the CIDR simply tells you how many addresses are available when you set a certain subnet mask.
For example a /24 gives you 254 available addresses (256 but the first and the last bit is unavailable, they are the network and the broadcast address respectively).
The /24 mask is [`255.255.255.0`](https://255.255.255.0) which means the last set of numbers equals 256 available addresses. It is usefull to set up smaller networks and not use up a huge numbers of ip-addresses that would go unused if all you need is a maximum of 254.
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