AWS VPC Subnet Sizing: Usable IP Addresses Per CIDR Block

A "/24 gives 254 usable hosts" is the textbook subnetting answer. Inside an AWS VPC it's wrong by 3 — AWS reserves 5 addresses per subnet, not the 2 (network + broadcast) that standard subnetting math accounts for. Here's the exact count for every common VPC subnet size.

Why AWS reserves 5, not 2

Standard IPv4 subnetting reserves two addresses per subnet: the network address (all host bits 0) and the broadcast address (all host bits 1). AWS reserves three more on top of that, per AWS's VPC sizing documentation:

The network address (standard)
The VPC router, at network address + 1
The DNS server (reserved by AWS), at network address + 2
A reserved-for-future-use address, at network address + 3
The broadcast address (standard) — even though VPC doesn't support broadcast traffic, the address is still reserved

That's 5 total, so a subnet's AWS-usable count is 2^(32 − prefix) − 5 instead of the standard 2^(32 − prefix) − 2.

Usable addresses by subnet size

CIDR	Total addresses	Standard usable	AWS VPC usable	GCP VPC usable
/16	65,536	65,534	65,531	65,533
/17	32,768	32,766	32,763	32,765
/18	16,384	16,382	16,379	16,381
/19	8,192	8,190	8,187	8,189
/20	4,096	4,094	4,091	4,093
/21	2,048	2,046	2,043	2,045
/22	1,024	1,022	1,019	1,021
/23	512	510	507	509
/24	256	254	251	253
/25	128	126	123	125
/26	64	62	59	61
/27	32	30	27	29
/28	16	14	11	13

Range shown is /16 to /28 — AWS's allowed VPC subnet size range. GCP reserves 3 addresses (network, a second address, and broadcast) per GCP's VPC subnet documentation, so its usable count sits between the standard and AWS figures.

Need a size that isn't in this table, an IPv6 subnet, or the reverse calculation (two IPs → smallest covering CIDR)? The full calculator below handles any prefix length and both address families.

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