Methodology for calculating subnet masks

1. Why subnet?

• To divide a large network into several smaller networks (subnetworks).
• To optimize the use of IP addresses.
• To isolate services or departments within a network (security, organization).

2. The subnet mask

It indicates how many bits are used for the network and host parts.

Two possible notations:

• Decimal: 255.255.255.0
• CIDR: /24

3. Calculation methodology – step by step

Objective: Find the mask suitable for N machines in a subnetwork

1. Know the number of hosts required (e.g., 50)
2. Add 2 reserved addresses (network + broadcast)
So here: 50 + 2 = 52
3. Find the closest higher power of 2:
2⁵ = 32 not goof
2⁶ = 64 good
4. Deduce the number of bits for the hosts: here 6 bits
5. Calculate the CIDR mask:
32 bits (total) - 6 bits = /26
6. Convert to decimal mask: /26 = 255.255.255.192
7. Check:
Number of usable hosts = 64 - 2 = 62

4. Useful correspondence table