What is CIDR and Subnetting?
CIDR, or Classless Inter-Domain Routing, is the modern way to write IPv4 network size. A value like 192.168.1.10/24 means the first 24 bits identify the network and the remaining 8 bits identify addresses inside that network. Subnetting uses this prefix length to divide a larger address space into smaller, predictable ranges. Administrators use subnets to separate offices, VLANs, cloud environments, VPN pools, and infrastructure services without wasting address space.
Private vs Public IP Ranges explained (RFC 1918)
Private IPv4 addresses are reserved for internal networks and are not routed directly on the public internet. RFC 1918 defines three common blocks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Home routers commonly use 192.168.x.x, while cloud networks and enterprise LANs often use 10.x.x.x or 172.16-31.x.x. Public addresses sit outside those private ranges and must be globally unique. Network address translation lets private devices share public connectivity while keeping internal addressing flexible.
Subnet Mask Reference Table (/8 to /32)
Shorter prefixes create larger networks with more available host addresses, while longer prefixes create smaller networks that are easier to allocate precisely. The table below covers the common mask sizes used in LAN, cloud, firewall, and routing plans.
| Prefix | Mask | Usable Hosts |
|---|---|---|
| /8 | 255.0.0.0 | 16,777,214 |
| /9 | 255.128.0.0 | 8,388,606 |
| /10 | 255.192.0.0 | 4,194,302 |
| /11 | 255.224.0.0 | 2,097,150 |
| /12 | 255.240.0.0 | 1,048,574 |
| /13 | 255.248.0.0 | 524,286 |
| /14 | 255.252.0.0 | 262,142 |
| /15 | 255.254.0.0 | 131,070 |
| /16 | 255.255.0.0 | 65,534 |
| /17 | 255.255.128.0 | 32,766 |
| /18 | 255.255.192.0 | 16,382 |
| /19 | 255.255.224.0 | 8,190 |
| /20 | 255.255.240.0 | 4,094 |
| /21 | 255.255.248.0 | 2,046 |
| /22 | 255.255.252.0 | 1,022 |
| /23 | 255.255.254.0 | 510 |
| /24 | 255.255.255.0 | 254 |
| /25 | 255.255.255.128 | 126 |
| /26 | 255.255.255.192 | 62 |
| /27 | 255.255.255.224 | 30 |
| /28 | 255.255.255.240 | 14 |
| /29 | 255.255.255.248 | 6 |
| /30 | 255.255.255.252 | 2 |
| /31 | 255.255.255.254 | 0 |
| /32 | 255.255.255.255 | 1 |