IP range with prefix - less than /24?

wesley

wesley

wesley

Soldato
Joined
29 Jul 2003
Posts
7,685
hi guys

i understand how to work out how many IP range with prefix /24 and upward but not less than /24 so how to work out?

my understanding that you can add up the 8 (prefix of 24) bits of binary due limited to 255 (broadcast)

thanks :)
 
Easiest way to work it out, assuming you have everything above /24 committed to memory is a /23 is a /31 less than a /24...

So /31 is 255.255.255.254
and /23 is 255.255.254.0

a /19 is a /27 less than a /24

so /27 is 255.255.255.224
and /19 is 255.255.224.0

Thats my method anyway...
 
For me, I remember the block size increments that are consistant throughout each class of network (A,B,C) - these block increments are as follows

0, 128, 192, 224, 240, 248, 252, 254 = 8 possible masks for each classful network (another useful tip to remember if you are on the right track).

If you study the chart below carefully - you will see what I mean.

/8 = 255.0.0.0
/9 = 255.128.0.0
/10 = 255.192.0.0
/11 = 255.224.0.0
/12 = 255.240.0.0
/13 = 255.248.0.0
/14 = 255.252.0.0
/15 = 255.254.0.0

/16 = 255.255.0.0
/17 = 255.255.128.0
/18 = 255.255.192.0
/19 = 255.255.224.0
/20 = 255.255.240.0
/21 = 255.255.248.0
/22 = 255.255.252.0
/23 = 255.255.254.0

/24 = 255.255.255.0
/25 = 255.255.255.128
/26 = 255.255.255.192
/27 = 255.255.255.224
/28 = 255.255.255.240
/29 = 255.255.255.248
/30 = 255.255.255.252
/31 = 255.255.255.254
/32 = 255.255.255.255

*phew* - hope that helps you anyway :)
 
thanks guys but how did you work out?

for example : 136.185.86.112/18

IP range would be 136.185.64.1 to 136.185.127.254

but how to work that out?

thanks again

edit: corrected typo!!
 
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ok so a 'quick' way i was taught is to simply convert /18 into dotted decimal notation...which comes out as 255.255.192.0 (you know how this works, right?)...and then simply take 192 from 256 which gives you 64...so you know the networks go up in 64's starting in the third octet...make sense?

edit:
IP range would be 136.185.64.1 to 136.185.127.54
Click to expand...

you know it goes up to 254, right? that was just a typo?
 
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it just goes up in 64's, so the networks would be:

network 136.185.0.0, first host 136.185.0.1, last host 136.185.63.254, broadcast 136.185.63.255
network 136.185.64.0, first host 136.185.64.1, last host 136.185.127.254, broadcast 136.185.127.255
network 136.185.128.0, first host 136.185.128.1, last host 136.185.191.254, broadcast 136.185.191.255
network 136.185.192.0, first host 136.185.192.1, last host 136.185.255.254, broadcast 136.185.255.255
 
if its /18 then yeah...

all a /18 means is a /16 network, with 2 host bits borrowed for subnets, leaving 14 bits remaining for hosts...

vlsm/cidr notation is really straightforward, assuming you actually understand how the dotted decimal notation works?
 
no........LOL

i'm studying ccna exploration 4 at th moment via online, cisco academy connection site, it doesnt say how do it
 
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trust me when i tell you...subnetting is a complete pita...but just keep practicing and one day it will just 'click'...or, at least, it did with me... i'll try and find some resources that i used because now i just 'do it' without thinking... i've been where you are though, so don't worry...we'll get you through!
 
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