If basic subnetting is about splitting a network, advanced subnetting is about doing it efficiently, securely, and strategically to support growth, isolation, and troubleshooting. As a sysadmin, I use subnetting to design networks for branch offices, segment services like printers or VoIP, and avoid IP conflicts.
📌 Recap: What Is Subnetting?
Subnetting breaks a large network (like 192.168.0.0/16) into smaller chunks or subnets to improve performance, security, and management.
- CIDR Notation:
192.168.1.0/24→ 256 IPs (254 usable) - Subnet Mask: Defines how many bits are used for network vs host
🔍 Key Concepts to Master
1. CIDR Notation (/24, /25, /26…)
CIDR = Classless Inter-Domain Routing
It shows how many bits are used for the network.
| CIDR | Subnet Mask | Total Ips | Usable Hosts | Subnets from a /24 |
|---|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 | 1 |
| /25 | 255.255.255.128 | 128 | 126 | 2 |
| /26 | 255.255.255.192 | 64 | 62 | 4 |
| /27 | 255.255.255.224 | 32 | 30 | 8 |
| /28 | 255.255.255.240 | 16 | 14 | 16 |
| /29 | 255.255.255.248 | 8 | 6 | 32 |
| /30 | 255.255.255.252 | 4 | 2 | 64 |
Example:
If I need four VLANs with ~60 devices each, I’ll choose /26 → gives me 62 usable IPs per subnet.
2. Finding Subnet Ranges Manually (Layman Trick)
Let’s subnet 192.168.1.0/24 into 4 subnets (use /26):
- Block size = 64
(Formula:2^(32 - new CIDR)→2^6 = 64)
Subnets will be:
192.168.1.0 – 192.168.1.63192.168.1.64 – 192.168.1.127192.168.1.128 – 192.168.1.191192.168.1.192 – 192.168.1.255
Each has:
- 62 usable IPs (first = network address, last = broadcast address)
3. Subnetting a Bigger Network (e.g., /16)
Let’s say we’re given: 10.0.0.0/16
That’s 65,536 total addresses.
If I want 100 subnets, I ask:
How many bits do I borrow from host part?
2^7 = 128→ I need 7 extra bits- So, new subnet =
/23(16 original + 7 = 23)
Result:
- 100+ subnets, each with 510 usable hosts
4. Variable Length Subnet Masking (VLSM)
Not all subnets need the same size. VLSM lets you assign different subnet sizes based on the need.
Example:
- Office A: Needs 100 hosts →
/25 - Office B: Needs 50 hosts →
/26 - Office C: Needs 10 hosts →
/28
This way you avoid wasting IPs.
5. Subnetting for Security & Segmentation
Common use cases in real life:
| Subnet | CIDR | Purpose |
|---|---|---|
192.168.1.0/26 | /26 | Staff devices |
192.168.1.64/26 | /26 | VoIP phones |
192.168.1.128/26 | /26 | Printers & IoT |
192.168.1.192/26 | /26 | Guests (isolated VLAN) |
Helps in:
- Preventing malware spread
- Isolating traffic
- Applying firewall rules easily
🛠️ Real-World Example: Multi-Branch Office
You have:
- 5 branch offices
- Each needs:
- 1 subnet for users (60 devices)
- 1 subnet for VoIP (30 devices)
- 1 subnet for printers/IoT (15 devices)
Planning:
- Users →
/26(62 usable) - VoIP →
/27(30 usable) - IoT →
/28(14 usable)
You can design IP ranges like:
| Office | Subnet Purpose | Subnet Address |
|---|---|---|
| A | Users | 10.0.0.0/26 |
| A | VoIP | 10.0.0.64/27 |
| A | IoT | 10.0.0.96/28 |
| B | Users | 10.0.1.0/26 |
| … | … | … |
🚨 Common Mistakes to Avoid
- Overlapping subnets → causes conflicts
- Wrong subnet mask → devices can’t communicate
- Forgetting about reserved addresses → 2 IPs lost per subnet (network & broadcast)
- Underestimating future growth → always leave buffer IPs if possible
🧠 Summary
- Advanced subnetting is about optimizing address use and segmenting networks.
- CIDR, subnet masks, and block size are key.
- VLSM lets you custom-size subnets.
- Use subnetting for performance, security, and simplified management.
- Planning ahead helps avoid future renumbering.