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CIDR notation: what /24 means
CIDR notation: what /24 means An IP address has 32 bits. In 10.0.1.0/24 , the /24 says the first 24 bits are the network part. The rest are for hosts (devices). AWS keeps 5 IPs in every subnet: the first 4 and the last one. So a /24 subnet: 256 − 5 = 251 usable IPs /32 = only one IP, like 198.51.100
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What is CIDR?
An IP address has 32 bits. In 10.0.1.0/24, the /24 says the first 24 bits are the network part. The rest are for hosts (devices).
Why AWS gives 251, not 256
AWS keeps 5 IPs in every subnet: the first 4 and the last one.
So a /24 subnet: 256 − 5 = 251 usable IPs
/32 vs /0 in a firewall rule
/32 = only one IP, like 198.51.100.7/32.
0.0.0.0/0 = the whole internet. SSH (port 22) open to 0.0.0.0/0 is a common cloud mistake.
Before saving a rule, ask: “Who does this rule allow?”
Learn more in the lesson: 1.4 CIDR Notation and Subnet Masks