Cutting Down The Time Spent On Subnetting Questions
Figuring out the number networks increment by (Magic number).
Take 256 minus the subnet mask= magic number.
Example:
128.0.0.0
256 minus 128= 128
255.192.0.0
256 minus 192= 64
255.255.224.0
256 minus 224= 32
255.255.255.248
256 minus 248= 8
Figuring out which network a particular host belongs to.
Take
172.26.180.185
255.255.248.0 for example.
172.26 we don't worry about, and same with the .185
Take 256 minus the 248 mask = 8 (increments of 8 to be exact) Magic number!!!
Take 180 and divide by 8 = 22.5 All you care about is the whole number here.
drop the .5 and multiply 8 (increments) X 22 (your whole number) = 176
176 is your network number. 172.26.176.0.
Take
172.24.65.168
255.255.255.128 for example.
172.24.65 we don’t worry about here
Take 256 minus the 128 mask=128 (increments of 128 to be exact) Magic number!!!
Take 168 and divide by 128=1.3125 all you care about is the whole number here.
Drop the .3125 and multiply 128 (increments) X 1 (your whole number) = 128
128 is your network number. 172.24.65.128
What if they give you a slash notation instead of the whole subnet mask.
Take
191.12.180.12 /28
To figure out the subnet mask we need to figure out what a /28 is
Take
/28 were borrowing 4 bits
X.X.X.11110000
128+64+32+16=240 (subnet mask)
1st bit borrowed= 128
nd
2 bit borrowed= 192
rd
3 bit borrowed= 224
th
4 bit borrowed= 240
th
5 bit borrowed= 248
th
6 bit borrowed= 252
th
7 bit borrowed= 254
th
8 bit borrowed= 255
So we know the IP address is 191.12.180.12
The subnet mask is 255.255.255.240
From here follow the above examples
191.12.180. we don’t worry about
Take 256 minus 240 mask= 16 (increments of 16 to be exact) Magic number!!!
Take 12 and divide by 16= .75 all you care about is the whole number here.
Drop the .75 and multiply 16 (increments) X 0 (your whole number) = 0
0 is your network number. 191.12.180.0.
Take
172.21.98.116 /23
To figure out the subnet mask we need to figure out what a /23 is
Take
/23 were borrowing 7 bits
X.X.11111110.0
128+64+32+16+8+4+2=254 (subnet mask
1st bit borrowed= 128
nd
2 bit borrowed= 192
rd
3 bit borrowed= 224
th
4 bit borrowed= 240
th
5 bit borrowed= 248
th
6 bit borrowed= 252
th
7 bit borrowed= 254
th
8 bit borrowed= 255
So we know the ip address is 172.21.98.116
The subnet mask is 255.255.254.0
From here follow the above examples
172.21. we don’t worry about, and the same with 116
Take 256 minus 254 mask= 2 (increments of 2 to be exact) Magic number!!!
Take 98 and divide by 2= 49 all you care about is the whole number here.
multiply 2 (increments) X 49 (your whole number) = 98
98 is your network number. 172.21.98.0.
Figuring out how many subnets and hosts per subnet can you get from a network.
Take
192.168.95.0/27 for example
We know a 192.X.X.X address is a class C address. Or in this case 24 bits. (classful).
Class A 0-126, or 8 bits example 124.X.X.X
Class B 128-191, or 16 bits example 128.X.X.X
Class C 192-223, or 24 bits example 192.X.X.X

To figure subnets
So take the slash notation were given of /27 minus it from the number of bits in our classful IP addresss 24=3
(remaining bits) this gives us 2^3=8 (Number of subnets).

To figure out hosts
So take the slash notation were given of /27 minus it from the number of bits in an Ipv4 address 32=5 (remaining bits)
This gives us 2^5=32 minus 2=30(number of usable hosts).
For the network 192.168.5.0 /27 you have 8 subnets, and 30 hosts
Take
128.23.14.0/28 for example
We know a 128.X.X.X address is a class B address. Or in this case 16 bits. (classful).

To figure subnets
So take the slash notation were given of /28 minus it from the number of bits in our classful IP addresss 16=12
(remaining bits) this gives us 2^12=4096 (Number of subnets).

To figure out hosts
So take the slash notation were given of /28 minus it from the number of bits in an Ipv4 address 32=4 (remaining bits)
This gives us 2^4=16 minus 2=14(number of usable hosts).
For the network 128.23.14.0 /28 you have 4096 subnets, and 14 hosts.
Don’t let exponents fool you (2^x). It’s not as hard as they seem.
Take
2^10 for example
You will always start with the number 2 double the number until you reach the second number
2 (1st time)
4 (2nd time)
8 (3rd time)
th
16 (4 time)
32 (5th time)
64 (6th time)
128 (7th time)
256 (8th time)
512 (9th time)
1024 (10th time)
So we took the number 2 and doubled it 10 times for the number 1024.
Take 2^6 for example
2 (1st time)
4 (2nd time)
8 (3rd time)
th
16 (4 time)
32 (5th time)
64 (6th time)
2^6=64
So we took the 2 and doubled 6 times for the number 64
The main key to subnetting is practice, practice, practice. A great web site to practice is
http://www.subnettingquestions.com.
If you have any questions in reguards to subnetting feel free to email me at
shayne.cda1@yahoo.com