Subnetting class a is hurting my brain!

Network newbie321

Hello all, I need help!

I need to subnet a class a address of 60.25.0.0 in order to have at least 410 subnets.
I know I need a /17 mask to get 512 networks, but this means that the networks go up in 128's, so I only see 2 networks.

what am I missing?

Also what is the first network address? 60.25.0.0? So is the first usable address 60.25.0.1?

If the broadcast is [URL="tel:60.25.127.255"]60.25.127.255[/URL], isn't that just one massive network?

What are the addresses of subnet 1, subnet 2, subnet 400, subnet 405, etc?

thanks and confused,


Jim

krjay

/17 would get you 512 subnets.. 60.0.0.0 - 60.255.128.0.

/25 Would get you 512 subnets with 60.25.0.0 - 60.25.255.128

101010

I will take a stab at this as well, but as I am still learning, I may be completely wrong.

Also, the methods I use below can be found in this post: http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html (he does a great job of breaking down the process)

First, what is the mask of the address you are starting with? I will start with the assumtion that it is a /16, and that the address provided (60.25.0.0) is just a subnet of a true Class A address (60.0.0.0).

Now, to get the minimum of the 410 subnets you will have to figure you how many of the currently available host bits [16] you will have to borrow. So 2^9 = 512, thus we have to borrow 9 host bits for our new subnets. So our /16 shifts to a /25 [16+9 = 25].

Now, with that /25 we have to figure out our network hops. We will take our 25 and subtract it from 32 (the next boundary) with will give us 7. Now we take that to the power of 2 and we get 128 as our increment [2^7 = 128]. Once we have our network increment we can begin to layout the network boundaries.

So, 60.25.0.0 /25 breaks down into:

60.25.0.0
60.25.0.128
60.25.1.0
60.25.1.128
......
60.25.255.1
60.25.255.128

Conversely if you are actually working from a /8 address you would be jumping to a /17 and your increments would look like this:

60.0.0.0 /17

60.0.0.0
60.0.128.0
60.1.0.0
60.1.128.0
......
60.255.0.0
60.255.128.0


TLDR: What krjay said.

iamme4eva

Basically, what they said. ^^^^^^^^

The question is a little misleading. If you are doing a true class A subnet mask, then your first subnet is 60.0.0.0.

krjay

I meant to get back and edit my post earlier but I got busy at work. What I wanted to mention was I think that question is terrible, and misleading. Maybe someone with more experience knows better than I do though.