IIRC, it would have to go in from one NIC, and out another. I would say to
check http://www.obfuscation.org/ipf/ipf-howto.txt for a better answer...
Check about halfway down, labelled page 26.

Taken from there...
"...
An extremely important point must be made  about  rdr:   You
cannot easily use this feature as a "reflector".  E.g:

     rdr tun0 20.20.20.5/32 port 80 -> 20.20.20.6 port 80 tcp

will  not  work  in the situation where .5 and .6 are on the
same  LAN  segment.   The rdr function is applied to packets
..."

---   David A Woyciesjes
---   C & IS Support Specialist
---   Yale University Press
---   mailto:david.woyciesjes@yale.edu
---   (203) 432-0953
---   ICQ # - 905818


-> -----Original Message-----
-> From: mel kravitz [mailto:mel@switchpwr.com]
-> Sent: Thursday, October 26, 2000 11:54 AM
-> To: tech-net@netbsd.org
-> Subject: ipnat rdr rules on the same segment?
-> 
-> 
-> Can ipnat (rdr )redirect port 80 from www(x.y.z.b)machine to
-> FW(x.y.z.a)machine when both reside on the same outside segment? Is a
-> rule such as the following possible?
-> rdr fxp0 x.y.z.b port 80 -> x.y.z.a port 80
-> -Mel
-> 
-> 
->