Subnet Calculation Guide
Master IP subnetting, CIDR notation, and VLSM with practical examples
📊 CIDR Reference Table
Complete subnet mask reference for all CIDR notations
| CIDR | Subnet Mask | Usable Hosts | Total Networks | Class |
|---|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,214 | 1 | Class A |
| /9 | 255.128.0.0 | 8,388,606 | 2 | Class A |
| /10 | 255.192.0.0 | 4,194,302 | 4 | Class A |
| /11 | 255.224.0.0 | 2,097,150 | 8 | Class A |
| /12 | 255.240.0.0 | 1,048,574 | 16 | Class A |
| /13 | 255.248.0.0 | 524,286 | 32 | Class A |
| /14 | 255.252.0.0 | 262,142 | 64 | Class A |
| /15 | 255.254.0.0 | 131,070 | 128 | Class A |
| /16 | 255.255.0.0 | 65,534 | 256 | Class B |
| /17 | 255.255.128.0 | 32,766 | 512 | Class B |
| /18 | 255.255.192.0 | 16,382 | 1,024 | Class B |
| /19 | 255.255.224.0 | 8,190 | 2,048 | Class B |
| /20 | 255.255.240.0 | 4,094 | 4,096 | Class B |
| /21 | 255.255.248.0 | 2,046 | 8,192 | Class B |
| /22 | 255.255.252.0 | 1,022 | 16,384 | Class B |
| /23 | 255.255.254.0 | 510 | 32,768 | Class B |
| /24 | 255.255.255.0 | 254 | 65,536 | Class C |
| /25 | 255.255.255.128 | 126 | 131,072 | Class C |
| /26 | 255.255.255.192 | 62 | 262,144 | Class C |
| /27 | 255.255.255.224 | 30 | 524,288 | Class C |
| /28 | 255.255.255.240 | 14 | 1,048,576 | Class C |
| /29 | 255.255.255.248 | 6 | 2,097,152 | Class C |
| /30 | 255.255.255.252 | 2 | 4,194,304 | Class C |
| /31 | 255.255.255.254 | 2* | 8,388,608 | Class C |
| /32 | 255.255.255.255 | 1 | 16,777,216 | Class C |
🔒 Private IP Address Ranges
RFC 1918 private address space for internal networks
| CIDR Range | Start Address | End Address | Total Hosts | Network Type |
|---|---|---|---|---|
| 10.0.0.0/8 | 10.0.0.0 | 10.255.255.255 | 16,777,216 | Class A |
| 172.16.0.0/12 | 172.16.0.0 | 172.31.255.255 | 1,048,576 | Class B |
| 192.168.0.0/16 | 192.168.0.0 | 192.168.255.255 | 65,536 | Class C |
🎯 VLSM (Variable Length Subnet Mask)
Efficient IP address allocation using variable subnet sizes
Example: Company Network Design
Allocating 192.168.1.0/24 for different departments:
Sales Department
Hosts needed: 50
Subnet: 192.168.1.0/26
Range: 192.168.1.1-62
Engineering
Hosts needed: 25
Subnet: 192.168.1.64/27
Range: 192.168.1.65-94
Management
Hosts needed: 10
Subnet: 192.168.1.96/28
Range: 192.168.1.97-110
Server Room
Hosts needed: 5
Subnet: 192.168.1.112/29
Range: 192.168.1.113-118
WAN Links
Hosts needed: 2
Subnet: 192.168.1.120/30
Range: 192.168.1.121-122
💻 Understanding Subnet Masks in Binary
How subnet masks work at the binary level
IP Address:
192.168.1.100
Binary:
11000000.10101000.00000001.01100100
Subnet /24:
255.255.255.0
Binary:
11111111.11111111.11111111.00000000
Network:
11000000.10101000.00000001.00000000
Result:
192.168.1.0 (Network Address)
Subnetting Best Practices
- Always plan for growth: Reserve 20-30% extra IP addresses for future expansion
- Use VLSM efficiently: Allocate larger subnets first, then smaller ones
- Document everything: Maintain an IP address management (IPAM) spreadsheet
- Consider summarization: Design subnets that can be easily summarized for routing
- Reserve networks: Keep some subnet space unallocated for emergency use
- Use private ranges: Always use RFC 1918 addresses for internal networks
- Plan for services: Separate subnets for servers, users, management, and DMZ
- Remember the rules: Network and broadcast addresses cannot be assigned to hosts