Ipv4 subnetting reference
| CIDR | Netmask | Hosts / subnet | Classfull name | Typical Usage |
| Class A | Largest block allocation made by IANA | |||
| /10 | 255.192.0.0 | 4194304 | ||
| /11 | 255.224.0.0 | 2097152 | ||
| /12 | 255.240.0.0 | 1048576 | ||
| /13 | 255.248.0.0 | 524288 | ||
| /14 | 255.252.0.0 | 262144 | ||
| /15 | 255.254.0.0 | 131072 | ||
| /16 | 255.255.0.0 | 65536 | Class B | |
| /17 | 255.255.128.0 | 32768 | ISP / large business | |
| /18 | 255.255.192.0 | 16384 | ISP / large business | |
| /19 | 255.255.224.0 | 8192 | ISP / large business | |
| /20 | 255.255.240.0 | 4096 | Small ISP / large business | |
| /21 | 255.255.248.0 | 2048 | Small ISP / large business | |
| /22 | 255.255.252.0 | 1024 | ||
| /23 | 255.255.254.0 | 512 | ||
| /24 | 255.255.255.0 | 256 | Class C | Large LAN |
| /25 | 255.255.255.128 | 128 | Large LAN | |
| /26 | 255.255.255.192 | 64 | Small LAN | |
| /27 | 255.255.255.224 | 32 | Small LAN | |
| /28 | 255.255.255.240 | 16 | Small LAN | |
| /29 | 255.255.255.248 | 8 | ||
| /30 | 255.255.255.252 | 4 | "Glue network" (point to point links) | |
| /31 | 255.255.255.254 | 2 | "Useless Network", proposed for point to point links (RFC 3021) | |
| /32 | 255.255.255.255 | 1 | Host route |
Note that certain chunks of IPv4 address space are specially allocated by RFCs for special uses such as loopback, RFC 1918 networks, and Zeroconf usage, and are not available for allocation by RIRs.
Note also that not all sizes of prefix announcement may be routable on the public Internet: see routing, peering.
