NAT map no-op?

I'm interested in having a FreeBSD box route packets between my
various internal networks as well as providing NAT services when
routing my private networks to the Internet.  The network looks
something like:

  192.168.15.0 ---|
                  |
  192.168.16.0 ---|
                  |
  192.251.93.0 ---|
                  |
  Internet     ---|
                  |   
                  |-- re0 interface -- FreeBSD 7.1 running IP Filter: v4.1.28

I have a configuration which appears to work, however am wondering if
there's a better way.  Note: the re0 interface has multiple IP addresses
... one on each network.  Here's my ipnat.conf:

  # If it's from our private networks going to our networks,
  # then leave it alone ... i.e. map no-op
  map re0 from 192.168.15.0/24 to 192.251.93.0/24 -> 0/0
  map re0 from 192.168.15.0/24 to 192.168.16.0/24 -> 0/0
  map re0 from 192.168.16.0/24 to 192.251.93.0/24 -> 0/0
  map re0 from 192.168.16.0/24 to 192.168.15.0/24 -> 0/0

  # If it's from our private networks going outside the building,
  # then NAT it
  map re0 from 192.168.15.0/24 to any -> 0/32 portmap tcp/udp 40000:60000
  map re0 from 192.168.15.0/24 to any -> 0/32
  map re0 from 192.168.16.0/24 to any -> 0/32 portmap tcp/udp 40000:60000
  map re0 from 192.168.16.0/24 to any -> 0/32

What I'm not clear on is the best way to tell IP Filter that packets
going between my various internal networks should be left alone.  Currently
I have what should be a no-op map rule, however it still results in work
being done.  I.e. a NAT table entry ... ipnat -l shows things like:

  MAP 192.168.16.13   161   <- -> 192.168.16.13   161   [192.251.93.208 1031]

I'd like to short circuit the whole NAT process for internal traffic.

Thoughts / suggestions / recommendations?

Note: I am limited to getting the job done using this one box.

-- John
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|   Feith Systems  |   Voice: 1-215-646-8000  |  Email: [email protected]  |
|    John Wehle    |     Fax: 1-215-540-5495  |                         |
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