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SubjectRE: howto disable auto route setup?
Date
No.  255.255.254.0 is not legal for a Class C network (194.111.0) as a
subnet mask on an interface. You could have that "mask" in a routing
protocol that can use summaries or CIDR, such as OSPF, but you should not be
able to configure that on an interface. In effect, you can't have a
"reverse" subnet mask, it has to be at least as significant as the network
itself. An interface that is using an IP from a Class C network must have
at least a 255.255.255.0 mask. The resulting broadcast address that you
mention is also illegal.

If you were using a Class A or Class B network (1 - 191.255) a subnet mask
of 255.255.254.0 is perfectly fine. So, if you were using 190.111 instead
of 194.111.0 you could have a subnet mask of 255.255.254.0 with a broadcast
address of 190.111.1.255. Your first subnet would be 190.111.2.0/23 unless
you use the not-really-legal-but-supported-by-most-router-vendors option of
zero subnet masks
(and-not-really-advisable-unless-you-know-what-you-are-doing). Your last
subnet would be 190.111.252.0/23, as there is no "option" of
all-ones-subnet-mask from any vendor AFAIK and it would not make much sense
because, although you may be able to get away with using the all-zero's
subnet portion of the IP because broadcasts to "this" subnet are not used
often, you probably can't get away with all-one's because broadcast to "all"
subnets are used often.

Check out the RFC's if there is a question in the HOWTO's. They are
authoritative, while the HOWTOs are rewrites for the lay person that are not
necessarily authoritative.

Someone correct me if I'm wrong on any details.

What has this to do with the kernel?

Fred


> -----Original Message-----
> From: owner-linux-kernel@vger.rutgers.edu
> [mailto:owner-linux-kernel@vger.rutgers.edu]On Behalf Of Neil Conway
> Sent: Tuesday, February 02, 1999 8:53 AM
> To: Mike Jagdis
> Cc: Sam Mortimer; linux-kernel@vger.rutgers.edu
> Subject: Re: howto disable auto route setup?
>
> Vaguely on this topic, I've been wondering if it's
> legal/advisable/safe
> to have a netmask like 255.255.254.0 ?
>
> Anyone? The HOWTO is rather ambiguous about it. I think it's legal,
> but am not entirely sure. This also seems to imply a
> broadcast addr. of
> something like 194.111.1.255 for a network address of 194.111.0.0,
> presumably that's OK too?
>
> cheers
> Neil
>
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