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Cutting Down The Time Spent On Subnetting Questions
Figuring out the number networks increment by (Magic
number).
Take 256minus the subnet mask= magic number.Example:128.0.0.0
256minus 128= 128255.192.0.0 256minus 192= 64255.255.224.0 256minus
224= 32255.255.255.248 256minus 248= 8
Figuring out which network a particular host belongs to.
Take172.26.180.185255.255.248.0for example.
172.26 we don't worry about, and same with the .185
Take 256minus the 248mask = 8(increments of 8 to be exact) Magic
number!!!Take 180and divide by 8 = 22.5All you care about is the
whole number here.drop the .5 and multiply 8(increments) X 22(your
whole number) = 176176 is your network number. 172.26.176.0.
Take172.24.65.168255.255.255.128 for example.
172.24.65 we dont worry about here
Take 256minus the 128mask=128(increments of 128 to be exact)
Magic number!!!
Take 168and divide by 128=1.3125all you care about is the whole
number here.Drop the .3125 and multiply 128(increments) X 1(your
whole number) = 128128 is your network number. 172.24.65.128
What if they give you a slash notation instead of the whole
subnet mask.
Take191.12.180.12 /28
To figure out the subnet mask we need to figure out what a /28
is
Take/28 were borrowing 4 bits 1st bit borrowed=
128X.X.X.11110000 2
ndbit borrowed= 192
3
rd
bit borrowed= 224128+64+32+16=240 (subnet mask) 4thbit borrowed=
240
5thbit borrowed= 248
6thbit borrowed= 252
7thbit borrowed= 254
8thbit borrowed= 255
So we know the IP address is 191.12.180.12The subnet mask is
255.255.255.240From here follow the above examples191.12.180. we
dont worry aboutTake 256minus 240mask= 16(increments of 16 to be
exact) Magic number!!!
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Take 12and divide by 16= .75all you care about is the whole
number here.Drop the .75 and multiply 16(increments) X 0(your whole
number) = 00 is your network number. 191.12.180.0.Take172.21.98.116
/23
To figure out the subnet mask we need to figure out what a /23
is
Take/23 were borrowing 7 bits 1st bit borrowed=
128X.X.11111110.0 2
ndbit borrowed= 192
3rd
bit borrowed= 2244
thbit borrowed= 240
5thbit borrowed= 248
6thbit borrowed= 252
128+64+32+16+8+4+2=254 (subnet mask 7thbit borrowed= 254
8thbit borrowed= 255
So we know the ip address is 172.21.98.116The subnet mask is
255.255.254.0From here follow the above examples172.21. we dont
worry about, and the same with 116Take 256minus 254mask=
2(increments of 2 to be exact) Magic number!!!
Take 98and divide by 2= 49 all you care about is the whole
number here.multiply 2(increments) X 49(your whole number) = 9898
is your network number. 172.21.98.0.
Figuring out how many subnets and hosts per subnet can you get
from a network.
Take192.168.95.0/27for example
We know a 192.X.X.X address is a class C address. Or in this
case 24bits. (classful).Class A 0-126,or8bits example
124.X.X.XClass B 128-191, or 16bits example 128.X.X.XClass C
192-223, or 24bits example 192.X.X.X
To figure subnetsSo 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 hostsSo 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=32minus 2=30(number of usable hosts).For the
network 192.168.5.0 /27 you have 8subnets, and 30hosts
Take128.23.14.0/28for example
We know a 128.X.X.X address is a class B address. Or in this
case 16bits. (classful).
To figure subnetsSo 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=16minus 2=14(number of usable hosts).For the network
128.23.14.0 /28 you have 4096subnets, and 14hosts.
Dont let exponents fool you (2^x). Its not as hard as they
seem.Take2^10for exampleYou will always start with the number
2double the number until you reach the second number2 (1st
time)
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4 (2nd time)8 (3rd time)16 (4
thtime)
32 (5th time)64 (6th time)128 (7th time)256 (8th time)
512 (9th time)1024 (10th time)So we took the number 2and doubled
it 10times for the number 1024.
Take 2^6 for example2 (1st time)4 (2nd time)8 (3rd time)16
(4
thtime)
32 (5th time)64 (6th time)2^6=64So we took the 2and doubled
6times for the number 64
The main key to subnetting is practice, practice, practice. A
great web site to practice
ishttp://www.subnettingquestions.com.
If you have any questions in reguards to subnetting feel free to
email me [email protected]
http://www.subnettingquestions.com/http://www.subnettingquestions.com/mailto:[email protected]?subject=Subnettingmailto:[email protected]?subject=Subnettingmailto:[email protected]?subject=Subnettinghttp://www.subnettingquestions.com/
