Dynamic address translation, usually called dynamic Network Address Translation (NAT), temporarily assigns an external IP address to an eligible internal host when it needs to communicate beyond its network. The NAT device records the mapping while it is in use; when the applicable session or timeout ends, the address can return to a pool for another host. The same host is not guaranteed to receive that external address next time.
What is dynamic NAT?
Network Address Translation maps IP addresses between address realms so packets can travel between them. A NAT device, commonly placed at a network boundary, changes IP addresses in packet headers. Some forms also translate transport identifiers such as TCP or UDP ports. RFC 2663 describes NAT as a method for mapping addresses between realms to provide transparent routing to end hosts: RFC 2663.
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With dynamic NAT, an administrator defines which internal addresses may be translated and provides a pool of external addresses, or an interface address, for the translations. When an eligible internal host initiates a connection, the device assigns an available mapped address and records the binding. The binding lasts for the relevant session or timeout; after it is no longer needed, the address can be reused. Exact assignment and expiration behavior depends on the NAT implementation.
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How a dynamic address mapping is created and released
- Eligibility is configured. The administrator selects the internal addresses allowed to use translation and specifies the external address pool or interface address. Cisco describes this as a dynamic mapping between local and global addresses in its NAT FAQ.
- An internal host initiates traffic. The NAT device assigns an available external address and records the mapping for the connection.
- The mapping remains temporarily. It stays active for the applicable session or timeout. Once it expires, the host may receive a different external address on a later connection.
- The address becomes reusable. When no active session uses the binding, the address can be returned to the available pool.
Dynamic NAT vs. static NAT vs. PAT
| Type | How the mapping works | Address pool | How concurrent conversations are distinguished | Predictable inbound addressing |
|---|---|---|---|---|
| Static NAT | A persistent one-to-one mapping for the lifetime of the NAT operation, as defined in RFC 2663. | Not required for the persistent mapping. | Not established by the cited RFC terminology for this comparison. | The mapped address is persistent, but that alone does not establish that inbound traffic is permitted. |
| Dynamic NAT | A temporary address assignment based on usage and session flow; the address may change after the mapping expires. | Typically assigns an external address from a configured pool or interface address. | Uses assigned addresses for mappings; it is distinct from sharing an address through port translation. | Not reliably predictable after a mapping expires or when no relevant mapping exists. |
| PAT (overloading) | Multiple internal addresses can share one or more external addresses. | Uses one or more mapped external addresses rather than requiring a separate external address for each conversation. | Unique source ports distinguish conversations that share mapped addresses. | Port translation does not itself guarantee an inbound connection can reach an internal host. |
RFC 2663 distinguishes static and dynamic NAT, while Cisco explains PAT or overloading as using unique source ports to differentiate conversations sharing mapped addresses: Cisco NAT FAQ.
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What dynamic NAT means for inbound connections
A temporary outbound mapping is not the same as a permanent address for a service that remote users can always contact. Cisco’s guidance for Secure Firewall Management Center 7.1 says a dynamic translation is created when the real host initiates a connection; when the relevant entry is absent, outside hosts cannot reliably initiate a connection through it. An access rule still governs traffic while a mapping exists. This is documented behavior for that Cisco platform, not a guarantee about every vendor’s implementation: Cisco Secure Firewall Management Center 7.1 NAT documentation.
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What dynamic NAT does—and does not—provide
- It translates addresses so packets can move between network address realms.
- It assigns an external address temporarily according to configured eligibility and active use.
- It can make an address available for reuse after a mapping ends or expires.
- It does not promise that a host will retain the same mapped address across separate connections.
- Address translation is not, by itself, a complete security policy. Access rules and the specific device configuration still matter.
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