IPv4 subnetting reference
In the IPv4 address space certain address blocks are specially allocated or reserved for special uses such as loopback interfaces, private networks (RFC 1918), and state-less autoconfiguration (Zeroconf, RFC 3927) of interfaces. Such addresses may be used without registration or allocation from Regional Internet Registries (RIRs). However, these address ranges must not be routed into the public Internet infrastructure.
The netmask is a bitmask that can be used to separate the bits of the network identifier from the bits of the host identifier. It is often written in the same notation used to denote IP addresses.
Not all sizes of prefix announcement may be routable on the public Internet: see routing, peering.
| Class | Leading bits | Start | End | Default Subnet Mask in dotted decimal | CIDR notation |
|---|---|---|---|---|---|
| A | 0 | 0.0.0.0 | 127.255.255.255 | 255.0.0.0 | /8 |
| B | 10 | 128.0.0.0 | 191.255.255.255 | 255.255.0.0 | /16 |
| C | 110 | 192.0.0.0 | 223.255.255.255 | 255.255.255.0 | /24 |
| D | 1110 | 224.0.0.0 | 239.255.255.255 | not defined | not defined |
| E | 1111 | 240.0.0.0 | 255.255.255.254 | not defined | not defined |
While the 127.0.0.0/8 network is a Class A network, it is designated for loopback and cannot be assigned to a network.
Class D multicasting
Class E reserved, but also see [1].
| CIDR | available bits | Netmask | Hosts / subnet | Classful name | Typical usage |
|---|---|---|---|---|---|
| /8 | 24 | 255.0.0.0 | 16777216 = 224 | Class A (see this list) | Largest block allocation made by IANA |
| /9 | 23 | 255.128.0.0 | 8388608 = 223 | ||
| /10 | 22 | 255.192.0.0 | 4194304 = 222 | ||
| /11 | 21 | 255.224.0.0 | 2097152 = 221 | ||
| /12 | 20 | 255.240.0.0 | 1048576 = 220 | ||
| /13 | 19 | 255.248.0.0 | 524288 = 219 | ||
| /14 | 18 | 255.252.0.0 | 262144 = 218 | ||
| /15 | 17 | 255.254.0.0 | 131072 = 217 | ||
| /16 | 16 | 255.255.0.0 | 65536 = 216 | Class B | |
| /17 | 15 | 255.255.128.0 | 32768 = 215 | ISP / large business | |
| /18 | 14 | 255.255.192.0 | 16384 = 214 | ISP / large business | |
| /19 | 13 | 255.255.224.0 | 8192 = 213 | ISP / large business | |
| /20 | 12 | 255.255.240.0 | 4096 = 212 | Small ISP / large business | |
| /21 | 11 | 255.255.248.0 | 2048 = 211 | Small ISP / large business | |
| /22 | 10 | 255.255.252.0 | 1024 = 210 | ||
| /23 | 9 | 255.255.254.0 | 512 = 29 | ||
| /24 | 8 | 255.255.255.0 | 256 = 28 | Class C |
Large LAN |
| /25 | 7 | 255.255.255.128 | 128 = 27 |
Large LAN | |
| /26 | 6 | 255.255.255.192 | 64 = 26 |
Small LAN | |
| /27 | 5 | 255.255.255.224 | 32 = 25 |
Small LAN | |
| /28 | 4 | 255.255.255.240 | 16 = 24 |
Small LAN | |
| /29 | 3 | 255.255.255.248 | 8 = 23 | ||
| /30 | 2 | 255.255.255.252 | 4 = 22 | "Glue network" (point to point links) | |
| /31 | 1 | 255.255.255.254 | 2 = 21 | "point to point links (RFC 3021) | |
| /32 | 0 | 255.255.255.255 | 1 = 20 | Host route |
Note that in common usage, the "host all zeros" address is reserved for referring to the whole network, while the "host all ones" address is reserved as a broadcast address; this reduces the number of hosts available by 2, explaining the reference to /31 as "Useless Network".