% Pg 159 IP Octet to change? Subnetting

[mid4]

The question asks to sub net 255.255.0.0/16 this is a class A network.

T.L changes the second octet when adding the blocks surely the octet to change for each network would be the third?

This has completely lost me.

I thought when you have a subnet mask 255.255.248.0 you would change the third octet, anything with 255 you leave along and go to the next octet?

Thanks

[core21]

Hi mid4,

First let's make sure we have the correct terminology. The 255.255.0.0 is the subnet mask. You are not subnetting the subnet mask. You subnet a network address.

With that being said, let's look at the example 1A on page 159 that you are referring to.
The Class A default mask is 255.0.0.0. In this example, he is going to use the mask 255.255.0.0. So, he has taken 8 bits away from the host portion to use for network addressing. These are your 8 subnet bits. So he does the calculations based on that.

Hope this helps.

Edit: Just realized this thread is a year old. Oops.