How do I subnet a class B network

[John R. Morris]

Hi; I am fairly new to Unix and Suns and the "Net" so please bear with me.
I have a couple questions about subnetting.

I have some 4/110s and one 4/280 which were on a class B network.  The
4/280 now has two ethernet interfaces and I want to make it a router,
isolating all of my machines from the class B network.  I can't seem to do
it correctly and Sun-support is not very fast/helpful answering questions.
So I found out that this net has thousands of sun-gurus...

Anyway,
-------|                        |---------
       |                        |
       |     ----------         |
rest         |        |               my
 of    ------|sun4/280|----------   little
world        |        |             corner
  B    |     ----------         |     C
       |                        |
-------|                        |---------

On the 4/280 (running Sun OS 4.1) can I do the following:?
	ie0 (rest of world, Class B)
		inet 128.29.22.1 netmask 255.255.0.0 broadcast 128.29.255.255
	ie1 (my little corner, Class C?)
		inet 128.29.22.7 netmask 255.255.255.0 broadcast 128.29.22.255

If not, why?  Should I change the IP address on the class B side to
128.29.23.1  I'm told I should change the netmask to 255.255.255.0 on both
sides but I don't know why.  Then, what should my routing look like?  Why
is the following wrong?

Routing tables
Destination          Gateway              Flags    Refcnt Use  Interface
127.0.0.1            127.0.0.1            UH       1      26         lo0
default              Outside gateway      UG       lots   lots       ie1
129.83.0.0           128.29.22.1          U        0      100        ie1
128.29.22.0          128.29.22.7          U        27     19396      ie0
128.29.0.0           128.29.22.1          U        0      2          ie1

Any response besides comments on my compentency would be very appreciated.
	Thanks in advance
						John Morris
PS.  I'm  jmorris at mwunix.mitre.org (if I need to add that).