Re: plip with mulinux

From: Sven Conrad (
Date: Mon Sep 04 2000 - 20:23:19 CEST

Michele Andreoli wrote:
> On Sat, Sep 02, 2000 at 07:30:19PM +0200, Sven Conrad nicely wrote:

> > No good Idea, because netmask/interface is setup stuff!
> > Netmask/gateway is routing. This is totaly different! Don't
> > mix this up. The easy going one interface/network stuff,
> > netmask is in both cases identical, but only in this special
> > case.
> Ok. This is what was intended.
I can't believe this. A box with more than one interface make's
only in rare situations sense under this special conditions.

Ok, let's find a starting point. What means Netmask for a
interface? Not much. It is only for one purpose: Broadcasting.
If someone addresses the bradcast address, the physical layer
of the underlaying network must take care, that everyone with
a physical connection to this bus receives the packet.
Example: Ethernet normaly addresses the packet for a specific
mac address, all adapers with a different address don't touch
this paket. But there is a special mac address, which is
received on all adapters (If I remember correct, all ones).
Ok, but what is a netmask good for:
let's say, the destionation address is! Is this
a boradcast address? Yes if the network is
(= netmask, no if it is
(= netmask For all Bitnumbers less than
24 it is allso sure no Broadcast, for all nubers higher than
24 is it (if the address matches the network address) allso.

Netmask for interfaces have noting to do with routing. Often
seen, but not recommended: This netmask can be fauly and
the interface is still working! The reason is, programms mostly
build the broadcast address with kernel infos. When the resulting
address matches the interface and is recogniced as Broacast, all
is working.

Is this understandable? sorry I'am an Engineer and so no good
in teaching.

> >
> > Nope. Gateway routes can only address ONE interface per route. You can
> > have as
> > manny routes per inferface as you like.
> So, routing is a per-interface setting? I've to put them in, where
> x: plip ... eth?

Yes. Every route addresses one and exact one interface. But again, there
can be more than one route for a interface. Can be seen under windows.
you look at the output of "route print", M$ set a /32 route to the
and other obscure masks for the network.

> The problem is how to centralize the helps screen about recurrent
> network parameters as NETWORK, etc. Only a shell scripting problem.

As far as I see this discussion, there are not too manny common helps,
the default is allways different. Centralized helps are good for
and maintanace?
> Yes, it is true: pppd setup itself a route, when link is make up, but only
> if "defaultroute" parameter is on. Not a true problem, because the net
> effect is the same.
Ok. I don't got it, but I trust you. I have done only very limited
on ppp.
BTW: I have a new pppd and wvdial for kernel 2.0.x and libc5. This
is neccessary for newer german ISP. If somebody is interested, as me
for this.
> > Only for bus(eth) interfaces. Neither plip nor ppp have a netmask or
> > broadcast.
> But, why, why mon dieu? I can't route a network over plip, caused by
> the point-to-point specification?

Because plip allways addresses the interface on the other side. These
both are all the members on this "bus", so it is allso allways a
broadcast. ie. the netmask is allways and cannot
be changed. See the above explanation (which is more directed to

> >
> > Try to change your standpoint. There is nothing like a link between
> > interfaces.
> >
> It is strange, but I can better understand "ipfwadm", compared to "route".
> Can I imagine the "route" as a function like
> interface=route(ip,netmask)
> or this scenario is intrinsecally wrong?

looks good.
route is for setup "roules" with the question to which interface a
should be delivered in mind.
So there is a sorted rouing list, which is seeked linear from top to
and the test function looks like:

        if ((ip & netmask) == network) return interface;

(caution: Microsoft sort order is bottom up)

But take care of gateway entrys! A gateway is a kind of tunneling. You
a machine a packet for delivery, which seemed to know better, what to do
with it.

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