Subnet zero help please?

Subnet zero help please?
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Subnet zero help please? gregg johnstone 08-07-2006
Posted by BernieM on August 12, 2006, 3:04 am
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> 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?

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

> Am I correct in thinking 192.168.36.10 ,200.100.16.3 hasnt been subnetted
> ?

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.

>and 131.64.16.133 is the 1st usable ?

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

> BernieM wrote:
>> >
>> >> Just tried a few test questions-here was the subnet zero question
>> >> 131.64.16.133 mask /25
>> >> 192.168.36.10 mask /24
>> >> 156.100.11.37 mask /20
>> >> 200.100.16.3 /24
>> >>
>> >> Thanks alot
>> >>
>> >
>> > 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
>



Posted by gregg johnstone on August 12, 2006, 11:47 am
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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:
> > 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?
>
> 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
>
> > Am I correct in thinking 192.168.36.10 ,200.100.16.3 hasnt been subnetted
> > ?
>
> 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.
>
> >and 131.64.16.133 is the 1st usable ?
>
> 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
>
> > BernieM wrote:
> >> >
> >> >> Just tried a few test questions-here was the subnet zero question
> >> >> 131.64.16.133 mask /25
> >> >> 192.168.36.10 mask /24
> >> >> 156.100.11.37 mask /20
> >> >> 200.100.16.3 /24
> >> >>
> >> >> Thanks alot
> >> >>
> >> >
> >> > 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
> >


Posted by BernieM on August 12, 2006, 3:31 pm
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> 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?

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



Posted by Paul Matthews on August 12, 2006, 8:08 am
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gregg johnstone wrote:

>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?

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.

Posted by on August 12, 2006, 11:56 pm
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Paul Matthews wrote:
> gregg johnstone wrote:
>
> >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?
>
> 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.
good


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