SUMMARY: subnet problem

From: Frank Lowe (
Date: Fri Jun 26 1992 - 01:51:48 CDT

Many thanks to the following:
 Larry Chin <> (Abdi M. Oday)
 Mike Raffety <>
 Colin Macleod <> (Paulo L. de Geus) (Ricardo K. Aguilera) Tue May 26 22:22:32 1992

> We have recently subnetted our local class C network (192.82.225) as follows:
  (drawing changed from original to reflect reality 8-)
                                       netmask ffffff80
                                          darwin-gw ie1
                                            darwin | <
                               | < subnet 128
                                      domain nameserver | <
                                             ie0 |
             internet======netblazer----------| |
                | |
> | hosts
                                   subnet 0 > |
> | all netmasks ffffff80
                                     all netmasks ffffff80

> ftp, telnet, rlogin, mount, etc between both local subnets is ok.
> telnet, mail, and ftp from subnet 0 to the internet are ok.
> ftp and telnet from subnet 128 to internet do not work.
> ping from subnet 128 to internet hostname results in "unknown host".

The problem was with the netblazer which does not have a netmask as such
but a parameter which specifies the number of bits in the address to
use for routing. Once this was changed to 25, everything worked fine,
and the subnet 128 machines could then get to the internet. The en0
parameter is just our ethernet backbone.

SFBRNB1:Top>List> route
Dest Len Interface Gateway Metric 25 en0 1 24 en0 1

In retrospect, the whole process now seems straight-forward.

   1. Change ip addresses in /etc/hosts file on each machine to
       reflect the new net topology.

   2. Change the netmasks file on each machine to set the new netmask.
       In our case:

             # network netmask

   3. On NIS master, remake the NIS data base:
             darwin: cd /var/yp
             darwin: make netmasks

   4. Boot all machines.

   5. Use netstat -rn to check the routing tables and
       ifconfig -a to check the network interfaces.

Relevant FM References:
   Sun System & Administration Manual (Networking & DNS chapters)


Frank Lowe
Southwest Foundation For Biomedical Research
San Antonio, Texas

fax: (512) 674-2126
voice: (512) 674-1410
internet: (

Does the fact that the airplane is faster than the horse make
the world any better?

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