Table to help Subnetting

This table has the goal to make the manual subnet calculation more easy and fast. It gives us the number of bits necessary or the number of hosts depending on the needs.

Number of necessary bits for H 8 7 6 5 4 3 2 1
Number of Hosts 256 128 64 32 16 8 4 2 NA
Number of necessary Bits for N 1 2 3 4 5 6 7 8
Value of “Subnet Mask” 0 128 192 224 240 248 252 254 255
Number of Valid Networks 1 2 4 8 16 32 64 128 N/A

 

The Number of host is the total amount of hosts, valid and invalid.

e.g.:

We want to know the value of a subnet to host 10 machines.
To meet that aim our network administrator gives us the following network 192.168.1.0 to subnet.

Classical way is:

To host 10 machines we will need to use the following calculation formula: 2^n-2=10 being N the number of bits that we will need.

N=4

2^4-2 = 14

So to meet the goal our binary octet should look like this:

NNNNHHHH

4 Bits for Networking (N) and 4 bits for Hosts (H).

Our network would have a subnet mask of: 2^7+2^6+2^5+2^4=128+64+32+16=240

Numbers translated:

2^7 is the value of the firs N from left to right.

2^6 is the Value of the second N from left to right.

2^5 is the value of the Third N from left to right

2^4 is the value of the Fourth N from left to Right

Now Lets see how the table can make this faster:

First we need 10 hosts so it is necessary to search a value that is close to 10 but bigger.

So we see the value 16 Green also we could meet the number by searching the number of necessary bits 4.

Then we just go down in the column and check the values,  “Subnet Mask” 240  and the number of networks is 16.

Number of necessary bits for H 8 7 6 5 4 3 2 1
Number of Hosts 256 128 64 32 16 8 4 2 NA
Number of necessary bits for N 1 2 3 4 5 6 7 8
Value of “Subnet Mask” 0 128 192 224 240 248 252 254 255
Number of Subnets 1 2 4 8 16 32 64 128 N/A

 

Our network would be the 192.168.1.0 with the Mask 255.255.255.240 with the Network address of 192.168.1.0, first valid ip address 192.168.1.1 and the last valid ip address 192.168.1.14, the broadcast address will be the 192.168.1.15

Hope this helps everybody in their daily work.

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