Subnetting in networking with example

Subnetting is the process of dividing a single large IP network into multiple smaller, logical sub-networks called subnets.

Core Concepts 

• IP Address Components: An IPv4 address (32 bits) consists of two main parts: the Network ID (identifies the overall network) and the Host ID (identifies a specific device).
• Subnet Mask: A 32-bit number used to define where the network part ends and the host part begins. For example, 255.255.255.0 indicates that the first 24 bits are for the network.
• CIDR Notation: A shorthand for the subnet mask. For example, /24 means the first 24 bits are network bits.
• Borrowing Bits: Subnetting works by “borrowing” bits from the Host ID portion to create the Subnet ID. 

Formulas 

• Number of Subnets: 2n2 to the n-th power2𝑛, where nn𝑛 is the number of borrowed bits.
• Number of Hosts per Subnet:2h−22 to the h-th power minus 22ℎ−2, where hhℎ is the number of remaining host bits.
  • The “- 2” accounts for the Network Address (first) and Broadcast Address (last), which cannot be assigned to devices. 

Step-by-Step Example 

Scenario: You have the network 192.168.1.0/24 and need to divide it into 4 equal subnets. 

1. Identify Current State:
   • Network: 192.168.1.0
   • Subnet Mask: 255.255.255.0 (/24)
   • Total available host bits: 8 (32−24=832 minus 24 equals 832−24=8).
2. Calculate Borrowed Bits:
   • To get 4 subnets, solve 2n≥42 to the n-th power is greater than or equal to 42𝑛≥4.
   • n=2n equals 2𝑛=2 bits. You must borrow 2 bits from the host portion.
3. Find New Subnet Mask:
   • New Network bits: 24+2=2624 plus 2 equals 2624+2=26.
   • CIDR: /26
   • Decimal Mask: 255.255.255.192 (The last octet bits 11000000 equals 192).
4. Determine Subnet Ranges:
   • Each subnet will have 26=642 to the sixth power equals 6426=64 total addresses (32 total bits – 26 network bits = 6 host bits).
   • Subnet 1: 192.168.1.0 – 192.168.1.63 (Usable: .1 to .62)
   • Subnet 2: 192.168.1.64 – 192.168.1.127 (Usable: .65 to .126)
   • Subnet 3: 192.168.1.128 – 192.168.1.191 (Usable: .129 to .190)
   • Subnet 4: 192.168.1.192 – 192.168.1.255 (Usable: .193 to .254) 

Benefits of Subnetting 

• Reduced Congestion: Limits broadcast traffic to a smaller “domain”.
• Enhanced Security: Allows administrators to isolate departments (e.g., HR from Sales).
• Efficient IP Usage: Prevents wasting thousands of addresses in large class-based networks.