Subnet Masks

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Right here we go! I got this covered just want to make sure im doing it correctly and i didnt come to the answer by fluke.

Im given an address lets say:

10010010 01101011 00100111 10001000

and im told to convert that to dotted decimal and determine the subnet mask in CIDR notation so:

that comes out to 146.107.39.136 which is a class B address whose subnet mask is 255.255.0.0 so the final answer comes out to

146.107.39.136 /16

Now the question im asking is I found that /16 because i figured that the address i have is class B and i know the class B subnet mask so 16 is the answer! Is that the only way of working it out? I am correct btw!

Just confirm me while i hit lesson 3 on Chapter 2..... Subnetting and Supernetting of IP Networks!

 

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In the Microsoft world, where, even if they mention CIDR they really haven't come to terms with it, then yes - that is right.

In the real world the correct answer is that there isn't enough info to determine the mask, as 'Classes' don't exist any longer!
(CIDR = Classless Inter-Domain Routing - i.e. no classes)

EDIT: I suppose the getout here is the phrase

. This means they can ask for it in modern terms while using the mechanisms of last century!

Harry.

 

I don't know Zimbo. It's pretty difficult to say what the network address is in CIDR notation as there is no subnet mask. Network classes A, B, C, and D do not exist by definition in CIDR (Classless Inter Domain Routing).

 

so im right in how i got to /16?

 

Not with CIDR you're not!

You have determined that it is a /16 address using classfull methods, not classless.

With CIDR you could use the 146.107.39.136 address with a /24 address if you liked.

Confused? You will be!

 

the answer is /16 so how would you work it out? using just the binary?

 

First question - how do you know that the answer is /16?

As freddy and Harry have said it is difficult to determine the mask based purely on the IP address unless you have alreasy been told that it is /16 or 255.255.0.0.

In classfull addressing this is easy, in classless its extremely difficult without more info.

P.S. Boyce, If you're reading this I havent forgot about you!

 

Zim,

An IP address is worthless unless you have a mask with it. /16 means that the first sixteen bits belong to the network.

291 looks good!

Si

 

yeah, there seems to be some overlap/confusion when it comes to Classful and classless addressing. I bet the CCNA is a real belter!

Si

 

K i think let me try re-word what the question is asking:

1. Determine the dotted-decimal - we figured that one
2. Use CIDR notation to show the default subnet mask

all the info you are given is that binary

now i got the right answer BUT you guys are seriously confusing me!

 

Zim,

As far as i can see you haven't been given the mask.

You have been given the IP address in binary and the /16 means the first 16 bits are for network allocation.

 

and the way i worked it out was the address is class B which is subnet mask 255.255.0.0 OR /16

so the answer is 146.107.39.136 /16 i got that and the book had that...

 

like you said the 1st 16 bits is class B - 255.255.0.0

 

Zim,

As Harry pointed out, strictly speaking that address isn't in a class; it is classless, hence the /16

Si

 

so thats what im asking... if my method of finding the answer is wrong what is the right method using the given info?

 

Zimbo,

Take a look at my first post on this thread. There are no classes of networks in CIDR. It's that way by definition. Since CIDR has, by design, no classes, how can anyone say from an IP address alone what the subnet mask is?

What the question is asking you to do is take a binary IP address figure out what the dotted decimal equivalent is, figure out what the network address would be in the old classful networking scheme, and then put the CIDR equivalent at the end of the IP address. That's the only way possible to arrive at that answer. It's not possible using only CIDR notation.

Once again, look at the acronym, CIDR, and then what it stands for. Then you will start to understand what people are saying to you.

There are no default networks in CIDR other than the private address ranges that have been set aside. 146.107.39.136 could be in 146.107.39.136/9, or 146.107.39.136/28, or 146.107.39.136/21, or any other combination you can think of since CIDR doesn't start, by default, with /8, /16, and /24 when creating subnets.

 

Zim,

The purpose of classless addressing is for more scope.
I have attached a subnetting spreadsheet that the Transender technician sent me for N+.
Hope it helps. He pointed out to me that subnetting is one of those *specialist* areas.

Si

 

Great explanation, thanks Freddie

Si

 

so on that part im ok? right?

this is the part if you could explain please....


btw thanks to all of you guys... really hot here and got more confused!

 

Zim,

You could only refer it to *a class* if it were a classful address, which it isn't. When you see the / try and forget about A, B and C.

Not sure if you saw the list i attached; i double posted

Si