Nat/Napt - Siemens SIMATIC NET SCALANCE W780 Series Configuration Manual

Technical basics

4.11 NAT/NAPT

4.11 NAT/NAPT
What is NAT?
With Network Address Translation (NAT), the IPv4 address in a data packet is replaced by
another. NAT is normally used on a gateway between a private LAN and an external network
with globally valid IPv4 addresses. A local IPv4 address of the internal LAN is changed to an
external global IPv4 address by a NAT device at the gateway.
To translate the internal into the global IPv4 address, the NAT device maintains a translation
list. The address assignment is automatic. You configure the address assignment in "Layer 3
> NAT > Basic (Page 349)".
What is NAPT?
In "Network Address Port Translation" (NAPT) or "Port Address Translation" (PAT), several
internal source IPv4 addresses are translated into the same external source IPv4 address.
To identify the individual source nodes, the port of the source device is also stored in the
translation list of the NAT gateway and translated for the external address.
If several local clients send a query to the same external destination IPv4 address via the
NAT gateway, the gateway enters its own external source IPv4 address in the header of
these data packets. Since the forwarded data packets have the same global source IPv4
address, the NAT gateway assigns the data packets to the clients using different port
number.
Note
NAT/NAPT is possible only on layer 3 of the ISO/OSI reference model. To use the NAT
function, the networks must use the IP protocol.
When using the ISO protocol that operates at layer 2, it is not possible to use NAT.
If a client from the global network wants to use a service in the internal network, the
translation list for the static address assignment needs to be configured. You configure the
translation list for NAPT in "Layer 3 > NAT > NAPT (Page 352)".
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SCALANCE W780/W740 to IEEE 802.11n Web Based Management
Configuration Manual, 08/2018, C79000-G8976-C267-13