How many IPs does AWS reserve per subnet?

AWS reserves 5 addresses: .0 (network), .1 (VPC router), .2 (DNS server), .3 (reserved for future use), and .255 (broadcast). A /24 subnet therefore has 251 usable addresses, not 254.

How many IPs does a Kubernetes cluster need?

In VPC-CNI mode (AWS), each pod gets its own IP. A cluster with 100 nodes and 30 pods per node needs 3,000+ IPs. Use /20 (4,091 usable) or /19 (8,187 usable) subnets for Kubernetes workloads.

What happens when a subnet runs out of IPs?

New pods, instances, and services fail to launch with 'InsufficientFreeAddresses' errors. Resolution requires either migrating to a larger subnet (disruptive), adding secondary CIDR blocks, or reducing IP consumption.

Can I resize a subnet after creation?

In most cloud providers, you cannot resize an existing subnet. You must create a new larger subnet, migrate resources, and delete the old one. This is why planning for growth upfront is critical.

How many subnets can I create from a /16?

A /16 contains 65,536 addresses. It can be divided into 256 /24 subnets (254 hosts each), 16 /20 subnets (4,094 hosts each), or any other combination that fits without overlap.

Should I use private or public IPs?

Use private IPs (RFC 1918) for internal resources and public IPs only for internet-facing endpoints. NAT gateways allow private resources to access the internet. This conserves public IP address space and improves security.

IP Address Count Calculator

Calculate total and usable IP addresses for any CIDR range. Plan IP allocation for VPCs, subnets, and networks.

IP Version

Cloud ProviderReserves 5 addresses per subnet

CIDR Prefix Length

/24

Subnet Split (bits)Additional bits to subdivide

Target Utilization

Total Addresses

256.00

2^8 = 256.00

Usable Addresses

249.00

2.00 standard + 5 cloud reserved

Subnet Mask

255.255.255.0

/24 — 8 host bits

Target Capacity

199.00

80% of 249.00 usable addresses

Possible /24 Subnets

1.00

Number of standard-size (256 addr) subnets

Address Utilization

■ Usable: 199.00■ Reserved: 7■ Free: 50.00

IPv4 CIDR Reference Table

CIDR	Subnet Mask	Total IPs	Usable	Typical Use
/8	255.0.0.0	16,777,216.00	16,777,209.00	Class A
/10	255.192.0.0	4,194,304.00	4,194,297.00	Large ISP
/12	255.240.0.0	1,048,576.00	1,048,569.00	ISP block
/16	255.255.0.0	65,536.00	65,529.00	Class B / large org
/18	255.255.192.0	16,384.00	16,377.00	Data center
/20	255.255.240.0	4,096.00	4,089.00	AWS VPC default
/22	255.255.252.0	1,024.00	1,017.00	Medium subnet
/24	255.255.255.0	256.00	249.00	Standard LAN
/25	255.255.255.128	128.00	121.00	Half /24
/26	255.255.255.192	64.00	57.00	Small office
/27	255.255.255.224	32.00	25.00	Tiny subnet
/28	255.255.255.240	16.00	9.00	AWS minimum
/29	255.255.255.248	8.00	1.00	Micro subnet
/30	255.255.255.252	4.00	0.00	Point-to-point
/31	255.255.255.254	2.00	0.00	RFC 3021 p2p
/32	255.255.255.255	1.00	0.00	Single host

Planning notes, formulas, and examples

About the IP Address Count Calculator

IP address planning is fundamental to network architecture. Every device, container, load balancer, and service endpoint consumes an IP address. Running out of addresses in a subnet forces disruptive resizing operations.

This calculator helps planners determine how many IP addresses are available in a given CIDR range and how many subnets can be carved from a larger network block. It accounts for reserved addresses (network, broadcast, and cloud provider reservations) to show the truly usable count.

Cloud providers reserve additional addresses beyond the standard network and broadcast addresses. AWS reserves 5 addresses per subnet (.0, .1, .2, .3, and .255). Understanding these reservations is critical for accurate capacity planning.

When This Page Helps

Running out of IP addresses forces costly network reconfiguration. This calculator helps plan address allocation with sufficient headroom for growth.

How to Use the Inputs

Enter the CIDR prefix length for your network.
Enter the number of cloud-reserved addresses per subnet (default 5 for AWS).
Review the total, reserved, and usable address counts.
Plan your subnet sizes to accommodate projected growth.

Formula used

Total Addresses = 2^(32 − prefix)
Standard Reserved = 2 (network + broadcast)
Cloud Reserved = provider_reserved_count
Usable = Total − Standard Reserved − Cloud Reserved

Example Calculation

Result: 251 usable out of 256 total

A /24 has 256 total addresses. AWS reserves 5 addresses (.0 network, .1 VPC router, .2 DNS, .3 future use, .255 broadcast). Usable: 256 − 5 = 251 addresses for your resources.

Tips & Best Practices

AWS reserves 5 IPs per subnet; Azure reserves 5; GCP reserves 4.
Plan for 2–3x current need to accommodate growth.
Kubernetes pods can consume many IPs — use /20 or larger subnets for clusters.
Each ELB, NAT gateway, and ENI consumes an IP address.
Track IP utilization and alert when subnets reach 70% capacity.
Document IP allocations in an IPAM tool to prevent conflicts.

Cloud-Specific Address Reservations

Cloud providers reserve addresses for infrastructure services. AWS reserves 5 per subnet for routing, DNS, and future use. Azure reserves 5 for gateway, DNS, and broadcast. GCP reserves 4. These reservations reduce usable addresses, especially in small subnets.

Capacity Planning for Growth

Network architects typically plan for 3–5 years of growth. If you currently need 50 IPs, choose a subnet supporting 200+ to accommodate growth without reshuffling. For dynamic environments (auto-scaling, Kubernetes), planning is even more critical because IP consumption fluctuates.

Avoiding Address Exhaustion

Monitor IP utilization with cloud provider tools (VPC Flow Logs, IPAM services). Set alerts at 70% utilization. When approaching capacity, plan migration to larger subnets during maintenance windows to avoid emergency resizing.

Sources & Methodology

Last updated: February 8, 2026

Frequently Asked Questions

AWS reserves 5 addresses: .0 (network), .1 (VPC router), .2 (DNS server), .3 (reserved for future use), and .255 (broadcast). A /24 subnet therefore has 251 usable addresses, not 254.