Hi there!
I have a little problem in our dorm network. We have over 400 appartments in
this building. Since we started our network with only a few computers our
university gave us only one class C IP set for the beginning. Now we have over
250 users and so we got a second set. Our two sets look like this:
first set -> 50.xx
second set -> 34.xx
At first we were disappointed because the sets are split. But later one of our
sysadmins noticed that these sets can be united in one network, since the sets
have a difference of exactly 16=2**4. You just have to switch the fourth bit
of the third octett to toggle between the two IP sets. So he tried out to set
the network
134.130.34.0 netmask 255.255.239.0
on vmware, which worked fine! The same works on MacOS too (he complains, but
who cares :-) ).
Now we thought it wouldn't be a problem to set a unique network on all
computers and our dorm server, but Linux is refusing this netmask. After some
search in the kernel source (2.2.9) I found this in a header file;
include/linux/inetdevice.h line 90:
extern __inline__ int bad_mask(u32 mask, u32 addr)
{
if (addr & (mask = ~mask))
return 1;
mask = ntohl(mask);
if (mask & (mask+1))
/* ^^^^^^^^^^^^^^^^^ */
return 1;
return 0;
}
Who said that a netmask must begin with a closed set of "1"-bits!?! How shall
we set our network in Linux? Indeed, we can set a second route, but the nice
solution is the interrupted netmask. Arpwatch complains all the time when he
finds packets on the bus that don't fit into the device's network too, though a
second route is set that includes this network. We would appreciate it, if this
condition is removed from the kernel again, or at least if one can switch it
off in the kernel configuration. This "if condition" makes it even senseless to
save the netmask in a 32-bit word; 5 bit indicating how many "1" bits are
set were enough too!
If this netmask corrupts the Linux system, than why?
Carsten Jacobi
--
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Carsten Jacobi
Kastanienweg 6, App.1317
52074 Aachen
Tel.: +49 241 878117
Fax : +49 241 878189
ISDN: +49 241 878190
E-Mail: [EMAIL PROTECTED]
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