Issue
How is a subnet defined, and what role does it play in IP networking?
Product Line
EcoStruxure Access Expert, EcoStruxure Building Operation, EcoStruxure Security Expert, TAC I/NET
Environment
This explanation is based on principles outlined in TCP/IP Network Administration by Craig Hunt, published by O’Reilly & Associates, Inc
Cause
Subnetting is a method used to divide a larger IP network into smaller, more manageable segments called subnets. This segmentation helps optimize IP address usage, improve routing efficiency, and enhance network security and organization.
Resolution
What Is a Subnet?
A subnet (short for subnetwork) is a logical subdivision of an IP network. It is defined by applying a subnet mask to an IP address, which determines how the address is split into:
- Network portion – identifies the specific network.
- Host portion – identifies individual devices (hosts) within that network.
How Subnet Masks Work
A subnet mask is a 32-bit binary pattern that accompanies an IP address. It uses binary values to indicate which part of the IP address refers to the network and which part refers to the host:
- A bit set to 1 in the subnet mask means the corresponding bit in the IP address is part of the network.
- A bit set to 0 means the corresponding bit is part of the host.
Example
- IP Address:
192.168.1.10 - Subnet Mask:
255.255.255.0(binary:11111111.11111111.11111111.00000000) - CIDR Notation:
/24
This means:
- Network Address:
192.168.1.0 - Broadcast Address:
192.168.1.255 - Usable Host Range:
192.168.1.1to192.168.1.254
CIDR Notation
Modern networks often use CIDR (Classless Inter-Domain Routing) notation to define subnets. CIDR expresses the subnet mask as a suffix (e.g., /24), indicating the number of bits used for the network portion.
Common CIDR Blocks
| CIDR | Subnet Mask | Hosts per Subnet |
|---|---|---|
| /30 | 255.255.255.252 | 2 usable hosts |
| /24 | 255.255.255.0 | 254 usable hosts |
| /16 | 255.255.0.0 | 65,534 usable hosts |
Additional Notes
- Subnets are local constructs: They are meaningful within the internal network and typically not visible to external networks unless explicitly routed.
- Subnetting supports hierarchical addressing, which simplifies routing tables and improves scalability.
- Proper subnetting is essential for network performance, security segmentation, and efficient IP address management.
