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
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