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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; after the applicable session or timeout ends, the address may return to the pool for another host. That means a host is not guaranteed to receive the same external address every time.
What is dynamic address translation?
Network Address Translation maps IP addresses between address realms so packets can travel between them. A NAT device commonly sits at a network boundary and modifies IP addresses in packet headers. Depending on the NAT type, it may also translate transport-layer identifiers such as TCP or UDP ports.
In dynamic NAT, an administrator defines which internal addresses are eligible for translation and configures a source of external addresses, such as an address pool or interface address. When an eligible host starts a connection, the device assigns a mapped address and records the translation. The mapping lasts for the relevant session or timeout period, after which the address can be reused.
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RFC 2663 describes NAT generally as a way to map IP addresses from one address realm to another, providing transparent routing to end hosts: RFC 2663. The exact assignment and expiration behavior depends on the NAT implementation.
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How a dynamic NAT mapping is created and released
- Eligible addresses are configured. An administrator selects the internal addresses that may be translated and associates them with an external address pool or interface address.
- An internal host initiates traffic. When an eligible host starts a connection, the NAT device selects an available mapped address and records the translation.
- The mapping remains active temporarily. It applies for the session or relevant timeout period. The external address is not necessarily reserved for that host permanently.
- The address becomes available again. Once the last session using the binding ends, the address can be returned to the pool and assigned to another host.
Cisco’s FAQ describes dynamic NAT as allowing users to establish dynamic mappings between local and global addresses, and its terminology includes a pool or interface address as the source for mapped addresses: Cisco NAT FAQ.
Dynamic NAT vs. static NAT vs. PAT
| Type | How the mapping works | How ports are used | Address predictability |
|---|---|---|---|
| Static NAT | A persistent one-to-one mapping remains for the lifetime of the NAT operation, as described in RFC 2663. | Port translation is not the defining feature. | The mapped address is predictable while the static mapping remains configured. |
| Dynamic NAT | The device temporarily assigns an external address, typically from a configured pool, based on usage and session flow. | It assigns an address from the pool; port sharing is not what defines dynamic NAT. | The host may receive a different mapped address after a mapping expires. |
| PAT, or overloading | Multiple internal addresses can share one or more external addresses. | Unique source ports distinguish concurrent conversations that share mapped addresses. | Remote peers do not get a unique external IP address per internal host solely from the shared mapping. |
PAT and dynamic NAT are related forms of translation but are not interchangeable terms: PAT relies on port numbers to distinguish conversations, whereas dynamic NAT assigns an external address from a pool. Cisco explains this distinction in its NAT FAQ.
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What dynamic NAT means for inbound connections
A temporary mapping does not provide an internal host with a predictable, permanent external address for unsolicited inbound connections. Cisco’s Secure Firewall Management Center 7.1 guidance says a dynamic translation is created when the real host initiates a connection; when the relevant entry is absent, an outside host cannot reliably initiate a connection through that mapping. Access rules still determine whether traffic is permitted when a mapping exists. This describes the documented Cisco platform behavior, not a guarantee about every vendor’s implementation: Cisco Secure Firewall Management Center 7.1 NAT guidance.
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What dynamic NAT does—and does not—provide
- It translates addresses so packets can move between address realms.
- It assigns an external address temporarily according to configured eligibility and network use.
- It can return an address to a pool after the associated mapping ends.
- It does not guarantee that a host will keep the same external address after expiration.
- Address translation alone does not establish that traffic is secure; access rules and other network controls remain relevant.
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