In a VPC subnet, what is the purpose of the second IP address?

In a VPC subnet, the second IP address within the allocation typically serves as the default gateway for that subnet. This is an essential function because the default gateway is the IP address used by instances within the subnet to communicate with external networks, such as the internet or other subnets. In many networking configurations, the first usable IP address is often reserved for the network itself, while the second IP address is designated as the default gateway, providing a seamless routing path for outbound traffic.

The structure of subnets in IP addressing dictates that the lowest address is reserved for the subnet identifier, and the highest address is reserved for the broadcast address. Therefore, while the gateway may vary in its exact numbering based on subnet size, it is standard practice to use the second address for the gateway in many configurations to streamline routing processes. This enables ease of management and communication for instances within that subnet, as they know where to send packets that need to go outside their local network segment.