Network and broadcast addresses
Two addresses in every subnet cannot be given to a device.
A network address is the first address in a subnet and identifies the subnet itself. All host bits are zero. In 192.168.1.0/24 the network address is 192.168.1.0; it names the segment and is what appears in routing tables.
A broadcast address is the last address in a subnet. All host bits are one. In 192.168.1.0/24 that is 192.168.1.255. Traffic sent to it is delivered to every device on the segment.
A smaller example, 192.168.1.16/28 — 16 addresses:
- Network address: 192.168.1.16
- Usable host addresses: 192.168.1.17 to 192.168.1.30
- Broadcast address: 192.168.1.31
In a VPC
Section titled “In a VPC”AWS reserves five addresses in every subnet, not two. For a subnet 10.0.0.0/24 they are:
| Address | Reserved for |
|---|---|
| 10.0.0.0 | Network address |
| 10.0.0.1 | The VPC router |
| 10.0.0.2 | The Amazon-provided DNS resolver (VPC base + 2) |
| 10.0.0.3 | Future use |
| 10.0.0.255 | Broadcast address |
A VPC does not support broadcast traffic; the last address is reserved for consistency rather than because anything listens on it. The practical effect is that a /24 subnet gives you 251 usable addresses, and the smallest permitted subnet, a /28, gives you 11.