The subnet value - that is the portion of the address that is reserved
for the subnet - cannot be all ones or all zeroes. i.e
net address 135.148. 0.0
default netmask 255.255. 0.0 (class B)
subnet mask 0. 0.255.0
-------------
netmask 255.255.255.0
The third byte cannot be 'all ones' or 'all zeroes'(referring to the
binary subnet value - in this case whats in the 3rd byte).
255 (decimal) = 1111 1111 (binary), and obviously
0 (decimal) = 0000 0000 (binary)
The subnets 135.148.255.0 and 135.148.0.0 cannot be used. If you have a cisco
router, you may use the 135.148.0.0 subnet, if you set the 'subnet zero'
parameter - see the manual - but note that other routers on the network may
not forward the traffic to 135.148.0.0 if they have a subnet mask of
255.255.255.0 set for that net. The cause of the problem is RFC950,
(subnetting ip networks) which was written in the days when net addresses
were plentiful.
If you have a class C address - say 198.143.35.0, and you subnet with a
netmask of 255.255.255.192 (0xff.ff.ff.c0), you theoretically have the
following subnets available:
198.143.35.0
198.143.35.64
198.143.35.128
198.143.35.192
Without the 'subnet zero' feature, subnets 198.143.35.0 and 198.143.35.192
cannot be used. You just lost 50% of the address space. Another feature of IP!
There are other routers with the 'subnet zero' feature available, but I
don't have a list available.
Hope this helps.
Mark
Marol Consulting
On
Wed, 11 Oct 1995, Yannick Gravel wrote:
> Hi Net&Sys Security poeples,
>
> Something that everybody is talking about, but not everybody
> is saying the same thing about subnetting:
>
> Yes, everybody agree that we lose the first and last host of
> each subnet for net.iding and broadcasting.
>
> But, some are saying that I can use all subnet; but others are
> saying that we lose the first and last subnet...
>
> Whom truth is true..
>
> Thanks..
>
> Yannick Gravel
> System administrator -- yannick .
gravel @
planet-int .
net
>
Follow-Ups:
References:
|
|
