Understanding the Requirements
Network Address: 192.168.1.0/24
The /24 notation means a subnet mask of 255.255.255.0, providing 256 total addresses (192.168.1.0–192.168.1.255).
Usable hosts: 256 – 2 (network and broadcast) = 254.
Goal: Create 3 subnets, each with 30 hosts.
Each subnet needs enough addresses to accommodate 30 hosts, plus 2 reserved addresses (network and broadcast) per subnet.
Total addresses per subnet = 30 (hosts) + 2 (network/broadcast) = 32 addresses.
Subnetting Basics (Networking Fundamentals)
Subnet Mask: Determines how many bits are borrowed from the host portion to create subnets.
Original Mask: /24 (255.255.255.0) = 24 network bits, 8 host bits.
Formulae:
Number of subnets = 2^(number of borrowed bits).
Number of addresses per subnet = 2^(remaining host bits).
Usable hosts per subnet = 2^(remaining host bits) – 2.
We need:
At least 3 subnets.
At least 32 addresses per subnet (to fit 30 hosts + 2 reserved).
Step-by-Step Analysis
Determine Addresses Needed per Subnet:
32 addresses is a power of 2 (2^5 = 32).
This means each subnet requires 5 host bits (since 2^5 = 32 total addresses, and 32 – 2 = 30 usable hosts).
Calculate Remaining Bits:
Original network has 8 host bits (/24).
If 5 bits are left for hosts, we borrow: 8 – 5 = 3 bits for subnetting.
New Subnet Mask:
Original mask: /24 (24 network bits).
Borrow 3 bits: 24 + 3 = /27.
/27 = 255.255.255.224 (binary: 11111111.11111111.11111111.11100000).
Verify Requirements:
Number of Subnets: 2^3 = 8 subnets (meets the requirement of at least 3).
Addresses per Subnet: 2^5 = 32 addresses.
Usable Hosts per Subnet: 32 – 2 = 30 hosts (exactly meets the requirement).
Subnet Breakdown:
Increment: 256 – 224 = 32 (each subnet increments by 32 in the fourth octet).
Subnets:
192.168.1.0–192.168.1.31 (Network: .0, Broadcast: .31, Hosts: .1–.30)
192.168.1.32–192.168.1.63 (Network: .32, Broadcast: .63, Hosts: .33–.62)
192.168.1.64–192.168.1.95 (Network: .64, Broadcast: .95, Hosts: .65–.94)
(And 5 more subnets up to 192.168.1.255.)
Three subnets fit perfectly with 30 hosts each.
Evaluating the Options
A. 255.255.255.128 (/25):
Borrow 1 bit: 24 + 1 = /25.
Subnets: 2^1 = 2 (not enough, need 3).
Host bits: 7 (2^7 = 128 addresses, 126 hosts).
Why Not: Only 2 subnets, fails the requirement.
B. 255.255.255.192 (/26):
Borrow 2 bits: 24 + 2 = /26.
Subnets: 2^2 = 4 (meets 3).
Host bits: 6 (2^6 = 64 addresses, 62 hosts).
Why Not: 62 hosts exceeds 30, but it’s overkill; /27 is more efficient and still valid.
C. 255.255.255.224 (/27):
Borrow 3 bits: 24 + 3 = /27.
Subnets: 2^3 = 8 (meets 3).
Host bits: 5 (2^5 = 32 addresses, 30 hosts).
Why Yes: Perfectly fits 3 subnets with exactly 30 hosts each.
D. 255.255.255.240 (/28):
Borrow 4 bits: 24 + 4 = /28.
Subnets: 2^4 = 16 (meets 3).
Host bits: 4 (2^4 = 16 addresses, 14 hosts).
Why Not: Only 14 hosts per subnet, fails the 30-host requirement.
Why /27 (255.255.255.224) is Best
It provides exactly 30 usable hosts per subnet, avoiding waste while meeting the minimum requirement.
It allows 8 subnets, exceeding the need for 3, ensuring flexibility.
The study guide emphasizes efficient subnet design, and /27 balances host count and subnet availability.
CompTIA Network+ Context
Networking Fundamentals: Subnetting is a core skill, requiring understanding of CIDR, binary conversion, and address allocation.
Example from Study Guide: Similar problems calculate subnet masks for specific host counts, reinforcing /27 as a common solution for ~30 hosts.
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