Key Points
- Understand Subnetting: Subnetting divides large IP networks into smaller segments, improving routing efficiency and enabling more granular security controls.
- Interpret CIDR Notation: CIDR notation defines subnet size and flexibility, enabling efficient IP allocation and scalability.
- Calculate Subnet Outputs: Network address, host range, and broadcast address are derived from IP and subnet mask calculations.
- Match Subnet Sizes to Use Cases: Different subnet sizes fit specific environments, from point-to-point links to enterprise LANs.
- Apply Subnets for Security: Subnets enforce VLAN design, ACLs, routing policies, and security zones to simplify governance and reduce traffic.
Subnetting is the process of dividing a larger IP network into smaller, more manageable segments. This improves network organization and reduces unnecessary broadcast traffic, and understanding how it works and determining the optimal subnet size is integral to modern security policies.
In this article, we go over the fundamentals of modern subnetting and how you can customize the way your network is partitioned.
Design subnetting for better network management
Here’s how subnetting works and how your IT teams can interpret and select the appropriate subnet size for your organization.
📌 Prerequisites:
- Basic understanding of IPv4 addressing
- Familiarity with binary number basics
- Awareness of network architecture roles (routers, switches, VLANs)
Understand the purpose of subnetting
In essence, subnetting divides a large network into multiple, smaller ones to improve routing efficiency. Simply put, it organizes devices into logical groups (such as by department or location), making traffic easier to manage and route.
Since its introduction in 1985, this method has been implemented in networks worldwide, not only for its ability to reduce operational complexity but also to simplify troubleshooting across multiple device types and teams.
Subnetting also reduces traffic, allowing teams to receive data across the internet efficiently and maintain productivity levels. This makes subnetting a foundational tool for performance and stability.
Interpret CIDR notation
The Classless Inter-Domain Routing (CIDR) defines the size of your subnets, expressed in /n, where n is the number of bits used for the network prefix. The CIDR’s main value is its flexibility, as it allows administrators to choose how compartmentalized their networks can be.
For example, in IPv4 addresses (32-bit), 192.168.1.0/24 means that the first 24 bits are network bits, leaving only the 8 bits needed for the host addresses in a specific network.
CIDR enables more efficient use of IPv4 addresses by allowing flexible subnet sizes. However, IPv6 expands the available address space even further. enables efficient allocation and helps us optimize beyond IPv4’s limitations.
Calculate subnets from a given block
Subnetting is basically the process of borrowing host bits and converting them into additional network bits. This increases the number of smaller networks but reduces the size of each one.
Here’s how you can retrieve the IP/mask and calculate subnet details:
- Press Win + R, type cmd, then press Enter.
- Run ipconfig to find your current IP and subnet mask.
- Note your IPv4 address and subnet mask (Microsoft explains how the subnet mask separates local subnet vs remote network decisions).
- When configuring your network, the chosen prefix length (/n) determines how many subnets or hosts you can have.
- More subnets = increase prefix length (borrow host bits)
- More hosts = decrease prefix length (return bits to hosts)
- When analyzing a subnet, there are three key values you need to identify:
- Network address: The first address in the subnet. It represents the entire group, not a specific device
- IP = 192.168.1.130
Mask = 255.255.255.192 (/26)
Network Address = 192.168.1.128
- IP = 192.168.1.130
- Host range: The usable IP addresses that can be assigned to devices inside the subnet
- Network Address + 1 → Broadcast Address – 1
- Broadcast address: The last address in the subnet. It is reserved to send traffic to all devices within that subnet (IPv4 only).
- Network Address + (2^HostBits – 1)
- Network address: The first address in the subnet. It represents the entire group, not a specific device
Match subnet sizes to use cases
Subnetting is a capacity planning decision that impacts your entire IT environment. If you size them too small, you make routing more complex in your overall network, but if your subnets are too large, traffic containment and troubleshooting will become difficult.
Choose the subnet that fits your environment.
/30 or /31 (2 usable hosts)
- Point-to-point links between routers
- WAN uplinks where only two devices need IPs
- Minimizes wasted addresses in backbone connections
/29 (6 usable hosts)
- Small server clusters or DMZ segments
- Network appliances (firewalls, load balancers) with a handful of interfaces
- Small branch office LAN segments with limited devices
/28 (14 usable hosts)
- Small workgroups or lab environments
- IoT device clusters (sensors, cameras)
- Small isolated VLANs for testing or staging environments
/24 (254 usable hosts)
- Standard LANs in offices or schools
- Department-level segmentation (HR, Finance, Engineering)
- Common default subnet size in home and SMB networks
/23 (510 usable hosts)
- Larger departments or floors in enterprise buildings
- University labs or dorm networks
- Cloud VPC subnets needing mid-scale capacity
/22 (1,022 usable hosts)
- Large data center segments supporting many servers
- Large wireless LANs (stadiums, airports)
- Large cloud subnets supporting high-density workloads
/16 (65,534 usable hosts)
- Huge enterprise or campus networks (though rarely used today due to broadcast overhead)
- ISP customer pools before finer subnetting
- Often used as the maximum VPC size in cloud environments, then subdivided
Apply subnets to segmentation and security
Subnetting allows IT teams to enforce boundaries that make it easier to apply consistent security policies, eliminate unnecessary traffic, and monitor data transfers. Subnetting helps your administrators in:
- Virtual LAN design: Direct pairing with subnets ensures limited broadcast domains and logical function groups (for example, HR, Finance, IoT).
- Access Control Lists (ACLs): Traffic “gates” can be applied at the subnet level, allowing stricter control and enhanced least-privilege access.
- Routing Policy Partitioning: Subnets partition traffic, prioritize critical apps, and facilitate troubleshooting.
- Security Zone Definition: Subnets help define clear security boundaries, making it easier to apply consistent firewall rules and monitor traffic between different network segments.
🥷🏻| Detect threats, automate patches, and monitor performance from one platform.
Important considerations for your subnetting practice
Modern advancements and automated features should be taken into consideration when configuring your organization’s subnets. One of these is how network partitioning works in IPv6 environments.
With virtually limitless address space (128-bit compared to IPv4 32-bit), IPv6 subnets are typically allotted on /64 boundaries, making 2⁶⁴ host addresses that can accommodate nearly 18.4 quintillion devices.
IPv6 adoption is still ongoing, and it is often dual-stacked with its predecessor. But its scalability is something modern enterprises should leverage for easier hierarchical addressing.
Protocols like OSPF and EIGRP automatically propagate subnet information across routers. Simply put, these routing protocols exchange and organize route information, enabling consistent path selection and reliable data forwarding.
Lastly, subnet performance is dependent on its size. Remember that large subnets lead to high levels of broadcast traffic and higher overhead, while small subnets can run out of IP addresses, making it challenging to accommodate devices on your network.
Troubleshooting subnetting issues
Here are the most common subnetting issues IT administrators face, and how to fix them.
Overlapping subnets
When two different networks use the same IP address range, routers don’t know where to send the traffic. This causes routing problems and connectivity issues. To fix it, redesign the IP addressing so each network has a unique range, or use NAT in cases like VPN or multi-site connections.
Insufficient hosts
Subnets can run out of usable addresses, especially in smaller subnets. Recalculate the prefix length for more hosts (for example, /28 gives 14 hosts, /24 provides 254) to make space for more devices on your network.
Routing issues
Traffic forwarding can be disrupted or even stopped if subnet masks are misconfigured. Check your mask for any numerical errors to try to alleviate data transfers on your subnets.
Broadcast storms
Large IPv4 broadcast domains can increase the impact of broadcast traffic, which may degrade network performance. To mitigate this risk, use VLAN segmentation and features like Cisco Storm Control to limit excessive broadcast traffic.
How NinjaOne facilitates network management
NinjaOne provides visibility into endpoint network configurations and helps correlate device-level changes with system behavior. Administrators can monitor IP addressing details across managed endpoints and maintain consistent policies for laptops, servers, and other devices.
Subnetting refines network performance
Subnetting creates specialized compartments in your network to share data transfer workloads and ease traffic. Through careful planning, remote monitoring platforms, and subnet size optimization, your enterprise can more accurately monitor network performance for better performance and information security.
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