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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 mirror physical or geographical separations, such as you find between cities, buildings, floors or rooms. Most modern subnet definitions are created specifically with a concern of how many hosts will need to exist on the subnet now and in the future, what security controls are needed between networks, and the performance required for communications between hosts.
Legacy Subnets
Legacy subnets were not flexible, as 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. 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, or Classless Inter-Domain Routing, the classful definitions of subnet divisions was lifted. Any network address could be defined just as you could define any of the classful subnets of the past, all that is required is enough contiguous address space to cover all the ip addresses needed. Classless addresses also assisted in reducing the overall size of the global routing tables on network devices.
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