Subnet Information
Quick reference guide for IP subnetting, CIDR notation, and network calculations.
Quick Reference
Class A Default
/8
Class B Default
/16
Class C Default
/24
Total IPv4 Addresses
2³²
Subnet Formulas
Number of Subnets
2ⁿ (where n = borrowed bits)
Hosts per Subnet
2ʰ - 2 (where h = host bits, subtract 2 for network/broadcast)
Block Size / Subnet Increment
256 - subnet mask (in relevant octet)
CIDR Reference Table
| CIDR | Subnet Mask | Total Hosts | Usable Hosts | Class |
|---|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 | C |
| /25 | 255.255.255.128 | 128 | 126 | C |
| /26 | 255.255.255.192 | 64 | 62 | C |
| /27 | 255.255.255.224 | 32 | 30 | C |
| /28 | 255.255.255.240 | 16 | 14 | C |
| /29 | 255.255.255.248 | 8 | 6 | C |
| /30 | 255.255.255.252 | 4 | 2 | C (Point-to-Point) |
| /31 | 255.255.255.254 | 2 | 2* | Point-to-Point (RFC 3021) |
| /32 | 255.255.255.255 | 1 | 1 | Host Route |
Common Subnet Masks
Enterprise Networks
| Size | CIDR | Mask |
|---|---|---|
| Large Site | /16 | 255.255.0.0 |
| Medium Site | /20 | 255.255.240.0 |
| Small Site | /22 | 255.255.252.0 |
| VLAN/Dept | /24 | 255.255.255.0 |
Server Networks
| Purpose | CIDR | Hosts |
|---|---|---|
| DMZ | /28 | 14 |
| Web Servers | /27 | 30 |
| Database | /28 | 14 |
| Management | /29 | 6 |
Private IP Address Ranges (RFC 1918)
Class A - 10.0.0.0/8
10.0.0.0 - 10.255.255.255
16,777,216 addresses
Class B - 172.16.0.0/12
172.16.0.0 - 172.31.255.255
1,048,576 addresses
Class C - 192.168.0.0/16
192.168.0.0 - 192.168.255.255
65,536 addresses
Binary to Decimal Conversion
| Bit Position | 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 |
|---|---|---|---|---|---|---|---|---|
| Binary | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| Example: 192 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 |
| Example: 224 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 |
| Example: 240 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 |
Special Use IPv4 Addresses
| Address/Range | Purpose |
|---|---|
| 127.0.0.0/8 | Loopback (localhost) |
| 0.0.0.0/8 | Current network (source only) |
| 169.254.0.0/16 | Link-local (APIPA) |
| 255.255.255.255 | Limited broadcast |
| 224.0.0.0/4 | Multicast |
| 240.0.0.0/4 | Reserved for future use |