How do I subnet a class B network

[Joe Smith]

In article <1990Sep4.232423.17162 at rice.edu> jmorris at mwunix.mitre.org (John R. Morris) writes:
>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.
>-------|                        |---------
>       |     ----------         |
>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.  

Purpose of netmask:  When a packet is destined for an IP address and the
local host has multiple IP interfaces, the netmask is used to determine
which interface to send the packet to.  

  foreach interface do
    network1 = destination_address AND interface_netmask
    network2 = interface_address AND interface_netmask
    if ( network1 equals network2 ) then
      send packet though this interface
      break out of loop
    endif
  endfor
  no match, go consult the routing tables.

With ie0 having a netmask of 255.255.0.0, then all addresses in the form
of 128.29.*.* will match, and nothing nothing will go out ie1.  Simply
setting setting ie1's host address to 128.29.23.1 won't help; that address
still matches 128.29.*.*.  

You will have to set the netmask on both interfaces to 255.255.255.0.
This will make addresses in the form 128.28.22.* go to ie0, addresses in
the form 128.28.23.* go to ie1, and all other addresses will be compared
against the routing tables.  This last part includes addresses valid on
your class B network (such as 128.28.19.*) as well as addresses outside
your network (such as 128.83.*.*).

The route daemon does its thing automatically when it detects more than
one IP interface; you don't have to do anything other than set the network
address and netmask up properly.

Joe Smith (408)922-6220 | SMTP: jms at tardis.tymnet.com or jms at gemini.tymnet.com
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