A subnet is a smaller section carved out of your VPC. If a VPC is a big office building, subnets are the individual floors or departments inside it.

Basic Example

VPC:      10.0.0.0/16   → entire building (65,536 IPs)
Subnet 1: 10.0.1.0/24   → floor 1 (256 IPs)
Subnet 2: 10.0.2.0/24   → floor 2 (256 IPs)
Subnet 3: 10.0.3.0/24   → floor 3 (256 IPs)

Each subnet lives inside the VPC and gets a slice of its IP range.

AWS Reserves 5 IPs in Every Subnet

No matter what size subnet you create, AWS always holds back 5 IP addresses. You cannot use them.

Example using subnet 10.0.0.0/24:

IP Address Reserved For
10.0.0.0 Network address (building's main address)
10.0.0.1 VPC Router (AWS internal routing)
10.0.0.2 DNS Server (AWS DNS service)
10.0.0.3 Future use (reserved by AWS)
10.0.0.255 Broadcast address (not used, still reserved)

How Many IPs Can You Actually Use?

Formula:

Usable IPs = Total IPs - 5
Subnet Total IPs Minus Reserved Usable IPs
/24 256 5 251
/27 32 5 27
/28 16 5 11

How to Calculate Total IPs from a CIDR

IPv4 has 32 bits total. The number after the slash tells you how many bits are used for the network. The remaining bits are for host IPs.

Formula:

Total IPs = 2 ^ (32 - CIDR number)