Subnet zero help please?

Where is the question, don't see it....

The Dude

I think it's "which is an example of a subnet zero".  The answer being
"156.100.11.37 mask /20".

BernieM

Sorry,my fault-that is the correct answer -is it because .11 is the
first subnet? I always thought you werent allowed to use that subnet?
Am I correct in thinking 192.168.36.10 ,200.100.16.3 hasnt been
subnetted and 131.64.16.133 is the 1st usable ?
TIA
BernieM wrote:

here is Ciscos take on it
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml#subnetzero


gregg johnstone wrote:

I'm not sure what you mean.   ".11.37" is a host addr.  The actual subnet is
"156.100.0.0"

Host addr's are 156.100.0.1 through to 156.100.15.254
Broadcast addr is 156.100.15.255
Next subnet is 156.100.16.0



Yes.

Traditional 'classful' networks masks were:

'A-class' (8-bit mask) 1 through to 127
'B-class' (16-bit mask) 128 through to 191
'C-class' (24-bit mask) 192 through to 223

192.168.36.10 and 200.100.16.3 are both using their traditional 24-bit mask
so haven't been subnetted.



1st usable what?  The actual subnet is "131.64.16.128"

Host addr's are 131.64.16.129 through to 131.64.16.254
Broadcast addr is 131.64.16.255
Next subnet is 131.64.17.0

BernieM

My fault
1st usable what?  The actual subnet is "131.64.16.128"
I should have wrote it is in the first usable subnet- so far I have
been taught that the "1st and last" subnet are not usable-this is even
on Ciscos site when you play the subnetting slingshot game they
have-how many subnets-how many usable?
The subnet zero is something new to me - thanks for the help.

btw In  my first semester I was told that the first usable host should
always be the default gateway-then the next semester I  was told it
doesnt matter- is it a case of viva la difference?
BernieM wrote:

In reality it doesn't matter at all what the default gateway is assigned.
You could assign the middle host address.  It comes down to manageability
and best practice to avoid confusion but achieve consistency.

 I've found the most common practice is to assign the last host as the
gateway.  Quite often this might be a HSRP virtual address with the 2nd last
and 3rd last addresses assigned as real addresses to the primary and
secondary gateways.

BernieM

gregg johnstone wrote:



Real World: There is nothing *wrong* about using subnet zero, it will work just
fine BUT you need to be careful about always specifying masks as otherwise it
can get confusing.

As an example, you have /16 network 111.11.0.0 allocated to you.

You design your addressing scheme such that the first two bits available
indicate the region (you have four) then the next four indicate which building
and the last two in this octet indicate the floor.

So that means 111.11.0.0 /16 is the whole network. 111.11.0.0 /18 is the first
region, 111.11.0.0 /22 is the first building and 111.11.0.0 /24 the firat floor
within it.

That will work, but if you are not careful it can get VERY confusing, so if you
can it is good to avoid subnet zero.
--
Paul Matthews                                         CCIE #4063
Please post questions to the NG, NOT by e-mail.

Paul Matthews wrote:

good

thx for the advice -can you explain what you have written please?

 I've found the most common practice is to assign the last host as the
gateway.  Quite often this might be a HSRP virtual address with the 2nd
last
and 3rd last addresses assigned as real addresses to the primary and
secondary gateways.

esp. HSRP- I am only on Sem 3 thx
jameshanley39@yahoo.co.uk wrote:

gregg johnstone wrote:



Within a subnet there are two common practices - first or last as DG.
Personally I prefer first, but that is just a preference.

What I like to do is .1 - HSRP, .2 first real .3 secong real. If the network is
a transit network with firewalls, the convention we are settling on is firewall
vrrp last, then the previous two for real firewalls.
--
Paul Matthews                                         CCIE #4063
Please post questions to the NG, NOT by e-mail.

Paul Matthews wrote:

thanks

On Mon, 14 Aug 2006 13:16:07 +0100, Paul Matthews wrote:



At least take prime number as host address for 0/0 :-)

FW

Greg,

Post always on the bottom, buddy. Unless, you are really angry or mad :)

The Dude

The Dude wrote:


Will do :)