If one is running a mail server and has it inside their firewall and if using one IP then t has to use NAT. Couldn't one presumeably use the setup above and run a mail server on Plan 9 and bypass having to use NAT?  And also do the same thing for a web server?

Yes, I do that. The example you quoted creates two independent IP stacks, starting with the default '#I0' IP stack on ether0, then adding a new IP stack '#I1' connected to ether1 (#l1).
There is a separate TCP/IP, UDP/IP, ICMP etc for each stack. I also import /net from a Linux server via Inferno (on Linux) so I can send mail from a non-RBLd address.
You can create several types of virtual interface ("medium") on the IP stack, connected to a user-mode process. See pkg and netdev in ip(3)

  I still have a router with NAT though for non-Plan 9 machines. I never got round to writing a NAT for Plan 9 (which could work in user mode).

On Fri, May 8, 2020 at 7:55 PM Robert Sherwood <robert.sherwood@gmail.com> wrote:
I love the idea of importing the external interface to get outside the network. When I first read about this in Plan9, that's when the system really "clicked" for me.

On Fri, May 8, 2020 at 1:08 PM hiro <23hiro@gmail.com> wrote:
you can also have multiple ipstacks, working ipv6 and what have you.
cinap fixed a bunch of stuff in this regard.

it's much more like linux network namespaces now, no limits to your
creativity...

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