Home Forums Courses CompTIA Network+ Course Subnet mask

This topic contains 3 replies, has 4 voices, and was last updated by  dedeij 2 years, 1 month ago.

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• #89765

mfugale
Participant

Hi!

I really enjoy the class.(COMPTIA Network+)

I haven’t got you well about subnet mask. For example the range for class A is 1.0.0.0-126.0.0.0, now why is the subnet mask indicated 255, the highest number while the range is far below from it.

#89806

matthewbgage
Participant

I’m still learning all this too, but I’ll see if I can help!

The subnet mask designates the division of the IP address into the network portion and host portion. So, a subnet mask of 255.255.0.0 applied to an IP address of 10.1.2.3 tells us that the network is 10.1 and the host is 2.3

To appreciate the subnet mask, you need to see it in binary. 255.255.0.0 translates to 11111111.11111111.00000000.0000000 in binary. Where the 1s change to 0s is the division between network and host divisions. Maybe it will be clearer like this:

``````IP       10.1.2.3 = 00001010.00000001.00000010.00000011
Network       ^    Host ``````

So, why 255? Because that is the value of the binary 11111111 (128+64+32+16+8+4+2+1). That has nothing to do with the range, but is the binary value of the mask.

This gets more interesting when you get into subnets. Some examples:

``````255.255.255.0   = 11111111.11111111.11111111.00000000
255.255.255.128 = 11111111.11111111.11111111.10000000
255.255.255.192 = 11111111.11111111.11111111.11000000
255.255.255.224 = 11111111.11111111.11111111.11100000
255.255.255.240 = 11111111.11111111.11111111.11110000
255.255.255.248 = 11111111.11111111.11111111.11111000
255.255.255.252 = 11111111.11111111.11111111.11111100``````
• This reply was modified 2 years, 1 month ago by  matthewbgage.
#89813

Paul Rouk
Participant

You need to look at the definitions of class A/B/C networks to understand what is happening here. The simplest answer is simply that the class A network subnet mask is defined that way. Certain things will always be true for classful networks. By definition, a class A network will always begin with 0 as the first binary digit (0xxxxxxx) and will have 255.0.0.0 as the subnet mask.

You can also play around with an IP subnet calculator to get a better feel for how this works. There are a lot of these available on the Internet, but you have to be careful since some of them give bad results. The one at the link below seems to work well.

#89856

dedeij
Participant

There are 2 different things here…related yes. But not the same.

First is the ranges. Class A, means that the very FIRST bit in the entire address is set to 0.

there is a good guide here, I suggest looking at this,
specifically “Determining Address Class From the First Octet Bit Pattern”

If you still have questions after that, I’d be glad to help clear anything up.

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