What is a Subnet?

A subnet, or subnetwork, is a segmented piece of a larger IP network. The purpose of a subnet is to divide a single network into multiple smaller networks that are more manageable. Each subnet functions as its own isolated network, but still communicates with the rest of the system via routers.

Using subnets helps improve network performance and security, and reduces network congestion.

How do subnets work?

Subnets can be thought of as smaller components of a larger system - by breaking a larger network into parts (subnets), the entire system can be more efficient. Data traveling through a network without subnets may not necessarily take the most efficient route. With a subnet, data can travel in a way that makes sense; similar to POPs in a content delivery network (CDN), subnets help ensure data travels in a way that makes the most sense.

So how does subnetting work?

In order for subnetting to make sense, you first have to understand how IP addresses work. Each device on a network has an internet protocol (IP address) assigned to it. You can think of an IP address like a physical “address” denoting where the device lives; these “addresses” allow devices to find one another and communicate.

An IP address (let’s use 192.168.1.10 as an example) has two parts:

Network ID: Identifies the overall network; 192.168.1
Host ID: Identifies the specific device within that network; .10

Now, enter subnets… a subnet mask is a 32-bit number used in IP networking to separate the ‘network’ portion of an IP address from the ‘host’ portion. A subnet mask defines where the network ID ends and the host ID begins. In doing so, a subnet mask tells your computer or router:

Which IP addresses belong to the same local network
Which IP addresses require routing through a gateway (outside the network)

What are the benefits of subnetting?

Subnetting is a great way to help efficiently transmit and organize data across networks.

Some benefits of using subnetting include:

Improved efficiency: Breaking a large network into smaller ones minimizes traffic congestion and optimizes bandwidth usage.
Enhanced security: Subnets can isolate sensitive parts of a network, limiting access to only those who need it.
Simplified troubleshooting: Smaller networks are easier to monitor and manage, making issues quicker to identify and resolve.

Better use of IP addresses: Especially important with IPv4, subnetting helps conserve IP address space.

Who uses Subnetting?

Subnetting is used in a range of applications:

Corporate Networks: Departments can have isolated subnets - HR could remain separate from marketing, for example.
Data Centers: There can be separate subnets for public-facing servers vs. internal services.
Home Networks: You can isolate guest devices from home devices using different subnets.

How is a subnet different from a VLAN?

Subnetting and VLANs (Virtual Local Area Networks) are often used together but serve different purposes:

Subnetting is based on IP addressing and affects routing.
VLANs are implemented at the switch level and control broadcast domains.

Subnetting operates at Layer 3 (the Network Layer) of the OSI model and is based solely on IP addressing. It helps control how devices identify one another, and how traffic is routed to avoid congestion.

VLANs operate at Layer 2 (the Data Link Layer) of the OSI model and help group devices together based upon their function, security level or department - not physical location, like subnets.

Used together, each VLAN is usually assigned its own subnet. Traffic between VLANs must pass through a Layer 3 device (a router or switch), meaning that device traffic is given both Layer 2 isolation (VLAN) and Layer 3 routing (subnetting).

How do CDNs relate to subnets?

CDNs don’t actually ‘perform’ subnetting, but subnetting does play a critical role in helping CDNs optimize and route traffic. Here are the ways in which subnets and CDNs intersect and the benefits yielded by using both together:

Traffic ‘steering’ and geo-routing

CDNs can use subnet information to map user requests to their closest edge server - they detect a user’s IP and its associated subnet and use this to optimize the flow of ‘traffic’. This is called geo-IP mapping. Additionally, information from subnets can help a CDN identify large customer networks and ensure that they are served from dedicated edge clusters, yielding better performance.

More efficient IP address allocation

CDNs are responsible for managing enormous numbers of IP addresses across edge servers, worldwide. With subnets, CDNs can organize addresses strategically, helping to improve efficiency. Servers can be grouped by region or data center, IP blocks can be directed to nearby nodes and internal routing between edge locations can be dramatically improved, all resulting in a simpler more efficient flow of ‘traffic’.

Load balancing

Subnetting helps CDNs create logical groupings of servers, which allows the CDN to reroute to load balance, as needed.

Enhanced security and access control

Subnetting helps CDNs apply access controls. With a subnet, CDNs can restrict certain content to specific customer networks, or block malicious traffic by identifying IP ranges or specific subnets. Essentially, it makes it easier for CDNs to apply security decisions across vast swaths of traffic.

Optimized caching and delivery

By segmenting traffic by subnet, CDNs can help cache content closer to specific ISP networks and avoid duplicating cached content across different regions.

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