Subnetting a class A network

**Yirabeth** · October 13th, 2009, 09:53 AM

Hi all, I'm a networking student, and having grave difficulties with networking. This class is extremely important to my grade but, I'm just not getting it. I have spent the last week doing internet searches, questioning fellow students (who are as lost as I am) and going to the teacher and asking for additional help. He assured me he'd go over it again, then spent the next 2 hrs talking about routers, then said, see ya next week! and left. The homework was due yesterday, now I'm pretty much just wanting to know what the deal is as I wont be able to get a grade for it.

My situation is this -- I can figure out how to come up with a subnet mask, starting with the default subnet mask and the amount of networks I need. But I don't know how to apply that mask to my IP address and come up with network ID's, etc.

My homework assignment specifies a class A network starting with 10.50.0.0, and I need 75 networks. I need to know starting and ending network numbers, first and last workstation addresses on the third network(include 0 as the first), I need to know what network 10.52.90.8 is on. I need to know if I set the mask to /16, would I still meet the requirements listed in the objectives.(He doesn't specify how many hosts per network so this question is a bit odd in itself IMO, but I'm probably missing something)

I have figured I have a subnet mask of 255.254.0.0 and checked that with an online calculator as being correct.

Now..I really want to KNOW this, so you don't have to use those numbers, of course, but if someone could help me understand how to apply the subnet mask to an IP address and come up with the info I need, it would be greatly appreciated

Thanks in advance for any help

~Yira

**andyman** · October 13th, 2009, 11:50 AM

10.50.0.0 isn't a class A address. So I'm guessing you are either suppost to be using 10.0.0.0/8 or It's a class B and you are suppost to use 10.50.0.0/16. in either case you need to borrow 7 bits (2^7>=75). In order to match up a specific ip address to a network ID you use the subnet mask. Convert the ip address and the subnet mask into bits, then line them up with eachother. If a 1 corresponds to a 1 in the ip address you keep it, otherwise you use a 0. Then convert these new bits into an IP address and you have your network ID. In order to figure out what the broadcast ID is just figure out what the next subnet ID is and subtract one from the host field

**Yirabeth** · October 13th, 2009, 12:54 PM

Okay now I get confused..hehe isn't 1.x.x.x - 126.x.x.x Class A? Making 10.50.0.0 Class A?

In any case, here is what I did -- I took the default subnet mask, and using this formula (2^n - 2 = subnets) discovered I needed to "borrow" 7 places to get (at least) 75 networks. This gives me a subnet mask of 11111111.11111110.00000000.00000000 which translates to 255.254.0.0 At this point the subnet calculator (Online IP Subnet Calculator) agrees with me. Needless to say, this may all be wrong because I set the subnet calculator to class A, and therefore could have thrown that off as well. BUT!! If I did this part correctly ----

The problem is, I don't know what to do with those numbers (The 255.254.0.0 and the 10.50.0.0) to come up with the rest of the information I need to know to really know how to subnet a network. Somehow, I take that subnet mask, and combine/add/subtract/otherwise manipulate the starting IP address and come up with my networks, their hosts, etc etc. (as described in the first post.)

This is where I'm hung up.

In case I didnt explain it properly (wouldn't put anything past me by now!) Here's the teacher's exact paragraph on it ---

You are the network administrator for XYZ, corporation, and you have decided to change all of the IP addresses you use on your internal network to private addresses. Since your company has 75 locations worldwide, you have decided to use a private class A address (10.50.0.0) and subnet it to accommodate your needs.

This lab needs to be done individually and not as a group. Please do not use an online IP Subnet calculator, as I expect you to show your work.

Steps:

Based on the information provided above, I want you to perform the following. Don't forget, you need to show how you came up with your answers.

* Determine the subnet mask that is required to accommodate a minimum of 75 locations.
* Determine how many stations each subnet will have available for use.
* Determine the staring and ending subnet. What is the increment between subnets?
* What is the first available and last available workstation address in the third subnet? (Include Subnet 0 as the first)
* Given an IP address of 10.52.90.8, what subnet will the packet be delivered to? For this one, you need to use binary, and I do expect you to show this.
* Think outside of the box for a moment. If you were to set the subnet mask to a /16, could you still meet the requirements listed in the objectives? What advantages would this provide? You will need to give some thought to this one.

Again, I appreciate you taking the time to help straighten me out

~Yira

**Nude_Lewd_Man** · October 13th, 2009, 03:45 PM

Originally Posted by Yirabeth

Okay now I get confused..hehe isn't 1.x.x.x - 126.x.x.x Class A? Making 10.50.0.0 Class A?

Hi,

In theory you are correct, the 10.50.0.0 network does sit in the bracket that is generally used for Class A networks, but it is within what is known as a "Private IP Range" (Private network - Wikipedia, the free encyclopedia)...

This is in the same sort of way that 172.16.0.0 is a "Class B" and 192.168.0.0 us a "Class C" network....

(Bear in mind that I'm still trying to understand subnetting myself, so I might have gone slightly off)

**Yirabeth** · October 13th, 2009, 04:17 PM

Right, that part I understand

I'm trying to turn the private IP into subnetworks, and that's where the trouble starts, as the saying goes...hehe

Thanks though!

~Yira

**Nude_Lewd_Man** · October 13th, 2009, 04:23 PM

Just re-read your second post (oh, and BTW.... WELCOME TO TECHIMO - from the Folding@Home Team [#111]...!!) and saw the comment in there about the private IPs...

Have you had a look through this:
Subnetwork - Wikipedia, the free encyclopedia

I haven't had a chance to read it in full, but the quick glance I had seems that it could be of value...if you haven't seen it already....

**andyman** · October 13th, 2009, 05:35 PM

Originally Posted by Yirabeth

Okay now I get confused..hehe isn't 1.x.x.x - 126.x.x.x Class A? Making 10.50.0.0 Class A?

You are correct, I just wasn't clear in my explanation, if the teacher had given you the whole 10.0.0.0 network to work with then you would be dealing with a class A address. But he gave you 10.50.0.0 which is a fractional class A, or in other words you have the whole class B of 10.50.0.0/16 to work with, hope that clears things up. I would recommend buying a CCNA Study guide book like this one
Amazon.com: CCNA: Cisco Certified Network Associate Study Guide: Exam 640-802 (9780470110089): Todd Lammle: Books.

I used to teach a cCCNA class here in San Diego, and I think that you are going to need more detailed help then an online forum. Just don't give up, subnetting is hard at first, but once you get it it becomes very easy. It's also an esential part of any networking job

**Yirabeth** · October 13th, 2009, 05:44 PM

Thanks for the advice, not sure what I'll do as I'm not sure I need to understand the networking -- the class is required along with my programming for a programming degree(not sure why) and I thought, no harm, could come in useful etc. But carrying 17-20 credits per semester I may have to judge this one as the one that's too much for *now* anyway...

It would be nice to know it, and be able to sit for the CompTIA exam, but if not, I guess it's not REALLY the direction I planned on going anyway....

Thanks again!

~Yira

**andyman** · October 13th, 2009, 05:49 PM

Okay, but if a I can just give you a little advise I'm a network Analyst here where I work and we have a department that does programming also. Whenever I walk through the programming department they look like they are bored to tears and hating life. And it's not like they get payed more then we do to deal with that. I guess what I'm trying to say is that I would keep my options open if I were you and not rule out a career in networking