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2.7.2. Subnetting with VLSM and CIDR
CIDR (Classless Inter-Domain Routing): Allows subnet masks independent of address class. A /26 provides 64 addresses whether it's from Class A, B, or C space.
VLSM (Variable Length Subnet Mask): Using different subnet sizes within the same network. Allocate /24 where you need 200 hosts, /30 for point-to-point links, /27 for small segments.
Common Subnet Sizes:
| CIDR | Subnet Mask | Total IPs | Usable Hosts | Use Case |
|---|---|---|---|---|
| /30 | 255.255.255.252 | 4 | 2 | Point-to-point links |
| /29 | 255.255.255.248 | 8 | 6 | Small segments |
| /28 | 255.255.255.240 | 16 | 14 | Small department |
| /27 | 255.255.255.224 | 32 | 30 | Medium segment |
| /26 | 255.255.255.192 | 64 | 62 | Larger segment |
| /25 | 255.255.255.128 | 128 | 126 | Half Class C |
| /24 | 255.255.255.0 | 256 | 254 | Standard segment |
Subnetting Skill: Given an IP and mask, identify network address, broadcast address, and usable host range. Practice until automatic—this is tested both in multiple-choice and performance-based questions.
Written byAlvin Varughese
Founder•15 professional certifications