real IP and rfc1918 IP on the same machine - how?

[freebsd-lists at potato.growveg.org]

On Sun, Jan 25, 2015 at 08:16:11AM -0700, Matthew Hardcastle wrote:
> 
> Hi John,
> 
> The setup you’re describing is pretty standard and should “just work” 
> when configured correctly. It sounds like you may have misconfigured 
> one of your interfaces. The output of `ifconfig`, `netstat -rn`, and 
> `grep ^ifconfig /etc/rc.conf` would be helpful in diagnosing your 
> issue.
> 
> Also, I noticed you say your wired interface is a /28 but then state 
> it’s configured as a /24. Perhaps this is your issue?

Ah sorry I've probably worded it wrongly. 

em0 = wired interface = real ip on a /28 = 255.255.255.240 netmask
ath0 = wireless interface = 192.168.100.100 netmask 255.255.255.0

Something just occurred to me. If the default route i.e. 0.0.0.0/0 -> my 
real router ip then how am I going to exclude 192.168.100.0/24 ?
-- 
John