NAT Pool Planner Calculator

Inputs

Planning Mode

PAT scales with ports. One-to-one scales with IPs.

Private Network CIDR (optional)

Used for a host-count cross-check only.

Devices Requiring Translation

Count of internal clients that will NAT.

Demand Model

Devices × Sessions

Direct Translations

Choose how you want to specify demand.

Avg Concurrent Sessions per Device

Typical simultaneous outbound connections per device.

Total Concurrent Translations

Use when you already know peak translations.

Port Range Start

Commonly starts at 1024 or higher.

Port Range End

Commonly ends at 65535.

Reserve Percent

Keeps spare ports for bursts and retries.

Headroom Percent

Adds growth buffer to required public IPs.

Target Utilization Percent

Used for a conservative sizing recommendation.

Public Pool Input

Provide the public address inventory.

Public CIDR Block

Example ranges use documentation addresses.

Usability Rule

Exclude network and broadcast when applicable

For many routed blocks, all IPs may be usable.

Pool Start IP

Pool End IP

After submitting, your results appear above this form.

Example Data Table

Scenario	Mode	Devices	Sessions/Device	Port Range	Reserve	Public Pool	Usable IPs	Required IPs (Headroom)
Branch Internet Egress	PAT	500	40	1024–65535	10%	198.51.100.0/27	30	2
DMZ Mapped Services	One-to-one	18	—	—	—	203.0.113.64–203.0.113.95	32	22

Examples use documentation IPv4 blocks and simplified assumptions.

Formula Used

PAT (many-to-one) sizing

This planner treats one active translation as one source port.

Required Translations = Devices × Sessions/Device (or direct input)
Raw Ports/IP = (PortEnd − PortStart + 1)
Usable Ports/IP = floor(Raw Ports/IP × (1 − Reserve%))
Capacity = Usable Pool IPs × Usable Ports/IP
Required Public IPs = ceil(Required Translations ÷ Usable Ports/IP)
Headroom = multiply demand by (1 + Headroom%)

One-to-one NAT sizing

This mode assumes one public IP per mapped device.

Capacity = Usable Pool IPs
Required Public IPs = Devices
Headroom = ceil(Devices × (1 + Headroom%))
Utilization = Required ÷ Capacity
Target Util sizing = ceil(Required ÷ TargetUtil%)

Note: Real NAT behavior can vary by platform, protocol mix, and timers. Use measured peak values when available.

How to Use This Calculator

Select a planning mode that matches your design.
Enter device count and your preferred demand model.
For PAT, set a realistic concurrent session estimate.
Adjust port range, reserve percent, and headroom percent.
Provide your public pool using CIDR or range input.
Submit to view required IPs, utilization, and warnings.
Download CSV or PDF to share assumptions and results.