VPC Subnets

• A range of IP addresses for your VPC.

• VPC Subnet is attached to an Availability Zone - Subnet can span only a single Availability Zone.

• To create a subnet, you specify the CIDR block for the subnet, which is a subset of the VPC CIDR block.

• Each VPC subnet is in a different Availability Zone and together, they span across all the Availability Zones in the region.

• The first 4 IP addresses and last IP address in each subnet CIDR block are not available for use:

  • For example, in a subnet with CIDR block 10.0.0.0/24, the following five IP addresses are reserved:

    • 10.0.0.0 = Network address.

    • 10.0.0.1 = For the VPC router.

    • 10.0.0.2 = Reserved for DNS Server.

    • 10.0.0.3 = For future use.

    • 10.0.0.255 = Network broadcast address; AWS do not support broadcast in a VPC, so they reserve this address.

  • So, if you need 29 IP addresses for a VPC, a /27 CIDR block will only give you 32 IP addresses but you actually need 34 IP addresses, so you'd require a /26 CIDR block with 64 IP addresses.