CIDR (classless inter-domain routing) is used to represent a subnet mask in a more compact way. You may see subnets notated in CIDR notation, where a subnet such as the is written as which just means it includes both the subnet prefix and the subnet mask.

Remember an IP address consists of 4 bytes or 32 bits, CIDR indicates the amount of bits used as the network prefix. So means that the first 23 bits are used. Well what does that mean? How many hosts is that?

A simple trick is to subtract the total of bits an IP address can have (32) from the CIDR address (23), so that leaves 9 bits, 2^9 = 512, but we have to remove 2 addresses (subnet address and broadcast address) so we have 510 usable hosts.


No exercises for this lesson.


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