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CIDR notation
CIDR notation like this: 10.10.1.32 with a mask of 18 bits allows 14 bits (18 + 14 = 32) to be used for host addresses. This notation is important when defining rules for a security group in order to control the inbound traffic that's allowed to reach your instances.
In CIDR notation, an IP address is represented as A.B.C.D /n, where "/n" is called the IP prefix or network prefix. The IP prefix identifies the number of significant bits used to identify a network.
For example, 192.9.205.22 /18 means, the first 18 bits are used to represent the network and the remaining 14 bits are used to identify hosts. Common prefixes are 8, 16, 24, and 32.
A network access control list (ACL)
A network access control list (ACL) is an optional layer of security that acts as a firewall for controlling traffic in and out of a subnet. You might set up network ACLs with rules similar to your security groups in order to add an additional layer of security to your VPC.
A network ACL is a numbered list of rules that we evaluate in order, starting with the lowest numbered rule, to determine whether traffic is allowed in or out of any subnet associated with the network ACL. The highest number that you can use for a rule is 32766. We suggest that you start by creating rules with rule numbers that are multiples of 100, so that you can insert new rules where you need to later on.
A network ACL has separate inbound and outbound rules, and each rule can either allow or deny traffic. Your VPC automatically comes with a modifiable default network ACL; by default, it allows all inbound and outbound traffic.
Each subnet must be associated with a network ACL; if you don't explicitly associate a subnet with a network ACL, the subnet is automatically associated with the default network ACL. Network ACLs are stateless; responses to allowed inbound traffic are subject to the rules for outbound traffic.
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