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Introduction of Classful IP Addressing

Last Updated : 04 Aug, 2025

Classful IP addressing is an obsolete method for allocating IP addresses and dividing the available IP address space across networks. It was used from 1981 to 1993 until the introduction of CIDR (Based on Prefixes rather than classes). Classful method categorizes IP addresses into five classes (A, B, C, D, and E), each defined by the first few bits of the address and serving specific ranges of purposes. The functionality of classful networking also extended to ease of configuration. Network administrators could set up networks with default subnet masks without the need for detailed subnetting strategies, which was particularly beneficial in less complex networking environments.

Some points to be noted about dotted decimal notation: 

• The value of any segment (byte) is between 0 and 255 (both included).
• No zeroes preceding the value in any segment (054 is wrong, 54 is correct).

As the Internet grew, classful addressing led to significant IP wastage and routing inefficiencies

• Classful addressing provided a straightforward method to allocate and manage IP addresses based on fixed classes, simplifying the administrative burden associated with IP address distribution.

• Network equipment of the time, such as routers, could be more easily programmed to handle a limited number of fixed classes, speeding up the routing process because the class of an address could be quickly identified from its first few bits.

• While it did have limitations, classful addressing allowed for scaling of networks within the bounds of each class size. Larger networks could use a Class A or B address, and smaller networks could operate efficiently within a Class C subnet.

• By standardizing address ranges, classful addressing facilitated the implementation of uniform network protocols, which was important for the interoperability of network devices across different networks and platforms.

• Classful addressing was a cost-effective solution that supported the early expansion of the Internet by avoiding the need for complex subnetting schemes that require additional computing resources and more sophisticated network management tools.

The 32-bit IP address is divided into five sub-classes. These are given below:

• Class A
• Class B
• Class C
• Class D
• Class E

Each of these classes has a valid range of IP addresses. Classes D and E are reserved for multicast and experimental purposes respectively. The order of bits in the first octet determines the classes of the IP address. 

The class of IP address is used to determine the bits used for network ID and host ID and the number of total networks and hosts possible in that particular class. Each ISP or network administrator assigns an IP address to each device that is connected to its network. 

Note: 

• IP addresses are globally managed by Internet Assigned Numbers Authority (IANA) and Regional Internet Registries (RIR).
• While finding the total number of host IP addresses, 2 IP addresses are not counted and are therefore, decreased from the total count because the first IP address of any network is the network number and whereas the last IP address is reserved for broadcast IP.

IP addresses belonging to class A are assigned to the networks that contain a large number of hosts. 

• The network ID is 8 bits long.
• The host ID is 24 bits long.

The higher-order bit of the first octet in class A is always set to 0. The remaining 7 bits in the first octet are used to determine network ID. The 24 bits of host ID are used to determine the host in any network. The default subnet mask for Class A is 255.x.x.x. Therefore, class A has a total of:

• 2²⁴ - 2 = 16,777,214 host ID

IP addresses belonging to class A ranges from 0.0.0.0 - 127.255.255.255. 

IP address belonging to class B is assigned to networks that range from medium-sized to large-sized networks. 

• The network ID is 16 bits long.
• The host ID is 16 bits long.

The higher-order bits of the first octet of IP addresses of class B are always set to 10. The remaining 14 bits are used to determine the network ID. The 16 bits of host ID are used to determine the host in any network. The default subnet mask for class B is 255.255.x.x. Class B has a total of: 

• 2¹⁴ = 16384 network address
• 2¹⁶ - 2 = 65534 host address

IP addresses belonging to class B ranges from 128.0.0.0 – 191.255.255.255.

IP addresses belonging to class C are assigned to small-sized networks.

• The network ID is 24 bits long.
• The host ID is 8 bits long.

The higher-order bits of the first octet of IP addresses of class C is always set to 110. The remaining 21 bits are used to determine the network ID. The 8 bits of host ID are used to determine the host in any network. The default subnet mask for class C is 255.255.255.x. Class C has a total of:

• 2²¹ = 2097152 network address
• 2⁸ – 2 = 254 host address

IP addresses belonging to class C range from 192.0.0.0 – 223.255.255.255.

IP address belonging to class D is reserved for multi-casting. The higher-order bits of the first octet of IP addresses belonging to class D is always set to 1110. The remaining bits are for the address that interested hosts recognize.

Class D does not possess any subnet mask. IP addresses belonging to class D range from 224.0.0.0 – 239.255.255.255.

IP addresses belonging to class E are reserved for experimental and research purposes. IP addresses of class E range from 240.0.0.0 – 255.255.255.255. This class doesn’t have any subnet mask. The higher-order bits of the first octet of class E are always set to 1111.

169.254.0.0 – 169.254.0.16 : Link-local addresses
127.0.0.0 – 127.255.255.255 : Loop-back addresses
0.0.0.0 – 0.0.0.8: used to communicate within the current network.

Host IDs are used to identify a host within a network. The host ID is assigned based on the following rules:

• Within any network, the host ID must be unique to that network.
• A host ID in which all bits are set to 0 cannot be assigned because this host ID is used to represent the network ID of the IP address.
• Host ID in which all bits are set to 1 cannot be assigned because this host ID is reserved as a broadcast address to send packets to all the hosts present on that particular network.

Hosts that are located on the same physical network are identified by the network ID, as all host on the same physical network is assigned the same network ID. The network ID is assigned based on the following rules:

• The network ID cannot start with 127 because 127 belongs to the class A address and is reserved for internal loopback functions.
• All bits of network ID set to 1 are reserved for use as an IP broadcast address and therefore, cannot be used.
• All bits of network ID set to 0 are used to denote a specific host on the local network and are not routed and therefore, aren’t used.

In the above table No. of networks for class A should be 127. (Network ID with all 0 s is not considered)

The problem with this classful addressing method is that millions of class A addresses are wasted, many of the class B addresses are wasted, whereas, the number of addresses available in class C is so small that it cannot cater to the needs of organizations. Class D addresses are used for multicast routing and are therefore available as a single block only. Class E addresses are reserved.

Since there are these problems, Classful networking was replaced by Classless Inter-Domain Routing (CIDR) in 1993. We will be discussing Classless addressing in the next post.

• The network ID is 24 bits long.
• The host ID is 8 bits long.
• 2²¹ = 2097152 network address
• 2⁸ - 2 = 254 host address
• Within any network, the host ID must be unique to that network.
• Host ID in which all bits are set to 0 cannot be assigned because this host ID is used to represent the network ID of the IP address.
• Host ID in which all bits are set to 1 cannot be assigned because this host ID is reserved as a broadcast address to send packets to all the hosts present on that particular network.
• The network ID cannot start with 127 because 127 belongs to the class A address and is reserved for internal loopback functions.
• All bits of network ID set to 1 are reserved for use as an IP broadcast address and therefore, cannot be used.
• All bits of network ID set to 0 are used to denote a specific host on the local network and are not routed and therefore, aren't used.

Here is the main difference between Classful and Classless Addressing:

• Network
• Host 
• Subnet

• Host 
• Subnet

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