I don't think the BSD networking code in 2.10 worked on systems with only 256K bytes of memory. The kernel is too big even without networking and a lot of work was done to push things out of the kernel. [The 17th bit (split I/D) really only mattered for user space, the kernel mapped things around - but with only 18 bits of address map there is not much space]. Adding networking and in particular the space for the mbuf's becomes a real issue.
Here are some thoughts..
1.) Easiest/Cheapest solution might be to front end the system with an RPi, Intel Edison or the like and run the IP stack on it and then use one or more serial ports from the micro to 11.
2.) See if you can dig up an old copy of the 3 COM's first product - UNET - which was the original TCP/IP for V7. It's old and not very sexy, but the kernel requirements are minimal and if all you want it telnet & FTP that will work.
3.) If you have real hardware, see if you can find an old Able "Enable" board which will allow you to put 4Megs of memory in an 40 class processor (you get a cache plus a new memory MAP with 22 bits of address like the 45 class processors). I had 2.X working on that years ago (and wrote a USENIX paper on it). The Enable board support was in the BSD 2* distributions (I put there) but I doubt its been tried in many years.
4.) Ultrix-11 should boot on a 40 class system, but I do not remember if on the 18 bit machines you could configure networking. Armando might remember if Fred ever made that work. If any one could have or would have, it would have been Fred. As I said, there just is not a lot of space and frankly there is not going to be a lot of space left for user programs.
Clem
PS I remember running V6 on 11/34 with 48K bytes of memory for a few months as our memory for that system was back ordered. It was slow, but it worked and we were happy. It was our machine!!