Subnetting touble

Cilia

Hi Everyone,

I just need reassurance I need somebody to tell me if I need 3 subnets from network 10.1.0.0/24 is the number of usable hosts I get 30.

Thank you very much

sprkymrk

If you need three subnets out of a class C network (as you infer with the /24), you would have to actually get 4 networks with 62 hosts each.

Cilia

Thanks sprkymrk,

Where do you get 4 subnets and 62 hosts from if we have a prefix 24 mask?

From my understanding 10.1.0.0/24 leaves me only last octet left to subnet, I don't know why I get confused with class A subnetting but I can subnet perfectly with class B and C.

So, if I take 3 bits out of 8 for subnetting I get 32-2=30 usable hosts, right?

darwinism

A /24 means you have a class C network and are using 8 bits for the host portion of the subnet mask. Also each octet is 8 bits.

binarysoul

If you need three subnets out of a class C network (as you infer with the /24), you would have to actually get 4 networks with 62 hosts each.

I'm not sure if I understand. If you need 3 subnets out of class C, then you need to borrow 3 bits (8-2=6 subnets). That leaves 5 bits for hosts, e.g. 32-2=30 usable. Any comments?

Cilia

Hi darwinism,

So I am right,right?all I need is to know if I am right and if not if you have time explain why I am wrong.
Thank you

Thank you binarysoul

Finally somebody that speaks my language,I get confused by looking at an A class address,but I believed that no matter what address you have you have to deal with the subnet mask,like in this case,class A address, class C subnet mask equals class C subnetting.

Bless

darwinism

10.1.0.1 to 10.1.0.62
10.1.0.65 to 10.1.0.126
10.1.0.129 to 10.1.0.190
10.1.0.193 to 10.1.0.254



62 hosts a piece.

sprkymrk

binarysoul wrote:

If you need three subnets out of a class C network (as you infer with the /24), you would have to actually get 4 networks with 62 hosts each.

I'm not sure if I understand. If you need 3 subnets out of class C, then you need to borrow 3 bits (8-2=6 subnets). That leaves 5 bits for hosts, e.g. 32-2=30 usable. Any comments?

You don't need to borrow 3 bits for 3 networks. Borrowing 2 bits will give you 4 networks (you don't subtract 2 here), leaving 6 bits for hosts per network, or 2 to the 6th power minus 2 = 62.
It's been wayyyy too long since I have subnetted for CCNA, so forgive me if the explanation is poor.

binarysoul

You don't need to borrow 3 bits for 3 networks. Borrowing 2 bits will give you 4 networks (you don't subtract 2 here),

sprkymrk,

That is possible, but all CCNA study materials I've seen always subtract 2 from total number of host and subnets. So I am not sure what makes you think "you don't subtract 2 here".

darwinism

Because you subtract them off the addresses not the bits. There is a big difference in bits and ip's.

8 bits = 255

Cilia

Ok, let's focus here,

I am asked to subnet 3 networks out 10.1.0.0/24, you are saying that I only need 2 bits and that I have 6 bits left for hosts so I get my 3 subnets plus 1 I don't want/need to use and 62 usable host IPs per subnet,if it's like this then I finally get it,but I am still adjusting to the news because I have to admit, until today I also had the idea that calculating subnets and hosts was down to the 2n-2 and I am not the only one.

Thank you guys

TheShadow

Cilia wrote:

Ok, let's focus here,

I am still adjusting to the news because I have to admit, until today I also had the idea that calculating subnets and hosts was down to the 2n-2 and I am not the only one.

Thank you guys

It really depends, questions on the exam will define it for you.
If you search Cisco for this document if it is still there; Cisco - Subnet Zero and the All-Ones Subnet
I can't quote it all without violating the fair use rules.

"Subnet-Zero
Use of subnet zero for addressing was discouraged because of the confusion inherent in having a network and
a subnet with indistinguishable addresses.
With reference to our example above, consider the IP address 172.16.1.10. Now, if you calculate the subnet
address corresponding to this IP address, the answer you would arrive at would be subnet 172.16.0.0 (subnet
zero). Note that this subnet address is identical to network address 172.16.0.0, which was subnetted in the first
place, so whenever you perform subnetting, you get a network and a subnet (subnet zero) with
indistinguishable addresses. This was formerly a source of great confusion.
Prior to Cisco IOS® Software Release 12.0, Cisco routers, by default, did not allow an IP address belonging
to subnet zero to be configured on an interface. However, if a network engineer working with Cisco IOS
version older than 12.0 finds it safe to use subnet zero, the ip subnet-zero command in the global
configuration mode can be used to overcome this restriction. As of Cisco IOS version 12.0, Cisco routers now
have ip subnet-zero enabled by default, but if the network engineer feels that it is unsafe to use subnet zero,
the no ip subnet-zero command can be used to restrict the use of subnet zero addresses.
In versions prior to Cisco IOS version 8.3, the service subnet-zero command was used.
[...]
The All-Ones Subnet
Use of the all-ones subnet for addressing has been discouraged in the past because of the confusion inherent
in having a network and a subnet with identical broadcast addresses.
With reference to our example above, the broadcast address for the last subnet (subnet 172.16.224.0/19) is
172.16.255.255, which is identical to the broadcast address of the network 172.16.0.0, which was subnetted in
the first place, so whenever you perform subnetting you get a network and a subnet (all-ones subnet) with
identical broadcast addresses. In other words, a network engineer could configure the address 172.16.230.1/19
on a router, but if that is done, he can no longer differentiate between a local subnet broadcast
(172.16.255.255 (/19)) and the complete Class B broadcast (172.16.255.255(/16))."

So before doing this you have to know what the rest of the infrastructure is doing. Windows really does not like some of this.

binarysoul

Subnetting can be a subjective topic, but with respect to CCNA, the n-2 rule is very clear: if you need to create 7 subnets out of a class C address for example, you MUST borrow 4 bits and not 3. If you do, you will get zero unless the question specifcially tells you igonre the n-2 rule

So long story short, I'm gonna stick with n-2 unless somebody provides evidence from Cisco-approved CCNA materials

TheShadow

That is the exact reason why I posted Cisco's response. Note in the quoted text is direct from Cisco and that as of IOS 12.0 subnet zero is on by default. That is why the questions will tell you what to assume. There are no tricks in the test but one needs to be aware of the possibilities.

If one is not aware of it then one can be confused by all the helper software that is available. Boson's version 1.2 subnet calculator assumes subnet zero is true, I think Johan beta makes it optional, several flash card sites offer two versions of the cards one with -2 and one without. I have not specific opinion on study material and am only suggesting that like all things in IT one should not cast their thoughts in stone.

Cheers

BubbaJ

darwinism wrote:

A /24 means you have a class C network and are using 8 bits for the host portion of the subnet mask. Also each octet is 8 bits.

No. The network started with a 0 in the first bit position so it is a Class A network. In this case, the /24 means that it is a subnet of a Class A network.

As I recall, it is important to know the differences between the classes for the CCNA exam:

Class A has a 0 in the first bit, /8 for a full network
Class B has 10 in the first two bits, /16 for a full network
Class C has 110 in the first three bits, /24 for a full network

mcockrell

BubbaJ wrote:

darwinism wrote:

A /24 means you have a class C network and are using 8 bits for the host portion of the subnet mask. Also each octet is 8 bits.

No. The network started with a 0 in the first bit position so it is a Class A network. In this case, the /24 means that it is a subnet of a Class A network.

As I recall, it is important to know the differences between the classes for the CCNA exam:

Class A has a 0 in the first bit, /8 for a full network
Class B has 10 in the first two bits, /16 for a full network
Class C has 110 in the first three bits, /24 for a full network

the class of network isnt really revelant to his question. yes, 10.x.x.x are considered class A nets, but /24 is considered a class C mask. the net class doesnt matter for this question. 99% of private networks that use 10.x.x.x that ive encountered use a /24 mask so if your planning on taking these studies to the real world, it would be advisable to get familiar with this type of scenario.

to answer the original question: the original poster is correct as far as cisco testing is concerned. ALWAYS ASSUME THERE IS NO ZERO DOT UNLESS EXPLICITLY TOLD TO USE ZERO DOT.

as a field engineer, nobody i know uses zero dot unless they are in between subnets, like the one posed here, and even then most wont use it.

hope this helps,
michael

BubbaJ

mcockrell wrote:

BubbaJ wrote:

darwinism wrote:

A /24 means you have a class C network and are using 8 bits for the host portion of the subnet mask. Also each octet is 8 bits.

No. The network started with a 0 in the first bit position so it is a Class A network. In this case, the /24 means that it is a subnet of a Class A network.

As I recall, it is important to know the differences between the classes for the CCNA exam:

Class A has a 0 in the first bit, /8 for a full network
Class B has 10 in the first two bits, /16 for a full network
Class C has 110 in the first three bits, /24 for a full network

the class of network isnt really revelant to his question. yes, 10.x.x.x are considered class A nets, but /24 is considered a class C mask. the net class doesnt matter for this question. 99% of private networks that use 10.x.x.x that ive encountered use a /24 mask so if your planning on taking these studies to the real world, it would be advisable to get familiar with this type of scenario.

to answer the original question: the original poster is correct as far as cisco testing is concerned. ALWAYS ASSUME THERE IS NO ZERO DOT UNLESS EXPLICITLY TOLD TO USE ZERO DOT.

as a field engineer, nobody i know uses zero dot unless they are in between subnets, like the one posed here, and even then most wont use it.

hope this helps,
michael

I was correcting darwinism who gave the original poster some bad information. Cisco will not be forgiving on the CCNA exam if you call it a Class C network when it is really a subnet of a Class A or B network.

My experience in the real world is the opposite of yours. I see subnet zero used all the time. I also hear people tell me that it can't be used, and I'm asked for a Class C network when using 10.x.x.x. We also use the appropriate sized subnets of 10.x.x.x since it has such limited address space for the size comapny that we are, and we encourage people to use classless notation when referring to subnets. It is much more difficult to right-size a subnet after the fact, but some software and hardware vendors only know how to deal with /24 networks even when our requirements are for a /27, /28, or /29. IBM, in particular, seems to always get it wrong while configuring their HA servers on our networks.

It's strange what people get into their heads and won't let go. The RFC allowing the use of subnet zero is now over 11 years old.

unwritt3n

ok... so what is the final descision??

do we do a 2^n-2 on both the network and host bits??

Or is it just a:

2^n for bits borrowed

&

2^n-2 for hosts

binarysoul

Here is an interesting question:

Which of the following could be a unicast?

A. 10.3.1.27/30
B. 255.255.255.255
C. 172.16.128.255/18
D. 192.168.10.48/29

The answer is C. Does anybody explain it in a simple way?

EdTheLad

A. 10.3.1.27/30
/30 x.x.x.252 subnets 0,4,8,12 ...28 i.e 27 broadcast address

B. 255.255.255.255
I dont think i have to comment on this address

C. 172.16.128.255/18
/18 x.x.192.0 subnets 64,128,192 in third byte,
host range 128.1 -> 191.254

D. 192.168.10.48/29
/29 x.x.x.248 subnets 0,8,16,32,48
192.168.10.48 is a subnet.

The answer is C.

jelexy

what do you mea by Unicast?

from What I understand 172.16.128.255 is a broadcast address for the 172.16.128.0 subnetwork


if what you meant was

which one of the following is abroadcast then answer C is correct

BubbaJ

jelexy wrote:

what do you mea by Unicast?

from What I understand 172.16.128.255 is a broadcast address for the 172.16.128.0 subnetwork


if what you meant was

which one of the following is abroadcast then answer C is correct

No. You would be correct if the subnet mask were a /24, but it is a /18. There will be 64 addresses that have .255 in the last octet for that subnet, and only the last one is the broadcast address for the subnet.