Subnetting Unveiled: Master the Art of Network Segmentation 2025

Subnetting allows a network administrator to create a smaller network known as sub-networks or subnets inside a large network by borrowing bits from the Host ID portion of the address. It implements and manages a practical IP addressing plan by partitioning a single physical network into more than one smaller logical sub-network (subnets), enhancing control and efficiency.

An IPv4 address contains a network portion and a host portion. Both portions of the IP address allow basic network groupings and help route packets between different networks. The router forwards packets based on the network portion of an IP address, and the host portion of the address allows for identification of the destination device. However, with network growth and the transition to IPv6 as of June 29, 2025, the two-level hierarchy is insufficient. IPv6 subnetting uses /64 prefixes, posing challenges like address planning and compatibility with legacy IPv4 networks.

Further dividing a network adds another level to the network hierarchy. Division creates three levels: a network, a subnetwork, and a host. Subnetting also allows an organization to add sub-networks without needing a new network.

Subnetting is possible by borrowing bits from the IP address’s host portion. The borrowing bits assign several smaller subnetworks inside the original network. Subnets also decrease network traffic and cover network complexity. Subnetting is necessary when allocating a single network number, but more Local Area Network (LANs) is needed.

All IP addresses are divided into different classes, and all the classes, such as Class A, B, and C, include the subnet mask known as the default subnet mask. Using a subnet mask, we determine the class and number of IP addresses required for a given local network. The default subnet mask for each class is the following: A: 255.0.0.0 or /8, B: 255.255.0.0 or /16, C: 255.255.255.0 or /24. Custom masks like /30 (255.255.255.252) are used for point-to-point links, providing 2 usable IPs, ideal for WAN connections.

We can partition a single Class A, B, or C network into smaller portions using subnetting. It is also possible to subnet a network repeatedly for more subnetworks.

Role of CIDR and VLSM in Subnetting

Subnetting extends beyond classful addressing with CIDR (Classless Inter-Domain Routing) and VLSM (Variable Length Subnet Masking), critical for CCNP students. CIDR eliminates class boundaries, using slash notation (e.g., /24) for flexible address allocation, reducing wasted IPs. VLSM allows different subnet sizes within the same network (e.g., /30 for point-to-point, /26 for LANs), optimizing address use. These techniques are essential for modern IP planning. Subnetting a network into several subnets provides benefits as follows:

Subnetting improves network performance and speed

A network is a logical broadcast domain in a computer network. Devices in the same network can reach each other by broadcasting at the data link layer. A broadcast domain can be within the same network segment. A single broadcast packet sends information to reach every device configured on the same network or sub-network. An extensive network negatively impacts internetwork switching device performance and your network’s overall performance and speed.

Subnetting allows subnetworks to maximize their speed effectively. By dividing the network’s broadcast domains, the network administrator can better control traffic flow, thus increasing network performance. Subnetting also limits the number of devices on the subnet, which helps to control the traffic flow and network overhead.

Flexibility

It allows any organization to subnet its network according to how it is structured in its physical network. We can also modify the number of subnetworks and the number of hosts per subnetwork for each organization, and they can decide on their subnet structure and change it as required.

Case Study

Subnetting’s flexibility shines in multi-office setups. Consider a company with three offices using 192.168.1.0/24:

• Office 1 (HQ): 50 hosts → /26 (62 hosts, 192.168.1.0 – 192.168.1.63).
• Office 2: 20 hosts → /27 (30 hosts, 192.168.1.64 – 192.168.1.95).
• Office 3: 10 hosts → /28 (14 hosts, 192.168.1.96 – 192.168.1.111).
• This VLSM approach saves addresses compared to a flat /24, adapting to growth (e.g., adding a /28 for a new office).

Routing Table size

The larger routing table size reduces the router’s efficiency, and the smaller routing table speeds up the entire routing process. We can minimize the size of the routing tables on the Internet since additional networks that are not required will not be added to the table.

Simplified Management

Subnetting makes the job of a network administrator a lot easier. Using subnetting, an administrator can create logical host limits, as opposed to classful addresses. Subnetting enables precise host allocation (e.g., /28 for 14 hosts), simplifies VLAN assignment for isolation, and facilitates targeted troubleshooting by segmenting networks, reducing administrative overhead..

In classful addressing for more than 254 hosts or devices, we need a Class B network, which can waste thousands of IP addresses. We can select the number of bits by subnetting according to our host requirement. Subnetted networks are easier to manage and troubleshoot.

Reduce network congestion

We can reduce network congestion by strategically planning subnets, reducing the network’s load, and integrating QoS policies (e.g., prioritizing VoIP traffic with mls qos) to manage bandwidth effectively, a key CCNP skill as of June 2025.

Boost network security

Subnetting a large network into multiple segments allows isolation of compromised areas. Integrating VLANs (e.g., vlan 10) enhances security by segregating traffic, preventing lateral movement of threats, a critical strategy as of June, 2025.

Broadcast Domain Segmentation

Subnetting enhances network performance by segmenting broadcast domains, a key concept for CCNA students. A broadcast domain is a network segment where a single broadcast packet (e.g., an ARP request) reaches all devices. In a /24 network (256 addresses), a broadcast to 192.168.1.255 impacts all 254 usable hosts, overloading switches. Subnetting into /26 (62 hosts per subnet) limits this to 62 devices, reducing switch processing load and improving speed.

For example, dividing 192.168.1.0/24 into four /26 subnets (e.g., 192.168.1.0/26, 192.168.1.64/26) isolates broadcast traffic, allowing better traffic control. CCNP students can further optimize with QoS policies to prioritize critical traffic.

Subnetting Tools and Practice

Enhance subnetting skills with tools and commands:

• Subnet Calculators: Use our online subnetting calculator to determine subnet ranges.
• Cisco IOS: Enable ip subnet-zero to use all-zero subnets, and verify with show ip interface brief to check configured subnets.
• Practice Tip: Simulate in Packet Tracer with 192.168.1.0/24 subnetted into /28s.