IPv4 has been defined 1981 in the RFC 791. It is the first version, which has been used worldwide to connect far away computers and was a main factor of the development of the “internet“.
The IPv4 address consists of 32 bits which allows a maximum of 4.294.967.296 (232) unique addresses.
Network- and Host devision
An IP address consists of 2 parts – a Network and a Host part.
The devision is defined via the so called “subnet mask”.
Example: 24-Bit network
| Subnet | = | 11111111.11111111.11111111.00000000 | (255.255.255.0) |
| Network part | = | 11000000.10101000.00000000 | (192.168.0) |
| Network address | = | 11000000.10101000.00000000.00000000 | (192.168.0.0) |
| First address | = | 11000000.10101000.00000000.00000001 | (192.168.0.1) |
| Last address | = | 11000000.10101000.00000000.11111110 | (192.168.0.254) |
| Broadcast | = | 11000000.10101000.00000000.11111111 | (192.168.0.255) |
Amount of available addresses: 28 − 2 = 254
It is 28, because the whole IP address has 32 bit and 24-Bit have been “occupied” by the subnet. Thats why we only have 8 bit left.
The network address (192.168.0.0) and the broadcast address (192.168.0.255) are always subtracted from the maximal amount of available addresses because these 2 shouldn’t be used as client addresses.
Example: 16-Bit network
| Subnet | = | 11111111.11111111.00000000.00000000 | (255.255.0.0) |
| Network part | = | 11000000.10101000 | (192.168) |
| Network address | = | 11000000.10101000.00000000.00000000 | (192.168.0.0) |
| First address | = | 11000000.10101000.00000000.00000001 | (192.168.0.1) |
| Last address | = | 11000000.10101000.11111111.11111110 | (192.168.255.254) |
| Broadcast | = | 11000000.10101000.11111111.11111111 | (192.168.255.255) |
Amount of available addresses: 216 − 2 = 65,534
Reserved IP address spaces
Not everything of the maximum 232 available IP addresses have the same “functionality”. Instead some areas have a special cause. The most import are:
| Address space (Prefix) | Usage | Referenz |
|---|---|---|
| 0.0.0.0/8 | Current network | RFC 1122 |
| 10.0.0.0/8 | 1 private 8-Bit network | RFC 1918 |
| 127.0.0.0/8 | Loopback (Local computer) | RFC 1122 |
| 169.254.0.0/16 | Private network (link local), APIPA | RFC 3927 |
| 172.16.0.0/12 | 16 private 16-Bit networks | RFC 1918 |
| 192.168.0.0/16 | 256 private 24-Bit networks | RFC 1918 |
| 255.255.255.255/32 | Limited Broadcast | RFC 919, RFC 922 |
Most of the here mentioned IP address spaces are only used for private usage and therefore for the “LAN” space.
Through this and other predefined IP address spaces the maximum amount of usable IPv4 addresses is decreased significantly.