What Is a Subnet?
A subnet (subnetwork) is a logically segmented portion of a larger IP network. Subnetting divides a large network into smaller, more manageable segments – improving security, reducing broadcast traffic, and simplifying administration.
Every subnet is defined by two values:
- Network address – the first address of the subnet
- Subnet mask – determines how many addresses the subnet contains
Understanding CIDR Notation
CIDR notation (Classless Inter-Domain Routing) writes an IP address with a slash followed by the prefix length:
192.168.1.0/24The number after the slash indicates how many of the 32 bits of the IP address belong to the network part. The remaining bits are available for hosts.
| CIDR | Netmask | Usable Hosts | Typical Use |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,214 | Large organizations |
| /16 | 255.255.0.0 | 65,534 | Medium enterprises |
| /24 | 255.255.255.0 | 254 | Home network, small offices |
| /25 | 255.255.255.128 | 126 | Department network |
| /26 | 255.255.255.192 | 62 | Small department |
| /28 | 255.255.255.240 | 14 | Small segment |
| /30 | 255.255.255.252 | 2 | Point-to-point link |
Calculating a Subnet Step by Step
Example: 192.168.1.0/24
Step 1: Determine the netmask
With /24, the first 24 bits are set:
11111111.11111111.11111111.00000000 = 255.255.255.0Step 2: Calculate the network address
IP address AND netmask (bitwise AND):
192.168.1.100 = 11000000.10101000.00000001.01100100
255.255.255.0 = 11111111.11111111.11111111.00000000
= 11000000.10101000.00000001.00000000
= 192.168.1.0 ← Network addressStep 3: Calculate the broadcast address
Set all host bits to 1:
192.168.1.255 ← Broadcast addressStep 4: Usable host range
First usable address: 192.168.1.1
Last usable address: 192.168.1.254
Number of usable hosts: 2^8 - 2 = 254Wildcard Mask
The wildcard mask is the inverse of the subnet mask – primarily used in ACLs (Access Control Lists) on Cisco devices:
Netmask: 255.255.255.0
Wildcard mask: 0.0.0.255Calculation: invert all bits, or compute 255.255.255.255 - netmask.
Private IP Ranges
The following ranges are reserved for home and internal networks (RFC 1918):
| Range | CIDR | Addresses |
|---|---|---|
| 10.0.0.0 | 10.0.0.0/8 | ~16 million |
| 172.16.0.0 | 172.16.0.0/12 | ~1 million |
| 192.168.0.0 | 192.168.0.0/16 | ~65,000 |
These addresses are not routable on the internet and can be freely used in private networks.
Supernetting – Aggregating Multiple Networks
The opposite of subnetting: supernetting (also called CIDR aggregation) combines several smaller networks into a larger one. This reduces the size of routing tables.
Example: The networks 192.168.0.0/24 and 192.168.1.0/24 can be summarized as 192.168.0.0/23.
Calculate Subnets Online
Use our free Subnet Calculator to instantly:
- Calculate network address, broadcast and netmask
- Determine the usable host range
- View the binary representation of IP and mask
- Test different CIDR prefixes with quick-select buttons
Last updated: March 2024