doc: net: Update NAT options for Internet connectivity

[jukkar]

The host NAT options for QEMU/native_sim Internet connectivity needs updating.

[sense-Jo]

Hi @jukkar, thank you for this PR, which helped me to get a working internet connection in Qemu!
However, there was one thing I had to change: At least on my system, after following the guide on Networking with QEMU, the name of the network interface isn't zeth, but tap0.
Isn't the interface only named zeth when using the net-setup.sh script, while with the ./loop-socat.sh and ./loop-slip-tap.sh, which the guide for Qemu advises to use, the interface would usually be named tap0?

[jukkar]

Isn't the interface only named zeth when using the net-setup.sh script, while with the ./loop-socat.sh and ./loop-slip-tap.sh, which the guide for Qemu advises to use, the interface would usually be named tap0?

Hi @sense-Jo, there are two guides for setting up qemu, one is with tap0 https://docs.zephyrproject.org/latest/connectivity/networking/qemu_setup.html#networking-with-qemu and the other is with zeth https://docs.zephyrproject.org/latest/connectivity/networking/qemu_eth_setup.html#networking-with-eth-qemu
So both zeth or tap0 are valid interfaces. We try to guide people to use zeth instead, as it is somewhat easier to setup in the host side.

[sense-Jo]

@jukkar I can see that there are multiple options of connecting a Zehpyr (Qemu) application to a network.
I just now understood that NAT/masquerading is required for both Qemu networking options, and apparently required as well when using the native_* targets.
Thus I wonder if it would make sense to move the instructions on NAT/masquerading to a separate page, maybe under Networking and titled something like "Internet access from hosted applications"?

[jukkar]

Thus I wonder if it would make sense to move the instructions on NAT/masquerading to a separate page, maybe under Networking and titled something like "Internet access from hosted applications"

Could you propose a PR for that, we can then review and see what it looks like.