> I remember simething flew past a few weeks ago on the topic of using subnet
> masks other than 16 or 24 bit. If anyone out there has the summary I would
> be grateful if they would send me it. We are contemplating this and it always
> helps to know the traps in advance.
I received 3 responses, one said:
From: Todd Pfaff <email@example.com>
I've tried using a netmask of 4 bits on our subnet, but anything other than
a multiple of 8 seems to be coerced to the 8 bit boundary.
From: Tom Conroy <firstname.lastname@example.org>
Netmasks are not required to exist on byte boundaries.
In order to have 10 bits of host address space, we are currently using
a netmask of 0xfffffc00 (255.255.252.0) on our class B network. We are
also subnetting a few class C networks for router testing, but I will
save that story for another time ...
In a class B network "aaa.bbb.0.0", this would give such subnets as
that composed of all addresses between aaa.bbb.16.0 - aaa.bbb.19.255
or aaa.bbb.64.0 - aaa.bbb.67.255 or aaa.bbb.68.0 - aaa.bbb.71.255.
On SunOS 4.0 and higher, the "all ones" broadcast address above must be
As far as pitfalls, I know of none - this is a really "normal"
thing to do.
Totally conflicting n'est pas? Then I got:
From: Steve Jay <email@example.com>
We are using a 25 bit netmask here (0xffffff80 or 255.255.255.128). We
have found a few bugs, including one nasty one in routed which prevented
things from working at all. We fixed routed by building a new version
from the BSD Tahoe source code, including a 1 line fix to the source to
make it handle netmasks correctly.
Routed also has a bug that causes it to mishandle network names that
translate into 25 bit network numbers with the low order bit set when
they appear in the /etc/gateways file, but that's a relatively little
used feature. You can get around it by specifying the network by number
rather than name in /etc/gateways.
"netstat -r" displays the number, rather than name, of a network with
the low order bit of the network number set, if the network number is
> 24 bits.
I think all of these problems occur only with netmasks > 24 bits. Most
of them happen because of the way the inet_*() routines work. They basically
don't work for network numbers > 24 bits.
After we fixed routed, our 25 bit netmask seems to be working ok on a
network with Sun's, SGI's, Convex, IBM MVS, IBM VM, IBM 370/AIX,
IBM RS6000, and PC-NFS.
I don't know of any problems if you use a 17 - 23 bit netmask.
It therefore appears that you get problems if you use a netmask greater that
24 bits and that this is a bug. Todd Pfaff's problem is unclear to me, but I
think it may be related to a problem which I have encountered myself relating
to the ifconfig command in the Sun rc.local file. This sometimes says:
ifconfig le0 netmask +
whereas it should say:
ifconfig le0 `hostname` netmask +
The former fails to set the broadcast in line with the new netmask.
As we were looking at a 20 bit netmask we ought to be ok. If anyone has any
more knowledge on this topic it would be very welcome.
This archive was generated by hypermail 2.1.2 : Fri Sep 28 2001 - 23:06:11 CDT