Here is a last octet calculator that will allow you to find bits barrowed and last address(subnet broadcast) faster without doing binnary dance.

class C subnet (last octet)
barrow bits left to right |------>

---------2 bits----3 bits----4 bits----5 bits---6 bits
----------192------224------240------248-----252----
-------|128 + 64|192 + 32|224 + 16|240 + 8|248 + 4|

|-128-|---64----|---32---|---16---|---8---|--4----|---2----|---1----|

host bits right to left <-----------|
----255------127------63------31-------15--------7--
|128 + 127|64 + 63 |32 + 31 | 16 + 15 | 8 + 7 | 4 + 3 |
----3------1----
| 2 + 1 | 0 + 1 |
(to find last addressable host substract 1 from host address)

Class C example: 220.17.154.101 255.255.255.224

1st) looking at above calculator 3 bits are barrowed for subnet address: 220.17.154.96/27 leaving 5 right most bits for hosts range 1 to 30, 31 is a broadcast.

worksheet: 101-32=69-64=5
add 32+64= 96 which is the subnet address for the given example. 5 is the decimal bits used for the host.

1st addressable host is 96 + 1 = 97 or 220.17.154.97/27
last addressable host (96 + 31)-1=126 or 220.17.154.126/27
Broadcast address is 96 + 31 = 127 or 220.17.154.127/27

In this example we have 3 left most bits for subnet addresses or bit 32, 64, 128. Best way to calculate is in 32 bit ranges or:
32 * 0 = 0 (or major network address)
32 * 1 = 32
32 * 2 = 64
32 * 3 = 96
32 * 4 = 128
32 * 5 = 160
32 * 6 = 192
32 * 7 = 224 (This is a subnet that has major network broadcast address or as some call it directed Broadcast.)

Then 5 right most bits are for hosts addresses and the subnet broadcast address or as some call it the Directed broadcast.

Hope this will help those that have problems with binnary conversions.

Last edited by GilGraber on 04-17-02 at 05:40 PM

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... or see www.mcsefreak.com/subnetting.htm for my free IP addressing and subnetting guide

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Or use a calculator with:
..(n)
2^ - 2

Darn white space

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This is my subnet calculator

1. 128 64 32 16 8 4 2 1
2. 192 224 240 248 252 254 255
3. 2 6 14 30 62 126 254



1.bit value
2.subnet mask
3.no of subnet

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you should be able to subnet in your head or as my teacher for MCSE and CCNA say "subnet in your sleep" i can look at a ip adress and tell you the subnets if you tell me how many networks you want or hosts required.

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