It begins with a starting IP address and is separated by a forward slash (the “/” character) followed by a number. The number at the end specifies how many of the bits of the IP address are fixed.
Example
192.168.1.0/24
The first 24 bits of the IP address are fixed. The rest 8 bits (32 bits - 24 bits) are flexible, meaning the range has 8^2 possible addresses are available. (IPv4 has 32 bits)
The higher the number after the /, the smaller the number of IP addresses in your network.
Amazon Virtual Private Cloud (Amazon VPC) enables you to launch AWS resources into a virtual network that you’ve defined. This virtual network closely resembles a traditional network. Each VPC spans multiple Availability Zones within the Region you choose. Each VPC can have up to four /16 IP ranges.
Source: https://docs.aws.amazon.com/vpc/latest/userguide/vpc-example-web-database-servers.html
Think of subnets as smaller networks inside your base network—or virtual area networks (VLANs) in a traditional, on-premises network. A subnet is a range of IP addresses in your VPC, which must be a subset of the VPC CIDR block.
In an on-premises network, the typical use case for subnets is to isolate or optimize network traffic. In AWS, subnets are used for providing different connectivity options for your resources: Use a public subnet for resources that must be connected to the internet, and a private subnet for resources that won’t be connected to the Internet.
Each subnet resides entirely within one Availability Zone and cannot span multiple Availability Zones.
AWS reserves five IP addresses in each subnet. These IP addresses are used for routing, Domain Name System (DNS), and network management.
Think of this gateway as similar to a modem. A modem connects your computer to the internet, the internet gateway connects your VPC to the internet. After you create an internet gateway, you then need to attach it to your VPC.
A virtual private gateway allows you to connect your AWS VPC to another private network.