Wookey wrote:
> On Tue 28 Sep, Bob Chiodini wrote:
>
> > When I initially setup Linux and pppd to my ISP, they (my ISP) told me
> > that the netmas k would be 255.255.255.252. This mask allows a subnet
> > with two hosts, you and your ISP. I really do not understand a subnet
> > mask of 255.255.255.255, this would imply zero hos ts (invalid?).
> > Maybe someone else could enlighten me.
>
> Well, I'm not an expert, but I think you have it nearly right, (4, not 2)
> and Sam Gyepi:
>
> > A netmask of 255.255.255.252 does not mean that you and the ISP share a
> > subnet but rather that the associated IP address belongs to a network
> > with a maximum of 251 hosts.
>
> has this wrong.
>
> I can't find the doc right now, but the point is that the netmask is a
> _mask_. Sensible values are ones with all ones on the left and all zeros
> on the right. IPs deemed to be on the local net are ones that fit in the
> mpart of the field that is zero.
> So (considering just the last byte) we get this table:
> last byte number of
> of netmask IPs in local network
> 255 1
> 254 2
> 252 4
> 248 8
> 240 16
> 224 32
> 192 64
> 128 128
> 0 256
>
> Now I'm not exactly sure what happens at the 'edges' so it may be + or -
> 1, (ie maybe 2 local IPs for a 255 netmask?) but that's essentially it.
>
> Hopefully a genuine expert will be along shortly to give a full
> explanation.
>
> Wookey
The "wire network" annswer rang a bell! The 255.255.255.252 netwask would be
correct for connecting router to router over a dedicated line allowing finer
subnetting should one have multiple point to point circuits from one router
to many others, and not wanting to waste an entire class b or class c subnet
just to connect two routers over a deidicated line. Sorry about the
confusion. In my case the sunet mask on the ppp interface does not seem to
matter (to me) both the 255 and 252 netmasks work. I've got an email into my
ISP to see what their thoughts are. They are a Linux shop. Hopefully the
results will be forthcoming.
Bob...
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Bob Chiodini [EMAIL PROTECTED]
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