NAT Rule Inputs
Example Data Table
| Use Case | Type | Original | Translated | Protocol | Notes |
|---|---|---|---|---|---|
| Publish HTTPS to internal web server | DNAT | 203.0.113.10:443 | 192.168.10.20:443 | TCP | Typical inbound web publishing rule. |
| Outbound internet using a fixed public IP | SNAT | 10.10.0.0/16:any | 203.0.113.50:any | Any | Common for egress IP consistency. |
| Many clients share one interface address | PAT | 10.20.30.0/24:53 | Interface:any | UDP | Port overloading for constrained address pools. |
Formula Used
The generator applies deterministic translation rules to produce repeatable templates.
- DNAT: destination becomes TranslatedIP; optional destination port becomes TranslatedPort.
- SNAT: source becomes TranslatedIP; ports often remain any for dynamic allocation.
- PAT: multiple sources share one egress address; sessions use ephemeral ports to prevent collisions.
- Service match: for TCP/UDP, match is Protocol + OriginalPort; for ICMP, both ports are any.
- Validation: IPv4/CIDR parsing, port range 1–65535, and protocol-to-port consistency checks.
How to Use This Calculator
- Pick NAT Type (DNAT for inbound, SNAT/PAT for outbound).
- Select Protocol, then provide ports only for TCP/UDP.
- Enter Original and Translated IP values (IPv4/CIDR).
- Optionally set zones, interfaces, logging, and a short audit comment.
- Click Generate Rule; review output above and export CSV/PDF.
Operational accuracy in NAT changes
NAT modifications are high‑risk because a single mismatch can expose services or break outbound access. This calculator standardizes inputs for original IP, translated IP, protocol, and ports, then renders predictable templates for review. Validation rejects invalid IPv4/CIDR syntax and enforces the 1–65535 port range, reducing typing errors. It streamlines reviews and rollback planning.
DNAT publishing and service control
For inbound publishing, DNAT maps a public address and optional service port to a private host. When TCP/UDP is selected, the generator binds the match to an original port and can translate to a different destination port, such as 443 → 8443. The source and destination selectors support “any”, a single IPv4, or a CIDR range, letting you constrain exposure to trusted networks. ICMP rules automatically require ports set to “any”, preventing invalid service entries.
SNAT and deterministic egress identity
SNAT is commonly used to present a stable egress identity for internal ranges and to simplify external allowlists. By pairing a source CIDR (for example, 10.10.0.0/16) with a translated address, the tool produces a repeatable template that can be audited later. Zone and interface fields document the intended traffic direction, which helps reviewers catch asymmetric routing risks. Logging and enabled state are captured per rule, aligning with operational controls and monitoring expectations.
PAT scaling for limited address pools
PAT (overload) supports many internal clients sharing a single egress address by using unique ephemeral ports per session. This approach is common where IPv4 space is constrained, but it complicates troubleshooting. The calculator flags risky PAT inputs, such as “any” original port on TCP/UDP, because service‑specific PAT is usually explicit. These guardrails encourage least‑privilege translations and reduce surprises during incident response.
Exports, evidence, and review workflow
Each generated rule is saved in a session list and can be exported as CSV for ticket attachments, spreadsheet review, or batch validation. The PDF export captures a compact table of recent rules for approvals, CAB notes, and audits. Copy buttons speed CLI pasting while keeping comments short and non‑sensitive. As a best practice, compare the rendered template to your firewall’s object model and confirm that a security policy permits the translated flow.
FAQs
1) What is the difference between DNAT and SNAT?
DNAT changes the destination address and optionally the port for inbound flows. SNAT changes the source address for outbound flows to control how traffic appears externally.
2) When should I use PAT instead of SNAT?
Use PAT when many internal hosts must share one egress address. Use SNAT when you need a fixed translated address per host or subnet, or when allowlists require stable egress IPs.
3) Why does ICMP require ports set to “any”?
ICMP does not use TCP/UDP port numbers. Forcing “any” prevents generating templates that would not apply cleanly on common platforms.
4) Can I translate to a different destination port?
Yes. For TCP or UDP DNAT, set the original port to the public service port and the translated port to the internal service port. Use “any” for address‑only translations.
5) What does the CSV export include?
The export includes timestamps, NAT type, protocol, match criteria, translation values, zones, interfaces, logging, enabled state, and comments for ticketing and audit evidence.
6) Are the vendor outputs production‑ready commands?
They are practical templates, but names and dependencies vary. Validate in a lab or maintenance window, then use peer review before production deployment.