Introduction
IP addressing is one of the pillars of networking and therefore when properly configured, it is vital for the efficient operation of networks. IP addresses allow devices on the internet to communicate with each other. However, effective address management is required in large-scale networks. It enters the concept of "subnet". A subnet divides a large IP network into smaller, manageable pieces, enabling more efficient utilization of the network. In this article, we will provide exhaustive and detailed information on the concept of subnet, subnetting mask, CIDR (Classless Inter-Domain Routing) and subnet structure.

1. What is Subnet?
Subnetworking is the subdivision of an IP network into smaller pieces (subnets). This process enhances management, promotes security, and makes the network traffic more efficient. Each subnet has specified blocks of IP addresses to which the devices in that respective block can communicate with each other. With subnetting, network administrators can create more adaptive structures and administer networks easily.

As, for example, creating a separate subnet for each department in an organization will isolate network performance and allow fast communication.

2. IP Address and Subnet Mask
IP addresses are unique addresses that allow communication between network devices. The IP addresses may be in the form of either IPv4 or IPv6. An IPv4 address is, therefore, made up of 32 bits and represented usually in four octets (8 bits) with each octet carrying a value between 0 and 255.

IP Address Format
An example of an IPv4 address: 192.168.1.10

Each of those octets here (192, 168, 1, 10) has a length of 8 bits totaling a complete address of 32 bits.

IP Address Category
IP addresses are usually classified into four major classes:

Class A: between 0.0.0.0 and 127.255.255.255 (Usually used for very big networks).

Class B: between 128.0.0.0 and 191.255.255.255 (Used for medium-sized networks).

Class C: between 192.0.0.0 and 223.255.255.255 (Used for small networks).

Class D: Multicast (For multimedia broadcast).

Class E: Reserved IP addresses.

Outside IP addresses, a subnet mask is used to determine which of the IPs belong to a network.

What is Subnet Mask?
Subnet Mask defines the portion of an IP address that refers to the network address and the portion that refers to the device address (host)—thus it refers to those devices that share the same address. In conjunction with an IP address, a subnet mask will help identify the devices that are in the same network.

Subnet Mask Format
Subnet Mask is also composed of four octets like IP Address. An entire subnet and devices identification gets done with the combination of an IP Address and Subnet Mask.

An example of a subnet mask: 255.255.255.0

Here, 255s indicate a network part, while 0s indicate a host part. Therefore from the above-stated, 192.168.1.0/24 refers to the network by 192.168.1.0, while addresses from 192.168.1.1 to 192.168.1.254 define the host addresses.

3. CIDR: Classless Inter-Domain Routing
CIDR handles the inter-domain addresses as it works at the level of the bits of addresses and does not even take account of octets that make those addresses.