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Subneting

The trick to Subneting

I always had this trick since long time ago when I did CCNA I saw it somewhere, I wrote it but could never find it. Now I have it and I document it for all.

First we have to draw the magic table,
just remember 1,128,128,0 then go from there…

bit(n) 1 2 3 4 5 6 7 8
bit 128 64 32 16 8 4 2 1
Subn 128 192 224 240 248 252 254 255

Sub(#s) 0 2 6 14 30 62 126 254

Important to remember about the class
NetworkHosts
Class C 248, Class B 1616 Class A 8 24

The most asked quesitons are:
Q1. Find the number of bits needed for the host
2^x = 1000(hosts) , 2^10 = 1024 n=10 bits(host)

2. Subnet Bits left for the network
32-x, 32-10 = 22 bits(net)

3. Determine the number of bits borrowed
Sub bit – 8 (octets), 22-8-8= 6 bits borrowed

4. The Subnet
bits 8.8.8.6.0 (ref table) = 255.255.255.252.0

Q1. 172.30.0.0 needs 55 subnets, how many hosts for each?
1. ref table # sub 62 or 6 bits + 16 bits(classB) = 22

2.Bits for the host part
32-22 = 10 bits (2^n)-2, (2^10)-2 =? 1022 hosts per subnet

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