[SOLVED] Subnet 255.255.255.128 Help!

[Nazumorg]

Hi guys, so i'm studying for my CCNA through Todd Lammle's book. For one of his examples with subnetting a class B network.

The example is 255.255.255.128 / 25 as the subnet mask

With a network of 172.16.0.0

He breaks down this subnet using his method

2 to the 9th power (number of binary bits on) = 512 subnets

2 to the 7th power (number of off bits) - 2 = 126 Hosts

Valid subnets (this is where i get confused) He does 256 - 255 = 1, therefore the valid subnets go 0, 1, 2, 3 etc. for the third octet.

But then in the next example

for 172.16.0.0 with a subnet mask of 255.255.255.192 /26

2 to the 10th power = 1024

2 to the 6th power - 2 = 62

when calculating the valid subnets he does 256- 192. Why does he subtract the 4th octect from 256 instead of the third octet in the previous example. His explanation for this is that there is a 255 in the 3rd octet position. However the previous example 255.255.255.128 ALSO had a 255 in the third octet position. Is this a typo? What am I not understanding here. I really appreciate any help!

 

[bill001g]

Not sure I never used that method. I just brute force memorized everything though about /22.

Unless you are looking at how far you can break them down farther A /25 has 2 subnets x.x.x.0 and x.x.x.128 each with 126 usable addresses...assuming you are using ethernet other technology can use all 128.

This tends to be hard to learn when you first start. Find the method that works best for you. Other than for certification tests it is not used much. In private industry almost everyone uses private ip for the lan. Since equipment is so fast and there is no need to conserve IP pretty much you see /24 used or even /22 just because it is the lazy way to do it and never run out. Simple and stable tends to out way efficient allocation of ip when you have no concern about running out.

The only place you see actual subnet masks is in ISP internet routers and nothing goes more than a /24 on those.