Subnetting

Cidr / - 1 bits in subnet mask Magic number(increments) = 256 - last interesting octet in mask S = Subnet bits. Example 255.255.192.0 = 1.1.1100000.0, so S = 2 squared H = power of 2, then subtract 2(broadcast and network) = Host bits. So 255.255.192.0 = 14 to the power of 2 - 2 = 16382

CIDR: To quickly find cidr if subnet mask known, count 1’s in subnet mask. So 255.255.192 = 18 = 11111111.11111111.11000000.0000000

1 1 1 1 1 1 1 1 128 64 32 16 8 4 2 1

Subnetting

Core Concepts

  • Subnetting allows network administrators to divide a single network into multiple smaller networks (subnets) for better organization, security, and efficient IP address utilization
  • Key principle: Borrowing bits from the host portion to create additional network segments
  • CIDR (Classless Inter-Domain Routing) notation expresses network prefix length as /x where x = number of consecutive 1 bits in subnet mask

CIDR Calculation Methods

Method 1: Counting 1-bits in Subnet Mask

  • Count all consecutive 1 bits from left to right in binary subnet mask
  • Example: 255.255.192.0 = 11111111.11111111.11000000.00000000 = /18 CIDR

Method 2: Quick Binary Reference

  • Each octet position represents powers of 2: 128, 64, 32, 16, 8, 4, 2, 1
  • Add values where 1-bits exist to verify subnet mask accuracy

Magic Number Method

  • Magic Number = 256 - Last Interesting Octet
  • “Interesting octet” = rightmost octet in subnet mask that isn’t 255 or 0
  • Magic number determines subnet increment values and network boundaries
  • Example: 255.255.192.0 → Magic Number = 256 - 192 = 64
  • Networks increment by 64: 0.0, 64.0, 128.0, 192.0

Subnet and Host Calculations

Subnet Bits (S)

  • S = number of bits borrowed from host portion for subnetting
  • Number of subnets = 2^S
  • Example: /18 mask on Class B network borrows 2 bits → 2² = 4 subnets

Host Bits (H)

  • H = remaining bits available for host addressing
  • Number of hosts per subnet = 2^H - 2
  • Subtract 2 for network address and broadcast address (unusable for hosts)
  • Example: /18 leaves 14 host bits → 2¹⁴ - 2 = 16,382 hosts per subnet

Vocabulary

Term Definition
CIDR Classless Inter-Domain Routing - method for allocating IP addresses using prefix notation
Magic Number Increment value calculated as 256 minus the interesting octet
Interesting Octet Rightmost subnet mask octet that contains both 1s and 0s
Network Address First address in subnet range (all host bits = 0)
Broadcast Address Last address in subnet range (all host bits = 1)
Subnet Mask 32-bit value distinguishing network portion from host portion

Practical Examples

Example 1: 192.168.1.0/26

  • Subnet mask: 255.255.255.192
  • Magic number: 256 - 192 = 64
  • Subnets: .0, .64, .128, .192
  • Hosts per subnet: 2⁶ - 2 = 62 hosts
  • Use case: Small office networks requiring isolation between departments

Example 2: 10.0.0.0/22

  • Subnet mask: 255.255.252.0
  • Magic number: 256 - 252 = 4 (in third octet)
  • Networks: 10.0.0.0, 10.0.4.0, 10.0.8.0, 10.0.12.0
  • Hosts per subnet: 2¹⁰ - 2 = 1,022 hosts
  • Use case: Medium enterprise networks with multiple VLANs

Notes

  • Always verify calculations by ensuring network + broadcast addresses don’t overlap between subnets
  • When subnetting, the network grows vertically (more subnets) at the expense of horizontal growth (fewer hosts per subnet)
  • VLSM (Variable Length Subnet Masking) allows different subnet sizes within the same major network for optimal address utilization
  • For CCNA exam: Practice identifying subnet boundaries quickly using magic number method
  • Common mistake: Forgetting to subtract 2 from host calculations for network and broadcast addresses
  • Zero subnets and all-ones subnets are valid in modern networking (older Cisco IOS versions disabled by default)
  • Use ip subnet-zero command on legacy equipment to enable subnet 0 usage