nat (outside,inside) source static obj-10.140.4.10 obj-10.66.23.39 destination static obj-209.51.47.8 obj-209.51.51.136
This NAT can be configured in either section 1 (manual NAT) or section 3 (after-auto manual NAT), and it is matched in a top down fashion (so the order they appear in the configuration matters).
This format is basically a policy NAT with the ability to also translate the destination IP or subnet. So here you can specify that if traffic sourced from 10.140.4.10 has a destination of 209.51.47.8, change the source IP to 10.66.23.39 and change the destination IP to 209.51.51.136.
This NAT (twice NAT) has several use cases.
1. you need to NAT the source address to avoid IP conflict or you have a requirement that the source IP needs to be a specific IP that cannot be assigned to the PC for whatever reason.
2. This format is also used for NAT exempt for VPN configurations
3. You need to NAT the destination IP or subnet, this is usually done to either avoid IP conflict or you have a requirement that your PCs need to user a specific IP or subnet to reach a destination but the destination is not able to actually use this other IP.
nat (outside,inside) source static obj-10.140.4.11 obj-10.66.23.70
This NAT can be configured in either section 1 (manual NAT) or section 3 (after-auto manual NAT), and it is matched in a top down fashion (so the order they appear in the configuration matters).
This NAT has the same function as object NAT which is configured in section 2 (auto NAT). This type of NAT just translates the source IP or subnet.
Use cases:
1. Translate server source IP to a dedicated public IP, or private IP depending on what you are trying to achieve.
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