What is a Subnetmask?
When we write a Subnetmask in binary, we will see - no matter, which Subnetmask we use - that there are allways some "ones" followed by "zeros". For example
11111111.11111111.11111111.11100000 = 255.255.255.224
11111111.11111111.00000000.00000000 = 255.255.0.0
The number of the used "zeros" is a indicator, how many hosts can take place in that subnet, means how many IP-addresses are useable in that subnet for host-PCs.
If we take a look at the example network
192.168.10.0
255.255.255.0
We have here ONE network, with 254 useable IPs for Client-PCs.
If we write down the Subnetmask in binary, we will see why.
The Number 255 is in binary
11111111 = 128+64+32+16+8+4+2+1 = 255
The whole Subnetmask in the example is in binary
11111111.11111111.11111111.00000000
If we calculate all the 8 zeros as powers of 2, we will come to the result.
2^8=256
But - from this 256 IPs we must take one IP for the Broadcast-IP, and one for the Network-IP. So there are 254 IPs left, useable for Host-PCs in that Network.
This rule is also known as Formula
2^n-2
The IP Range of this network is
|
First IP 192.168.10.1 |
Last IP 192.168.10.255 |
So we can give PCs in that network IPs like
| 192.168.10.1 |
Host PC 1 |
| 192.168.10.2 |
Host PC 2 |
| 192.168.10.3 |
Host PC 3 |
and so on, till the end of the network is reached with 192.168.10.254.
If we want to divide this network in two parts, we need two things: a router and some switches - and of cause someone, who can calculate subnets.
With Subnetmask 255.255.255.128 we would divide the network in two parts.
192.168.10.1 - 192.168.10.127
192.168.10.128 - 192.168.10.254
The Subnetmask defines how big the subnet is. That means - how many Client-PCs will have place in that subnetwork.
A Subnetmask of 255.255.255.0 means in binary
11111111.11111111.11111111.0
So, what do we see?
4 Blocks, divided with a ".". Each of these blocks is also called "octett". Because - each Block has 8 bits.
To be able to do subnet-calculation, we first must understand binary calculation.
Lets take a deeper view at the first octett.
11111111
The first "1" stands for a 128.
The second "1" stands for a 64.
The third "1" stands for a 32.
The fourth "1" stands for a 16.
The fifth "1" stands for a 8.
And so on. That means:
11111111=255=128+64+32+16+8+4+2+1
11110000=240=128+64+32+16+0+0+0+0
11100000=224=128+64+32+0+0+0+0+0
If we see something like "/24", that means that 24 bits are set to "1", from the left side.
Examples:
/16 = 255.255.0.0 = 11111111.11111111.00000000.00000000
/20 = 255.255.240.0 = 11111111.11111111.11110000
Calculation of Subnetmask big enough for a specified number of Hosts
Question: "How to create a subnet with minimum 10 host IPs"
1. Step
calculate a power of two, that is minimum 10
2^3=8. That is not enough
2^4=16 That is higher than 10. Good.
2. Step
Now put the LAST 4 Bits of your subnetmask to 0.
11111111.11111111.11111111.11110000
That is in decimal
255.255.255.240
With THIS Subnetmask, you have minimum 10 Host-Ips for each Subnet, without wasting to much IP-Addresses.
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Other example
"Create a subnet with minimum 70 Host-IPs"
1. Step
Calculate a Power of 2 that is MINIMUM 70
2^6=64. Not enough.
2^7=128. Thats higher than 70. Good.
2. Step
Put the LAST 7 Bits of your Subnetmask to 0.
11111111.11111111.11111111.10000000
That is in decimal
255.255.255.128
You have a Subnet, with more than 70 Host-IPs.
Calculation what is the Broadcast-IP of a Subnet
"There is subnet 172.16.64.0/20. What is the BROADCAST ADDRESS of that Subnet?"
1. Step
/20 meens 255.255.240.0
2. Step
Now analyze the Subnet Oktett to find out the "network-jumps" or "increments".
240 means 11110000
The LAST of the 1s is under decimal 16. That are our "network jumps"
(128/64/32/16/8/4/2/1)
3. Step
Write down the network-jumps
172.16.64.0 - 172.16.79.255
+16 172.16.80.0 - 172.16.95.255
+16 172.16.96.0 - 172.16.111.255
+16 172.16.112.0 - 172.16.127.255
Because the NEXT Subnet in the example is 172.16.80.0, the broadcast must be 172.16.79.255, cause THAT is the IP BEFORE the next Subnet starts = the BroadcastAddress.
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Other example of Broadcast-IP calculation:
If it would be 172.16.64.0 /26
Same procedure
/26 means 255.255.255.192
192 is binary 11000000
The LAST 1 stands under the 64.
That are in that example our "net-jumps".
172.16.64.0 - 172.16.64.63 <<<in this example THIS is the broadcastaddress of first subnet
172.16.64.64 - 172.16.64.127
172.16.64.128 - 172.16.64.191
Calculating first and last possible IP for a Host
Question: "You have Network 192.168.20.32 /27
The very first IP is reserved for Default Gateway!
What is the first and last valid IP for a Host-PC?"
1.Step
/27 is 255.255.255.224
2.Step
224 means 11100000
The LAST 1 is under the 32. That are our "network-jumps" in this example
(128/64/32/16/8/4/2/1)
Valid IPs in that subnet:
192.168.20.33 - 192.168.20.62
192.168.20.63 is NOT useable, this is the very last IP and so the BROADCAST-IP.
192.168.20.64 is the network-IP of the NEXT Subnet!
So, because the very first IP is reserved for Default Gateway, our first Host-PC IP would be
192.168.20.34
The very last Host-PC IP would be
192.168.20.62
