25 Feb
2012
25 Feb
'12
3:49 p.m.
On Sat, 25 Feb 2012 11:10:56 +0200, "Marius Petrescu"
<marius(a)yo2loj.ro> wrote:
Hello,
It is one thing I don't really get about this rollover...
A subnet is supposed to be a broadcast domain, so every subnet, don't matter
how small needs a network and a broadcast address to work properly.
In the classic routing schemes that means that by subnetting e.g. a /24
subnet into 4 /22 subnets you loose theoretically 2 subnets, and can use
only x.x.x.64/22 and x.x.x./128/22.
And this means also that only subnetting in steps of 2 is possible (and also
/31 is not a valid subnetting scheme).
Classless approaches eliminate this and allow the use of all subnets created
by subnetting (and I think all modern routers and OSes support this).
But you still have to reserve (IP & Mask) for the network address and (IP |
(~mask)) for the broadcast address (e.g. 44.x.x.0 and 44.x.x.255 on a /24
subnet).
This means that the roll over on the subnet boundary would have to be e.g.
44.x.x.254 -> 44.x.x+1.1 or the routers on the subnet gateway may go havoc.
Correct me if I'm wrong...
Marius, YO2LOJ
You are correct. The broadcast addresses are 0 and 255 in each /24 and
x.x.s+1.255 in each /23 where s is subnet 2,4,6 etc. The point of the
scheme is to allocate subnets such that the LSB of the subnets are
allocated later, if at all, only when you run out of the upper bits.
Yes, the rollover actually occurs when you have assigned 44.x.x.254.
See my reply to Brian listing the /24 scheme.
-----Original Message-----
From: 44net-bounces+marius=yo2loj.ro(a)hamradio.ucsd.edu
[mailto:44net-bounces+marius=yo2loj.ro@hamradio.ucsd.edu] On Behalf Of Geoff
Joy
Sent: Saturday, February 25, 2012 08:29
To: AMPRNet working group
Subject: Re: [44net] On the Issue of Subnets
On Thu, 23 Feb 2012 17:56:42 -0800, Geoff Joy <geoff(a)windowmeister.com>
wrote:
By growing the subnets downward and the hosts
upward linearly, if a
subnet were to become overpopulated with hosts, it can expand upward
into another subnet:
2 00101100.00010010.00010000.11111111 44.18.16.0/24
rolls over into
x 00101100.00010010.00010001.00000000 44.18.33.0/24
Correction:
Subnet 16 rolls over into subnet 17.
2 00101100.00010010.00010000.11111111 44.18.16.0/24
34 00101100.00010010.00010001.00000000 44.18.17.0/24
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