domain Routing (CIDR), a flexible system that replaced classful addressing in 1993.

Key Takeaway: Classless addressing (CIDR) solved IPv4 shortages by eliminating fixed network classes and enabling efficient subnetting.

This article, “Classful vs. Classless IP Addressing,” is the continuation of my previous articles about the IP addresses, which are the following:

Unicast, Multicast, and Broadcast Communication

Mastering Host Address, Network Prefix, Network ID, and Broadcast ID in 2025

IP address Classes- Exclusive Explanation

Positional Number System and Examples (Updated 2025)

Network and Host Portion of IPv4 Address

So, you need to study the above article to understand it better before reading it. If you have already covered the above topics, let’s dive straight into this article.

Classful Addressing

The IETF published the first major addressing scheme in September 1981 in RFC 790. The IP addressing scheme was 32 bits long and had three classes, A, B, and C, corresponding to 8-bit, 16-bit, and 24-bit prefixes. No other prefix lengths were allowed then, and there was no concept of nesting a group of 24-bit prefixes, such as within a 16-bit prefix.

Class D and E addresses were also defined, but neither of these two address classes was normally used. Class D addresses are reserved for multicasting, and Class E addresses are reserved for experimental and future use. The easiest way to distinguish between different address classes is to use the first decimal number in the IP address. Classful networks use the classful subnet mask according to the leading bits in the first block of the IP address. The figure below illustrates the key information of the Classful address scheme.

A comprehensive table explaining classful networks, focusing on IP address classes A, B, and C. The table highlights Class, First Octet Range, High Order Bit (HOB), Default Subnet Mask, Possible Networks, and Possible Hosts per Network. Class A supports 128 networks with 16,777,214 hosts each, Class B supports 16,384 networks with 65,534 hosts each, and Class C supports 2,097,152 networks with 254 hosts each. The table visually represents the characteristics and distinctions of classful networks, showcasing the foundational structure of IP address classification for networking.

Historical Context: Why Classful Addressing Failed

The 1980s: The Era of Fixed Classes

In the early days of the internet, IP addresses were divided into five fixed classes (A, B, C, D, E) based on their first few bits. This system wasted addresses. For example, a company needing 500 hosts had to take a Class B (65k hosts), wasting 64,500 addresses. By 1992, 49% of Class B addresses were allocated, risking IPv4 exhaustion (RFC 1338).

1993: RFC 1519 and the CIDR Revolution

CIDR introduced classless addressing, where networks could be split into subnets of arbitrary size using a variable-length subnet mask (VLSM). For example:

  • 192.168.1.0/24 = 254 hosts (subnet mask 255.255.255.0)
  • 10.0.0.0/16 = 65,534 hosts (subnet mask 255.255.0.0)

Result: ISPs could allocate precise address blocks, reducing waste by up to 70% (ICANN Report, 2000).

Technical Breakdown of Classful Addressing

Class A Networks (0.0.0.0 to 127.255.255.255): The Giants

The default subnet mask for this class is 255.0.0.0 or /8. This class supports an extremely large network with more than 16 million hosts. The first octet’s high-order bits of Class A addresses are zero, so the remaining 7 bits create 128 possible Class A networks. 0.0.0.0 is used for the default route, and the 127.0.0.0 network is reserved for local loop testing. So, the remaining network is from 1 – 126 total 126 networks.

  • Range: 1.0.0.0 to 126.255.255.255
  • Subnet Mask: 255.0.0.0 (/8)
  • Hosts: 16,777,214 per network
  • Use Case: Governments and telecom giants (e.g., MIT owns 18.0.0.0/8).

Limitation: Only 126 Class A networks existed globally, monopolized by early internet pioneers.

Class B Networks (128.0.0.0 – 191.255.255.255): The Middle Ground

The default subnet mask for the class B network is 255.255.0.0 or /16. Class B network support supports large networks with up to 65,000 host addresses. The high-order bits for the class B network are 10 in the first octet, and the remaining bits of the first 2 octets create over 16,000 networks. The network 169.254.0.0 is a special network for link-local addresses, also known as Automatic Private IP Addressing (APIPA).

  • Range: 128.0.0.0 to 191.255.255.255
  • Subnet Mask: 255.255.0.0 (/16)
  • Hosts: 65,534 per network
  • Use Case: Universities and mid-sized corporations.

Problem: Companies like Ford (19.0.0.0/8) hoarded Class A blocks, while smaller firms faced scarcity.

Class C Networks (192.0.0.0 – 223.255.255.255): Too Small for Growth

The default subnet mask for a Class C network is 255.255.255.0 or /24. Class C supports small networks with a maximum of 254 hosts. The first three bits of the octet indicate the high-order bit of the class. The remaining bits of the first three octets indicate the network, and the fourth indicates host addresses in this class. The high-order bit is 110. A Class C address has over 2 million possible networks.

  • Range: 192.0.0.0 to 223.255.255.255
  • Subnet Mask: 255.255.255.0 (/24)
  • Hosts: 254 per network
  • Use Case: Small offices.

Issue: Startups needing 300 hosts had to request multiple Class C blocks, complicating routing tables.

Class D (224.0.0.0 – 239.255.255.255)

The first four bits of the first octet in Class D IP addresses are high-order bits (HOB); the first four bits are 1110. The range of Class D addresses starts from 224.0.0.0 to 239.255.255.255. Class D is reserved for multicasting. In multicast communication, data is destined for multiple hosts, not for a particular host. The class has no subnet defined.

Class E (240.0.0.0 – 255.255.255.254)

The first five bits of the first octet are reserved HOB for Class E address. The HOB for Class E is 11111. The address range is 240.0.0.0 to 255.255.255.254. This class is reserved for experimental purposes only, such as R&D and study. Class E is also not equipped with a subnet mask like Class D.

Public IP Addresses

A public IP address range is defined for network devices, hosts, and servers like web servers and email servers to allow direct access to the Internet. Any server device using public IP addresses directly accessible from the Internet. A public IP address is globally unique and can only be assigned once to any device worldwide. Every device accessing the internet is using a unique IP address. Public IP addresses are also required for any publicly accessible network hardware, such as servers hosting websites. Public addresses are globally routed between different ISPs and routers. However, some addresses are not routable on the Internet. These addresses are called private addresses.

Private IP addresses

Private IPv4 addresses were introduced in 1990 because of reduced IPv4 addresses. The Private addresses are not unique and can be used repeatedly for internal networks. The computers at home, tablets, smartphones, network printers, and the computers within organizations are generally assigned private IP addresses. A computer with a private IP address can see and access the local network through its private IP address.

The computer and devices with a private IP address cannot directly access and communicate via the private IP address; however, using the router’s public IP addresses, the devices outside a private network can communicate. The NAT allows direct access to a local device assigned a private IP address. The range of private IP addresses is defined for all three classes.

10.0.0.0 /8 or 10.0.0.0 to 10.255.255.255

172.16.0.0 /12 or 172.16.0.0 to 172.31.255.255

192.168.0.0 /16 or 192.168.0.0 to 192.168.255.255

The Birth of Classless Addressing (CIDR)

Classless Addressing

CIDR replaces fixed classes with prefix lengths (e.g., /24, /17) to define networks. Classful addressing divides an IP address into the Network and Host portions along octet boundaries. It uses a fixed subnet mask, which is /8, /16 and /24, but classless addresses use a variable number of bits for the network and host portions of the address. The subnet mask is not fixed for a classless addressing system.

The classful addressing system assigned 50% of IPv4 addresses to Class A networks, 25% of IPv4 addresses to Class B, 12.5% of IPv4 addresses to Class C, and the remaining 12.5 % Shared to both Class D and E. The classful addressing plan wastes the most IP addresses, decreasing the availability of IPv4 addresses. For example, an organization with a network with more than 254 hosts would need a class B network with more than 65,000 addresses, wasting 64,700 IP addresses.

IETF introduced classless addressing to overcome the waste of IP addresses in 1993. There is no IP address class in a classless addressing system, but the addresses are still granted in blocks. In a classless addressing system, when an organization or individuals need connectivity to the Internet, it also grants a block or range of addresses according to the needs of the organization and individuals. For example, an individual requires only two addresses, and an organization is given thousands of addresses based on the number of its requirements.

Key Innovations:

  1. Aggregation: Combine multiple networks into a single route (e.g., 192.168.0.0/16 covers all /24 subnets).
  2. VLSM: Subnet a network into smaller chunks (e.g., split /24 into four /26 subnets).

Classful vs. Classless: Side-by-Side Comparison

FeatureClassful AddressingClassless Addressing (CIDR)
FlexibilityFixed classes (A, B, C)Custom prefix lengths (e.g., /24, /28)
SubnettingLimited to default masks (e.g., /8, /16)VLSM allows variable-sized subnets
Address EfficiencyHigh waste (e.g., 64k hosts for 500 needed)Minimal waste (allocate exact needs)
Routing TablesLarge tables (no aggregation)Compact tables (route aggregation)
Adoption Era1981–19931993–Present
Example150.10.0.0 (Class B, /16)150.10.0.0/22 (1,022 hosts)

Real-World Applications

Case Study 1: ISP Address Allocation

Comcast uses CIDR to allocate /29 blocks (8 addresses) to small businesses while reserving /20 blocks for enterprise clients.

Case Study 2: AWS VPC Subnetting

Amazon VPC allows users to create subnets like 10.0.1.0/24 for web servers and 10.0.2.0/28 for databases, optimizing security and cost.

To understand CIDR calculation, read our complete articles about IP address subnetting. Subnetting is essential for any networking technician and engineer. However, once you understand and pass your CCNA exam, then you can use our free online subnetting calculator for fast working.

FAQs

  • What is Classful IP Addressing?

    Classful IP Addressing divides IP addresses into fixed classes (A, B, C, D, E) based on their leading bits. Each class has a predefined subnet mask, which determines the division between the network and host portions. While simple, this method often leads to inefficient IP allocation in large networks.

Mastering IPv4 Address Classes with Confidence: 2025 Guide

Mastering IPv4 Address Classes with Confidence: 2025 Guide

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The IPv4 address is an identifying number for network devices. It is similar to a home or business address, which supplies that specific physical location with a particular address. Devices on a network are differentiated from one another through IPv4 addresses. In 2025, understanding IPv4 addressing is critical for CCNA preparation and managing networks with 5G, IoT, and high-speed 400G/800G interfaces, where address scalability remains a key challenge.

An IPv4 address allows a device to communicate with other devices over an IP-based network, like the Internet. There are two primary standards for IPv4 addresses: IPv4 and IPv6. In this article, I will discuss the IPv4 address. The IPv6 address will be discussed in later articles.

IPv4 Addressing

IPv4 is the fourth revision of IP and a widely used protocol for data communication over various networks. It is defined and specified in the IETF publication RFC 791. A connectionless protocol provides logical connections between network devices and a unique identification for each device.

Depending on the network type, we can configure the IPv4 addresses on the network devices manually or automatically. IPv4 uses 32-bit addresses for Ethernet communication in five IPv4 address classes: A, B, C, D, and E.

As of 2025, IPv4 address exhaustion drives the adoption of NAT and IPv6 transition strategies, especially in data centers supporting 100G/400G backbones.

Address Architecture

TCP/IP was introduced in the 1980s. At that time, the Internet used a two-level addressing scheme, which offered sufficient scalability. The 32-bit IPv4 address, introduced in the 1980s with a two-level scheme, now uses subnetting and CIDR in 2025 to support modern 400G/800G networks and IoT scalability. The figure below illustrates the 32-bit address architecture in the 1980s.

The image shows a diagram divided into two sections. The left section is labeled "Network (16 Bits)" and is colored in a darker blue shade. The right section is labeled "Host (16 Bits)" and is colored in a lighter blue shade. This diagram illustrates the division of an IPv4 address into network and host portions, each consisting of 16 bits.

The current IPv4 address is also 32 bits long and contains two portions: a network and a host portion. The network portion identifies the host’s network, and the host portion shows the host’s number in the network. The host portion identifies the device or the host. The router uses the network portion of the IPv4 addresses to make routing decisions and to facilitate communication between hosts that belong to different networks. Like routers and computers, humans can’t work with strings of 32 bits (1s and 0s).

Therefore, we express the 32-bit IPv4 address in dotted-decimal notation, as we know that 32-bit addresses are further divided into four groups of eight, called octets. An octet ranges from 000000000 to 11111111. Each octet is converted to decimal and separated by decimal points or dots. The range of each octet in decimal is from 0 to 255.

For example, 10.10.10.1, 172.16.16.254, and 192.168.0.254 are IPv4 addresses expressed in dotted decimal notation. If we can convert these addresses to binary. For example, the binary of the above addresses is 00001010. 00001010. 00001010.00000001, 10101100. 00010000.00010000. 11111110, 11000000. 10101000.00000000.11111110.

The 32-bit IPv4 structure is supplemented with subnetting and CIDR to optimize address allocation for dense IoT networks and high-speed 400G/800G deployments.

IPv4 Address Classes

At the start of TCP/IP, the IPv4 address classes define the network and host portions. IPv4 addresses are divided into five different IPv4 address classes. The first few bits of the first octet determine the IPv4 address classes. These bits are known as high-order bits (HOB). Although the IPv4 address classes can still be applied to IPv4 addresses, networks today often ignore the class rules in favor of a classless IP scheme. The five IPv4 address classes are A, B, C, D, and E.

Classes A, B, and C have different bit lengths for addressing the network host. Class D addresses are reserved for multicasting, while Class E addresses are reserved for future use. Depending on the address class, each of the four octets of an IPv4 address represents the network portion of the address. The figure below illustrates the network and host portions of the respective Class A, B, C, and D addresses.

The image is a table that categorizes different IPv4 address classes based on their octet structure. The table is titled "Class of Address" and "Octets" and is sourced from "Networkustad.com". The table has four rows for Class A, Class B, Class C, and Class D addresses, and four columns representing the octets. The transcription of the table is as follows: Class of Address: The different classifications of IP addresses (Class A, Class B, Class C, Class D). Octets: Class A: Network | Host | Host | Host Class B: Network | Network | Host | Host Class C: Network | Network | Network | Host Class D: Host | Host | Host | Host

The first three IPv4 address classes, A, B, and C, host addresses in any IP network. However, the Class D addresses are used for multicasting. Class E addresses are reserved for experiments in labs and medical equipment and are not shown in the above figure.

It is not easy to identify the network and host portions of any dotted-decimal IPv4 address because IPv4 addresses are not four numbers. They contain 32 different numbers or 32 bits (0 or 1s). The figure below illustrates all the important figures of IPv4 address classes.

A detailed table categorising IP address classes (A, B, C, D, E) by the value of the 1st octet, high order bit, default subnet mask, possible networks, hosts per network, and private IP address range. The table also includes notes about reserved addresses.

In 2025, classful addressing is largely replaced by classless inter-domain routing (CIDR), enabling efficient IP allocation for 5G and IoT devices across 100G/400G networks.

Class A

The most significant bit of the class A network is 0, also known as the high-order bit(HOB). The remaining seven bits in the first octet define the number of networks in class A addresses, and the three octets (24 bits) define the total number of IPv4 addresses available in the class. The figure below illustrates Class A’s address bit distribution.

If the first bit of an IPv4 address’s first octet is binary 0, the address is a Class A address. With the first bit being 0, the lowest number is 00000000 (decimal 0), and the highest number is 01111111 (decimal 127). Any address that starts with a value between 0 and 127 in the first octet is a Class A address. The numbers 0 and 127 are reserved and cannot be used as a network address.

Class A network can accommodate a very large number of hosts because only the first octet is reserved for the network number. The three octets, or 24 bits, are reserved for the host portion of the network. Each network of this class supports a maximum of 16,777,214 (2^24 -2) hosts per network. Figure 2 is subtracted because the first and last addresses are reserved as network IDs and broadcast IPs. The lowest address is 24, 0s in the host portion, and the highest is 24, 1s in the host portion.

This image is a diagram showing the bit allocation for a Class A IP address. It is divided into two main sections: Network Bits and Host Bits. The Network Bits section is highlighted in light green and spans from bit 0 to bit 7. The Host Bits section is highlighted in dark green and spans from bit 8 to bit 31. The diagram is labeled as "Class A Address Bits" at the top. The bit positions are marked at intervals: 0, 1, 7, 8, 15, 16, 23, 24, and 31. This image helps understand how IP addresses are structured in Class A networks, where the first 8 bits are used for the network identifier and the remaining 24 bits are used for host identifiers within that network.

The routing table uses network ID for the routing table. So, an address with all 0s in the host portion cannot be used with the individual host. For example, 10.0.0.0 is a Class A Address containing all 0s in the host portion. So it is network identification and cannot be configured on network hosts and devices.

Every network also requires a broadcast address, which can be used to send a message to each host on the network. This address has all 1s on the host portion. For example, a broadcast address for network 10.0.0.0 would be 10.255.255.255.

The total number of networks in Class A is 126. Each of the 126 Class A networks has almost 17 million host addresses, which make up 50 % of the entire IPv4 address space. The 0.0.0.0 address is also reserved for the default route. 127.0.0.0 is reserved for the local loopback address, which is why the Class A network range is from 1 to 126, and Class B addresses start from the address of 128.0.0.0.

A Class A network provides many possibilities for any organization or company. It is a huge global network with many hosts, and the hosts in such a network do not function as members of the same logical group.

Class A addresses creating a large broadcast domain, but network administrators need smaller logical groupings to control broadcasts, apply policies, and troubleshoot problems. This is possible by applying a subnet mask, which allows subnetting.

Subnetting breaks a large block of addresses into smaller groups called subnetworks. It avoids the waste of IPv4 addresses and is also helpful for better efficiency. The default subnet mask for a class A IPv4 address is 255.0.0.0. The private IPv4 address range for class A is 10.0.0.0 to 10.255.255.255.

Class B

The high-order bit of class B addresses starts with a binary 10 in the first 2 bits of the first octet. So, the lowest number of class B addresses is 10000000 (decimal 128), and the highest is 10111111 (decimal 191). So, any IPv4 address starts with a range between 128 – 191, and the first octet is a class B IPv4 address. A Class B network can provide an addressing scheme to medium-sized networks.

In Class B, the first two octets represent the network, and the other two represent the address’s host portion. So class B has 16384 (214) Network addresses and 65534 (216-2) Host addresses. So, each Class B network can support up to 65,534 hosts. The Class B network is much smaller than the networks of Class A. Just like Class A networks, we can subnet the Class B network to improve network efficiency. The figure below illustrates the 32-bit distribution of the Class B network.

The diagram illustrates the bit allocation for Class B IP addresses. The 32-bit address is divided into two main sections: Network Bits: These are represented in light green and span from bit 0 to bit 15. Host Bits: These are represented in dark green and span from bit 16 to bit 31. The diagram helps explain the structure of Class B IP addresses, where the first 16 bits are dedicated to the network identifier and the remaining 16 bits are used for host identifiers within that network.

The first octet of a Class B address has 64 possibilities ranging from 128 to 191. The second octet has 256 possibilities ranging from 0 to 255. That gives 16,384 networks. We can multiply 64 by 256 to get the total possible network in Class B. The default subnet mask for Class B IPv4 addresses is 255.255.0.0. Class B addresses also have a private IPv4 address range from 172.16.0.0 to 172.31.255.255.

Class C

The high-order bits of a class C address are binary 110. So, the lowest number that starts with binary 110 is 11000000 (Decimal 192), and the highest number that begins with binary 110 is 11011111 (Decimal 223). So, the class C range is from 192 to 223 in the first octet.

Class C addresses were initially planned for a small network. The first three octets of a Class C address represent the network portion of the IPv4 address, and the fourth octet represents the host portion. The figure below illustrates the distribution of Class C address bits.

The image is a diagram illustrating the bit allocation for a Class C IP address. The diagram is divided into two main sections: Network Bits and Host Bits. The Network Bits section spans from bit 0 to bit 23. The Host Bits section spans from bit 24 to bit 31. The diagram is labeled "Class C Address Bits" at the top. The Network Bits are highlighted in light green, while the Host Bits are highlighted in a darker green. The bit positions are marked at the top of the diagram, ranging from 0 to 31.

As we learn, the first 3 bits of a Class C address are always 110, so 21 bits are left in the network portion of the address, which results in 221 or 2,097,152 Class C networks. Each network contains 254 (28 -2) hosts. The first and last addresses are reserved for network identification and broadcasting. The default subnet mask for class C IPv4 addresses is 255.255.255.0. Class C addresses are also a private IPv4 address range starting from 192.168.0.0 to 192.168.255.255

Class D

The high-order bit of a Class D address is 1110 in the first octet. So, the lowest number starting with 1110 is binary 11100000 (Decimal 224), and the highest is 11101111 (Decimal 239). So, the range of Class D addresses is 224 to 239 in the first octet. Class D addresses are not used with individual hosts; they can be used with hosts called a host group or a multicast group.

It is used when a single message requires sending to many selected recipients, known as multicast or multicasting. Multicast is different from broadcast. Every device on a logical network must process a broadcast, whereas only devices configured to listen for a Class D address receive a multicast.

Class E

The high-order bits of a class E IPv4 address begin with 1111. The range of this class starts with 11110000 (decimal 240) and ends with 11111111 (decimal 255). So, the range of class E is from 240 to 255 in the first octet. Class E addresses are reserved for experimental purposes.

2025 IPv4 Addressing Strategies

In 2025, optimize IPv4 usage with NAT64 for IPv6 transition, implement CIDR for 400G/800G scalability, and use DHCPv6 for dual-stack networks in 5G environments.

Conclusion – IPv4 Address Classes

This 2025 guide provides a comprehensive foundation for mastering IPv4 address classes, equipping CCNA students and network administrators with essential knowledge to navigate modern networking challenges. With the integration of subnetting, CIDR, and dual-stack strategies, IPv4 remains vital in supporting 5G, IoT, and high-speed 400G/800G interfaces. As networks evolve, leveraging these techniques ensures efficient address allocation and scalability, preparing you to excel in both certification exams and real-world enterprise environments

FAQs

Self-Assessment - IP Address Classes