What is a Subnet?
A subnet is a logical organization of network address ranges used to separate hosts and network devices from each other to serve a design purpose. In many cases, subnets are created to serve as physical or geographical separations similar to those found between rooms, floors, buildings, or cities.
Most modern subnet definitions are created according to 3 main factors. These include:
- the number of hosts that needs to exist on the subnet now and in the future;
- the necessary security controls between networks; and
- the performance required for communications between hosts.
Legacy Subnets
Legacy subnets were not flexible because they had predefined limitations on their size and numbers. These were called "classful" networks because each network could be easily identified and placed into a specific class. Shown below is a table containing the original "classful" definitions for IP addresses:
| IP Address Range | CIDR Equivalent | Purpose | RFC | Class | Total # of Addresses |
|---|---|---|---|---|---|
| 0.0.0.0 - 0.255.255.255 | 0.0.0.0/8 | Zero Addresses | 1700 | A | 16,777,216 |
| 10.0.0.0 - 10.255.255.255 | 10.0.0.0/8 | Private IP addresses | 1918 | A | 16,777,216 |
| 127.0.0.0 - 127.255.255.255 | 127.0.0.0/8 | Localhost Loopback Address | 1700 | A | 16,777,216 |
| 169.254.0.0 - 169.254.255.255 | 169.254.0.0/16 | Zeroconf / APIPA | 3330 | B | 65,536 |
| 172.16.0.0 - 172.31.255.255 | 172.16.0.0/12 | Private IP addresses | 1918 | B | 1,048,576 |
| 192.0.2.0 - 192.0.2.255 | 192.0.2.0/24 | Documentation and Examples | 3330 | C | 256 |
| 192.88.99.0 - 192.88.99.255 | 192.88.99.0/24 | IPv6 to IPv4 relay Anycast | 3068 | C | 256 |
| 192.168.0.0 - 192.168.255.255 | 192.168.0.0/16 | Private IP addresses | 1918 | C | 65,536 |
| 198.18.0.0 - 198.19.255.255 | 198.18.0.0/15 | Network Device Benchmark | 2544 | C | 131,072 |
| 224.0.0.0 - 239.255.255.255 | 224.0.0.0/4 | Multicast | 3171 | D | 268,435,456 |
| 240.0.0.0 - 255.255.255.255 | 240.0.0.0/4 | Reserved | 1700 | E | 268,435,456 |
Classless IP Addresses
With the advent of CIDR (Classless Inter-Domain Routing), the "classful" definition of subnet divisions was lifted. Any network address could be defined just as any of the "classful" subnet of the past could be defined. All that is required is enough neighboring address space to cover all the IP addresses needed. Classless addresses also assist in reducing the overall size of the global routing tables on network devices.
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