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Network: IPv4 cheatsheet
IPv4 address design
Glossary
| Component | Description |
|---|---|
| IP address | Unique host identifier |
| Subnet mask | Specifies which part of the address is the network |
| Network address | Result AND (IP & mask) - network identifier |
| Broadcast address | Last address in the subnet (all host bits = 1) |
Supporting formulae
Classless interdomain routing (CIDR) abbreviation This is an abbreviated notation of the Mask in the form /XX where XX is the number of bits in the mask.
Formulas:
Formulas:
- Number of addresses = $2^{32 - {text{CIDR}}$ or for mask /24
255 - (Last octet of the mask) - Number of hosts = $2^{32 - \text{CIDR}} - 2 $ *(except for /31 and /32).
Summary of Masks and Number of Addresses
| CIDR | Subnet mask | Number of addresses | Description |
|---|---|---|---|
| /32 | 255.255.255.255 | 1 | Unusable mask (only used for assigning addresses that act as unique identifiers) |
| /31 | 255.255.255.254 | 2 | Non-useful mask |
| /30 | 255.255.255.252 | 4 | Point-to-point networks (e.g. router connections) |
| /29 | 255.255.255.248 | 8 | Small networks - e.g. router + several hosts |
| /28 | 255.255.255.240 | 16 | Small offices, branches |
| /27 | 255.255.255.224 | 32 | LANs with ~30 hosts |
| /26 | 255.255.255.192 | 64 | Small LANs |
| /25 | 255.255.255.128 | 128 | Class C half |
| /24 | 255.255.255.0 | 256 | Most popular for local networks |
| /23 | 255.255.254.0 | 512 | Two C classes - e.g. larger corporate networks |
| /22 | 255.255.252.0 | 1024 | |
| /21 | 255.255.248.0 | 2048 | Aggregation of several networks /24 |
| /20 | 255.255.240.0 | 4096 | Large campus networks |
| /19 | 255.255.224.0 | 8192 | |
| /18 | 255.255.192.0 | 16384 | |
| /17 | 255.255.128.0 | 32768 | |
| /16 | 255.255.0.0 | 65536 | Class C 192.168.0.0 - 192.168.255.255 |
| /15 | 255.254.0.0 | 131072 | Aggregation of two B classes |
| /14 | 255.252.0.0 | 262144 | |
| /13 | 255.248.0.0 | 524288 | |
| /12 | 255.240.0.0 | 1048576 | Class B 172.16.0.0 - 172.31.255.255 |
| /11 | 255.224.0.0 | 2097152 | |
| /10 | 255.192.0.0 | 4194304 | |
| /9 | 255.128.0.0 | 8388608 | |
| /8 | 255.0.0.0 | 16777216 | Class A 10.0.0.0 - 10.255.255.255 |
| /7 | 254.0.0.0 | 33554432 | |
| /6 | 252.0.0.0 | 67108864 | |
| /5 | 248.0.0.0 | 134217728 | |
| /4 | 240.0.0.0 | 268435456 | |
| /3 | 224.0.0.0 | 536870912 | |
| /2 | 192.0.0.0 | 1073741824 | |
| /1 | 128.0.0.0 | 2147483648 | |
| /0 | 0.0.0.0 | 4294967296 | Any IPv4 address (e.g. default route) |
Example of class-based subnetting from the 192.168.0.0/24 pool
If two devices have identical subnet mask values set, and the initial portions of their IP addresses of the length determined by the mask length match, then these devices are addressed in the same subnet.
source: Wikipedia Subnetting
Example of classless subnetting (VLSM) of pool 192.168.0.0/24
VLSM makes it possible to divide an address, e.g. class C (254 hosts, mask 255.255.255.0), into several smaller subnets containing different numbers of hosts. In order for network information to be well distributed between routers, the relevant routing protocols must exchange complete network information including masks between each other.
Source: Wikipedia VLSM
