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Subnet Masks (HELP!)


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#1 josh9990

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Posted 06 March 2015 - 11:09 AM

Hi there, first of all I'm new to this forum so if this is in the wrong place, go easy on me.

Could anyone help me understand subnet masking, in the most simplistic way possible, its for my course and I'm very confused about it.

 

also, can anyone explain this to me in more detail? http://prntscr.com/6dfrn9

 

thanks in advanced.

 

 



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#2 CaveDweller2

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Posted 06 March 2015 - 11:28 AM

This is sorta like when a child asks a parent, "Where do babies come from?" And the parent jumps right in with the truth and the kid just wanted to know if a stork brought them.

 

So, I will ask you, what do you understand about subnetting and the use of subnet masks?


Hope this helps thumbup.gif

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#3 josh9990

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Posted 06 March 2015 - 11:29 AM

This is sorta like when a child asks a parent, "Where do babies come from?" And the parent jumps right in with the truth and the kid just wanted to know if a stork brought them.

 

So, I will ask you, what do you understand about subnetting and the use of subnet masks?

honestly have no idea about it, its something new ive got to learn. i look online but i cant find anything usefull for me, i just need someone to explain both in the simplest way



#4 CaveDweller2

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Posted 06 March 2015 - 11:57 AM

Ok in the simplest terms - a subnet mask is what determines what is the network portion and the host/node of an IP address. We'll keep them both simple. It can get a lot more complicated. We'll just use 2 numbers for the subnet mask 255 and 0. But it is not always like that in subnetting so don't let that sink in too deep.

 

Think of it as a street with houses. The street is the network. The houses are host/nodes,  what is given to devices.

 

IP address -     192.168.1.54

Subnet mask - 255.255.255.0

 

In this example - anywhere you see 255 is the network/street and anywhere you see 0 is the hosts/houses

 

So this is the 192.168.1.0 network/street and the 54 host/house.

 

Does that help? The picture you asked about, would use the subnet mask of 255.255.0.0. Does it make more sense now? Just always keep in mind that subnetting isn't always at the decimal point.


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#5 josh9990

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Posted 06 March 2015 - 12:04 PM

very useful information, thank you. 

so i take it everyone who's connected to the network, can access all the networks created by the subnet mask?



#6 CaveDweller2

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Posted 06 March 2015 - 12:16 PM

Yes if they are on that street and configured right then they can access it. But you said networks. The next street over 192.168.2.0 with the mask 255.255.255.0 can't access 192.168.1.0 with the mask 255.255.255.0 without a cross street or router(again we're keeping this simple)

 

Just remember it's not the mask alone that makes the network, it's called a mask for a reason it goes over an IP address and shows how to divide that address into network and hosts.


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#7 josh9990

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Posted 06 March 2015 - 12:17 PM

Thank you very much. you're a great help!



#8 CaveDweller2

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Posted 06 March 2015 - 12:20 PM

I try. I wish I could find a free copy of the CBT Nuggets explanation of subnetting. It is the absolute best I have ever found on how to subnet.


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#9 josh9990

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Posted 06 March 2015 - 12:32 PM

i just want to make sure if this is right, so from my understanding

you're ISP gives you a assigned ip, 

so, if i was assigned a ip like 

163.135.181.63

the subnet mask would be 255.255.255.0

so the street is 163.135.181

and the last (63) is the node to connect to the internet?


Edited by josh9990, 06 March 2015 - 12:36 PM.


#10 CaveDweller2

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Posted 06 March 2015 - 12:59 PM

That is correct.


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#11 ChaseWheeler

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Posted 06 March 2015 - 06:19 PM

I find Andrew Crouthamel's episode on IPv4 subnetting in his CCNA training course a great resource, it's maybe a little bit more information than you need, but it's very indepth and will help you understand subnetting! 

 

Here's the episode on youtube. 


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#12 CaveDweller2

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Posted 06 March 2015 - 08:44 PM

That isn't bad but his explanation on actual subnetting is a bit convoluted. He needs to take it out further and show how, when you are in the 3rd octet, the numbers fall over into the 4th octet. 


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#13 ChaseWheeler

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Posted 06 March 2015 - 10:15 PM

I remember him talking about that either at the end of this video or maybe it was in one of the next few videos in the series But it's a great point to make About expanding or collapsing the size of subnets.

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#14 CaveDweller2

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Posted 06 March 2015 - 10:43 PM

It's still a good video tho that was my only issue. But if he explained it later in his series then good. 

 

But it's when you're messing in the 3rd octet that subnetting can get a little confusing. That whole carrying over to the 4th is like Do what now? lol


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#15 Jake001

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Posted 10 March 2015 - 09:00 AM

One thing you will see when working with subnet mask is a slash followed by the
number of digits belonging to the the network ID.
For example,
Subnet mask: 255.255.255.0 will be /24
Or
Subnet mask:255.255.0.0 will be /16

So you may see something like this:
208.123.45.18/16

This means it has the subnet mask of 255.255.0.0

This becomes very helpful when you have a a subnet like 255.254.0.0 because the
second octet has part of the network ID and part of the host ID. So this will have a /15
which will show you that the first 15 digits belong to the network ID without you having to
do any decimal to binary converting

I hope I explained it good enough. Let me know if you want a further explanation on this .

Edited by Jake001, 10 March 2015 - 09:03 AM.





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