atcomsystems.ca/forum Forums VOIP Phones & IP PBX Networking IP Addressing and Subnets.

You are starting to confuse represented data, like numbers and words and symbols that you and I read, for what a "mask" is.

To a computer, a "mask" is is a binary string to represent a pattern or grouping. They can also be called a bit-mask.

What you are thinking of is a "character set". What a character set does is define a translation path from binary strings to human-readable symbols like the words you are reading right now. For instance, in the ASCII character set (the standard for most lower-level systems), the value of 236 is represented as "001100100011001100110110". Morse code is also a good example of a binary character set.

They can also go by "Character Encoding", "Character Mapping", "Code Page", etc.

Hope that helps make sense.

Ohh, and as far as calculators go, I like to make use of this one when I feel lazy: https://www.subnet-calculator.com/cidr.php

Just to add to his point, any number can be converted to binary. Only a few numbers can represet a "mask". As Kumba pointed out, the binary value of 236 doesn't make a good mask, because it is full of holes.

The Class B mask
255.255.0.0 binary is:
11111111.11111111.00000000.00000000
<--------Network--|--Hosts---------->
Binary 1's represent the network
Binary 0's represent the hosts.

So is the 1st 2 Octets match, then they are on the same network. Within that network the HOSTS IP must be different.

On more example
255.255.240.0
11111111.11111111.11110000.00000000
1's and 0's are still together. No holes. A few more 0's in the subnet masks mean we have more IP addresses we can use for this network. The math gets a little fuzzier for mortals that don't work with binary every day, but for devices on the same network, the numbers in the Network part of the mask must be the same, and the Hosts must be different.

That is why only a few numbers can be used in the mask.

As Tito1411 said earlier in this thread:
"This is a very complex subject that I could spend days writing about"

"ahh...so the subnet does dictate what your starting IP address is. So you just cant throw in any range of addresses in the last octet. Rather in this case, since I have changed the value of the 3rd octet, I have to start at 192.168.0.1 and go up from there"

Just reread that. You dont HAVE to start at a number. I don't like 192.168.0.X because it is too common. Kumba mentioned this, the subnet is a pie slicer. Each slice is a fixed size depending on subnet mask. It has a fixed starting address and a fixed ending based on the mask. You could pick any IP within that range.

192.168.0.1/24 NOT on same network as 192.168.1.1/24
They will not even try to talk to each other directly.

but change subnet mask to 255.255.254.0:
192.168.0.1 CAN talk directly to
192.168.1.1
without a router because the PCs see each other as on the same network.

Am I overdoing it now? Sorry if I am clearifying something you already understand.

A subnet mask is a series of binary 1's followed by a series of binary 0's. The total number of binary digits in a subnet mask is 32. All the 1's must be on the left and all the zero's must be on the right. Here are all possible subnet masks:

00000000000000000000000000000000
10000000000000000000000000000000
11000000000000000000000000000000
11100000000000000000000000000000
11110000000000000000000000000000
11111000000000000000000000000000
11111100000000000000000000000000
11111110000000000000000000000000
11111111000000000000000000000000
11111111100000000000000000000000
11111111110000000000000000000000
11111111111000000000000000000000
11111111111100000000000000000000
11111111111110000000000000000000
11111111111111000000000000000000
11111111111111100000000000000000
11111111111111110000000000000000
11111111111111111100000000000000
11111111111111111110000000000000
11111111111111111111000000000000
11111111111111111111100000000000
11111111111111111111110000000000
11111111111111111111111000000000
11111111111111111111111100000000
11111111111111111111111110000000
11111111111111111111111111000000
11111111111111111111111111100000
11111111111111111111111111110000
11111111111111111111111111111000
11111111111111111111111111111100
11111111111111111111111111111110
11111111111111111111111111111111

an easier way to write the subnet mask is to just state the number of 1's it has. Using this notation, here are all possible subnet masks:

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32

Write these down so you don't forget them.

An IP addresss can be followed by a slash and the number of 1's in the subnet mask. For example:

192.168.1.1/24 means an ip address of 192.168.1.1 with a subnet mask of 11111111111111111111111100000000.

Some people like to divide that string of ones and zeros up into four equal parts like this:

11111111.11111111.11111111.00000000

The dots here are not decimal points. They are just convenient dividing points in one big binary number.

Some people like to convert these four smaller parts of the number into decimal. So then they get:

255.255.255.0

because 11111111(binary) = 255(decimal)

But the subnet mask is not four numbers, it is still one big number.

The formula used to figure out the number of hosts within a subnet is:

h = m^2 - 2
where
h is the number of hosts (usuable addresses)
m is the number of bits used for the mask
the -2 is there because 2 addresses are not usable the first address of any network is the network number and the last address is the broadcast address.

Using the example 192.168.0.0 255.255.255.0
this can also be written as 192.168.0.0/24 where 24 is the number of bits that identifies the network (all the 1s)
m=8 because the mask uses 8 bits 11111111.11111111.11111111.00000000
h = 2^m - 2 becomes
h = 2^8 - 2 = 256 - 2 = 254
There would be 254 usable addresses with 192.168.0.0 reserved for the network address
192.168.0.1 - 192.168.0.254 for hosts and 192.168.0.255 reserved as the broadcast address.

192.168.0.0 255.255.254.0 or 192.168.0.0/23
11111111.11111111.11111110.00000000
m=9 so
h = 2^9 - 2 = 512 - 2 = 510
There would be 510 usable addresses with 192.168.0.0 reserved for the network address
192.168.0.1 - 192.168.1.254 for hosts and 192.168.1.255 reserved as the broadcast address.
Note that with this mask 192.168.0.255 and 192.168.1.0 are usable addresses.

Thanks again guys. While it might take me a long time to be an expert at this. All the information you guys have provided gives me enough information to at least understand the basic rules and principles of IP addresses and subnets. I also downloaded a couple of subnet calcs for my iphone and ipad.

I believe we might be able to put this topic to bed!!!

Thanks again guys!!

...
31
32

Other than for illustrative purposes you aint going to do much communicating on a /31 or /32 subnet.

And once you strip out the 0 and broadcast address, a /30 subnet really has room for only 2 devices!

For most regular business' you will usually get a /29 or /28. Only residential really gets anything smaller.

write the ip address in binary digits. Then underneath it write the subnet mask in binary digits too. Using 192.168.0.1 as an example:

11000000.10101000.00000000.00000001 =ip address
11111111.11111111.11111111.00000000 =subnet mask

now you can see that the binary 1's in the subnet mask are underneath the part of the ip address that is the number of the network. and the binary 0's are under the part of the ip address that is the number of the host.

So this is the network number: 11000000.10101000.00000000

and this is the host number:
00000001


That's all there is to it...

LOL, that's the short and sweet version alright. Only problem is that the people who can make sense of it in that form aren't the ones who need help figuring out what it means.

At least in IPv6 subnetting will almost never change for most end users considering everyone pretty much gets a /64 (LOL!).