I wrote:
> When I go to the 192.156.206.3 machine and attempt to telnet to
> 205.245.6.2, (or anywhere else on the net, for that matter,) the
> connection fails to open.

tcpdump -e -i le0 (the internal Ethernet) during the telnet attempt to
usr09.primenet.com shows:

05:24:32.601474 0:80:19:b:e:4b 0:80:10:0:16:d7 ip 60: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>
05:24:34.157400 0:80:19:b:e:4b 0:80:10:0:16:d7 ip 60: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>
05:24:37.283746 0:80:19:b:e:4b 0:80:10:0:16:d7 ip 60: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>
05:24:43.515962 0:80:19:b:e:4b 0:80:10:0:16:d7 ip 60: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>

tcpdump -e -i ppp0 (the external PPP connection to the world) shows:

05:24:32.602065 O   48 03 0021: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>
05:24:34.157992 O   48 03 0021: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>
05:24:37.284339 O   48 03 0021: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>
05:24:43.516555 O   48 03 0021: unicorn.resun.com.11953 > usr09.primenet.com.telnet: S 1363820208:1363820208(0) win 4788 <mss 1460>

I infer from this that there is no IP address translation or port
mapping occurring.

I am running on a custom kernel.  Perhaps I have missed setting an
option in the kernel that is necessary for this to work?

Telnetting to 192.156.206.1 from 192.156.206.3 works.  (No surprise.)
-- 
  Don Phillips         bsd@blkhole.resun.com
  Research Unlimited
  Escondido, Calif.    My opinions are just that, and no more.