# Subnetting help ...

christopherfest
Posts:

**1**Member ■□□□□□□□□□
in CCNA & CCENT

have a few questions if someone coul clarify them and then give me a good subnetting resource webiste

How many subnets and hosts per subnet can you get from the network

192.168.245.0 255.255.255.240?

and

Network ID: 121.0.0.0

Subnet into 5000 networks

What is the broadcast address of the #3 network?

Thanks

How many subnets and hosts per subnet can you get from the network

192.168.245.0 255.255.255.240?

and

Network ID: 121.0.0.0

Subnet into 5000 networks

What is the broadcast address of the #3 network?

Thanks

0

## Comments

73Member ■■□□□□□□□□TechExams.net - IP Subnet Calculator

Online IP Subnet Calculator

I can not stress this enough you have to know how to do this with just pen and paper. And yes it is a pain in the behind but you have to know it.

723Member1. 16 subnets and 14 host/subnet

2. 121.0.23.255

Can someone please double check my answer? I want to practice subnetting as well.

Studying:Working on CCNA: Security. Start date: 12.28.10

Microsoft 70-640 - on hold (This is not taking me anywhere. I started this in October, and it is December now, I am still on page 221. WTH!)

Reading:Network Warrior - Currently at Part II

Reading IPv6 Essentials 2nd Edition - on hold

22Member ■□□□□□□□□□1. I also get 16 subnets, and 14 hosts per subnet.

2. At first I got 121.0.31.255, but looking at your answer I realised I didnt begin with the 121.0.0.0 network, instead starting with the 121.0.8.0 network. I need to get out of the habit of doing that.

111MemberI always see these Class C private address ranges using a /24 mask, can't remember if I've ever seen one that didn't. But I sat there and thought about it and isn't class C private 192.168.0.0/16?

Meaning 14 hosts per subnet but 4096(2^12) subnets?

69Member ■■□□□□□□□□class c is /24.

x.x.x.240 is a /28

which means 4 bits for the subnet and 4 bits for the hosts

means 2^4 subnets and 2^4 -2 hosts.

111MemberCheck the Odom ICND1 book. Or wikipedia. Or this.

Class C Subnet Mask | 6 of 10

723MemberStudying:Working on CCNA: Security. Start date: 12.28.10

Microsoft 70-640 - on hold (This is not taking me anywhere. I started this in October, and it is December now, I am still on page 221. WTH!)

Reading:Network Warrior - Currently at Part II

Reading IPv6 Essentials 2nd Edition - on hold

46Member ■■□□□□□□□□So what will happen to the networks between 0-8 if you are saying 121.0.31.255 is right

723MemberI just tried to answer the OP's 2nd question.

I am still in the learning process.

Studying:Working on CCNA: Security. Start date: 12.28.10

Microsoft 70-640 - on hold (This is not taking me anywhere. I started this in October, and it is December now, I am still on page 221. WTH!)

Reading:Network Warrior - Currently at Part II

Reading IPv6 Essentials 2nd Edition - on hold

69Member ■■□□□□□□□□Humm yea, good point. I never really thought about why the class c private range span across a /16.

22Member ■□□□□□□□□□I think its right:

1st subnet - 121.0.0.0

2nd subnet - 121.0.8.0

3rd subnet - 121.0.16.0 (bcast = 121.0.23.255)

4th subnet - 121.0.24.0

etc

2,331Member ■■■■■■■□□□With that network address, this gives you 13 subnet bits (8 in second octet, 5 in the thrid octet) and an increment of 8 (3 host bits in 3rd octet). Mask is a /21 or 255.255.248.0.

121.0.0.0 - 121.0.7.255

121.0.8.0 - 121.0.15.255

121.0.16.0 - 121.0.23.255 <--broadcast

121.0.24.0 - and so on

This is a great quick reference for doing quick subnet calculations:

http://packetlife.net/media/library/images/tn_IPv4_Subnetting.pdf.jpg

2,086Memberphoeneous

why do you say 2^13 is wrong? when 5000 can be taken from it

and 2^12 is short by 1000 subnets. 2^12 doesnt meet the requirements?

mask is /21 and 255.255.248.0 256248= 8 block size

256-240 = 16 block size

:

it is 2^13 is part of the answer...why did you say that? explain.

At least I dont get what you mean since it doesnt meet requirements..

thanks

yep I get

2,331Member ■■■■■■■□□□Ooppss, fixed

I typically like to short my subnets by 1000 hosts

443MemberIP Subnet Practice

Subnetting Quiz -- Steve Kehlet's Pages

I do about 30 mins every morning, just to keep it fresh.

Next Up: CCIE R&S Lab

12Member ■□□□□□□□□□5000 networks

bcast of 3rd network

no ip subnet zero:

1. 121.00000000.00000|000.00000000 <- 13 host bits borrowed

2. 121.00000000.00011|000.00000000 <- 3rd network (121.0.24.0)

3. 121.00000000.00011|111.11111111 <- bcast address of 3rd network (121.0.31.255)

ip subnet zero:

1. 121.00000000.00000|000.00000000 <- 13 host bits borrowed

2. 121.00000000.00010|000.00000000 <- 3rd network (121.0.16.0)

3. 121.00000000.00010|111.11111111 <- bcast address of 3rd network (121.0.23.255)

68Member ■■□□□□□□□□First, you want to memorize some things. I know, I know, memorizing sucks, but... trust me, you want to do this.As I am going over this, watch for the patterns that evolve throughout everything we do.

First, memorize your powers of 2.

Memorize higher if you like, I'm just stopping here because it nicely deals with an octet.

Now, memorize the relation between bit, decimal, and CIDR. I'm going to start from the last octet for a standard class C /24.

Now certainly you can go /23 and lower, but notice the binary and decimal repeat. So it is the same thing in every octet in its progression.

Now, here is the trick. Notice the powers of 2? and then look at the list we just did? See something sticking out? That's right, block sizes.

So if we memorize block size with this (and we also make sure we know how to count in 2's, 4's, 8's, 16's, etc...) we can blaze through these in seconds for pretty much any use we have.

So the question was how many networks and how many hosts for:

192.168.245.0255.255.255.240

First, its a class C so our focus is the last octet. We know that a 240 decimal is 4 bits on, 4 bits off, and a /28. We also know that if we see a class C which is /24 and we recognize that 240 is a /28 we can simply subtract /28-24 = 4 bits on. Once you have these basics memorized you can come at it from multiple angles depending on the need. Just make sure you can recognize backwards and forwards between them. In a class C, 4 bits on = /28 = 240 or /28 = 240 = 4 bits on, etc..

With that in hand, we only need to understand the basics of simple networking and how it relates to the subnet address and the broadcast address. For the number networks, we don't need to consider this unless we do not have "ip subnet zero" enabled which essentially is an old issue not common so much anymore, but basically it simply means that without it, you can't count the first and last subnet. It is on by default these days, so in most cases you don't have to worry about it and I think Cisco doesn't test on it anymore (someone correct me if I am wrong).

Anyway, so 4 bits are being used for the subnets. We know since we memorized that 4 bits is 2^4=16 which is our block size and if you have memorized this, you know that a 240 (/2 has a block size of 16. Notice that the number of networks is your block size and your bit multiple?

So if a 240, or a block size of 16, or rather a /28 is 4 bits on, 4 bits off, then we know exactly what we have for both.

We have 4 bits on for our network which is 16 subnets and we have 4 bits off available for our hosts. This is where identifying the subnets and hosts differ. You know that of those 4 bits 2^4=16 hosts, two of them have to be reserved to identify your subnet and your broadcast. So in this case, you have 16-2=14 hosts available.

So, we have 16 networks and 14 hosts per network.

If we start with each network and count in 16's (block size), we can identify the range of each network.

There we have it, 16 networks with each a block size of 16 and the host capacity of 14 each.

If you have memorized multiples of 2, your CIDR notation, block size, decimal, and bits on and off for just that single octet backwards and forwards, you can snap these out extremely fast.

It is the same for /8-/15 and /16-/23 as it is for /24-30 with the only difference being that you have 2 more bits to know for the class A and B networks which are easy to remember anyway. Point is, you can now subnet in your head through any address range and this will make doing VLSM's and route summarization all the more easier to understand and deal with. In fact, in many simple VLSM designs, you will also be able to do these in your head if you get good at it.

Sorry if I was long winded, or if I was a bit over redundant in my mention, It is just that this approach really helped me and maybe, if I haven't blinded people with walls of text, it can help any of you having trouble.

Also, I did this to make sure I remember it and am on track. If anyone sees any errors, by all means please correct me.

46Member ■■□□□□□□□□Nicely put. well done

2,086Memberdude great stuff thank I agree doing about 30 minutes a day to keep fresh

if not you kind of loose your mind tools super man I appreciate your extra stuff to add to my tool set

phoeneous

hahaha

I was like maybe I am wrong? ahhahaha good catch dude

I have done that too

Picker

that is excatly what I do: here is my **** sheet to know only: