subnetting

den1966

Or that "b******" subnetting, as i've begun to call it....

struggling here with it, any tips on how to get my head round this, the maths isn't the problem ...

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stryder144

Well, to be honest, we will need a bit more information if we are going to help. What is causing you the most problems?

Check this link out for possible assistance.

den1966

stryder144 wrote: »

Well, to be honest, we will need a bit more information if we are going to help. What is causing you the most problems?

Check this link out for possible assistance.

Everything , to be honest, i'm doing the course on line through a local a employment office, one of the questions was "how many hosts per subnet can you get from the network..172.18.0.0/28", i got the answer on a subnet calc but that's not much use when you cant use a calc in the exam !!

stryder144

I see. Click on the link I provided above. It might be helpful. Also, if you go to udemy.com, there is an IP Adressing and Subnetting course that is free (not misspelled, btw). Again, might be helpful. Also, look around on this forum, as there are a few threads that might be helpful.

http://www.learntosubnet.com/

http://www.semsim.com/ccna/tutorial/subnetting/subnetting.html

den1966

thanks for that...;)

Jon_Cisco

I found it helpful to go to this site and try a question couple times a day.
subnettingquestions.com - Free Subnetting Questions and Answers Randomly Generated Online

At first I always got it wrong and worked backwards from the answer to find my mistakes.

Once you figure it out it helps to practice ofter.

adam220891

1. First IP is the network ID. It cannot be used by a host.

2. Last IP is the broadcast. It cannot be used by a host. It is used for sending broadcasts (when a switch gets a broadcast, such as an ARP request, it will flood all interfaces with connected devices except the interface it received the frame on).

3. You're always concerned with the octet that isn't 255. Note that 11111111 in binary is 255. You should be familiar with binary 0-255, just write a **** sheet on an index card. While you're at it, you should write down the 'two to the tables.' Essentially 2^2 = 4, 2^3=8, etc up until maybe 2^20.

4. CIDR block (like /18 ) just means the number of 1s used in a subnet mask.

Normally, we have four octets.

Class A default mask (1-126) 255.0.0.0 is 11111111.00000000.00000000.00000000

Class B default mask (128-191) 255.255.0.0 is 11111111.11111111.00000000.00000000

Class C default mask (192-223) 255.255.255.0 is 11111111.11111111.11111111.00000000

So /18 would be 18 1s. 11111111.11111111.11000000.00000000. We can see the first two octets (sets of 8 binary numbers) are full so they are 255.255. And the third octet only uses 2 1s. This means You need to look at your binary chart and see what 11000000 is in binary (starting with the first digit, binary is 128, 64, 32, 16, 8, 4, 2, 1 so just add the corresponding number wherever there is a 1). In this case it is 128+64 so we know the third octet is 192. The last octet is all 0s, so that is easy to figure out, it's just 0.

The mask for /18 would be 255.255.192.0. But what does that mean? The 1s indicate the bits used for the prefix (basically the bits for the network and the subnet). The 0s are used for hosts. More 0s means less networks, but more hosts per network. How many hosts? That's easy to determine. Take your 0s, and raise 2 to the power of the number of 0s. In this case, 2^14 = 16,384 -2 = 16,382 possible hosts. You subtract two IP addresses because the first one is a network ID and the last one is the broadcast IP.

We can also determine the number of subnets. This is just 2^(the number of subnet bits). This is a little tricky, because you need to determine how many subnet bits there are, which can only be done by looking at what class the IP address would be if we were not subnetting. For example, an IP address of 172.16.2.4/18 is Class B. Class B's default mask is 255.255.0.0, so 16 bits are being used for the network, and 2 for the subnet. The number of subnets in this case is 2^2 = 4.

The only thing left to figure out is the range of addresses per subnet. We can do this by looking at the octet in the subnet mask that is not 255. In this case, 192. We calculate delta (or the magic number) by taking 256-192. This gives us 64, which means each the first subnet should be 172.16.0.1 to 172.16.63.254 (172.16.0.0 is the network ID and 172.16.63.255 is the broadcast). The next subnet would start at 172.16.64.0 and so on.

Quick Example:

Say we have an address 130.4.102.1/22.

This is Class B. The subnet mask is 255.255. (that would be /16) and the last part is .252 (11111100 in binary). Mask: 255.255.252.0

256-252 = 4 (magic number)

Subnets:
First: 130.4.0.0 (network ID) 130.4.0.1 - 130.4.3.254 (range) 130.4.3.255 (broadcast)

What subnet is 130.4.102.1 on?

130.4.100.0 (network ID) 130.4.100.1-130.4.103.254 (range) 130.4.103.255 (broadcast)

For your question:

"how many hosts per subnet can you get from the network..172.18.0.0/28"

Easy. The mask is 255.255.255.240. That means 11111111.11111111.11111111.11110000. There's 4 bits for the hosts. 2^4 - 2 = 14. You could also see that the total bits always adds up to 32, so 32-28 leaves 4 bits for hosts.

Your subnets in this case:

256-240 = 16

# subnets = 2^4 = 16

172.18.0.0 (network ID) 172.18.0.1-172.18.0.14 (range) 172.18.0.15 (broadcast)

172.18.0.16 (network ID) 172.18.0.17-172.18.0.30 (range) 172.18.0.31 (broadcast)

Continues for another 14 subnets. Your last subnet would be:

172.18.0.240 (network ID) 172.18.0.241 - 172.18.0.254 (range) 172.18.0.255 (broadcast).

If you can get your hands on the ICND 100-101 (CCENT official cert guide), chapters 11-14 cover subnetting in detail and will help you a lot. I'm sorry if any of the math here is wrong, I did it all quick with no paper/pencil, hopefully it's all accurate.

Datsyuk

Hey guys, not to jack a thread here but I have an assignment due this evening and am looking for some assistance. I am beginning a first year networking course and we were given an assignment where the network address is 220.126.15.0 and there are 100 nodes required. We then need to answer the attached question. I am not looking for someone to answer this for me, but rather point me in the right direction to figuring these out. I have tried a couple of subnet calculators but did not really understand it. I bookmarked the links that you all have posted for future reference. Thanks guys.

The Network Address is 220.126.15.0, and there are 100 required nodes.

a. What is the default subnet mask?
I believe it is 255.255.255.0 /24
b. What subnet mask is required to supply the required number of nodes per network?
c. How many new networks are created?
d. What are the first and last 2 networks created?
e. What is broadcast address of the first and last network created?

mikejwhat

Check out Professer Messers free N+ vids on ip addressing mate (1.3) Professor Messer's CompTIA N10-005 Network+ Training | Professor Messer - CompTIA A+, Network+, Security+, Linux, Microsoft Technology Training

Watch them in order

adam220891

Datsyuk wrote: »

Hey guys, not to jack a thread here but I have an assignment due this evening and am looking for some assistance. I am beginning a first year networking course and we were given an assignment where the network address is 220.126.15.0 and there are 100 nodes required. We then need to answer the attached question. I am not looking for someone to answer this for me, but rather point me in the right direction to figuring these out. I have tried a couple of subnet calculators but did not really understand it. I bookmarked the links that you all have posted for future reference. Thanks guys.

The Network Address is 220.126.15.0, and there are 100 required nodes.

a. What is the default subnet mask?
I believe it is 255.255.255.0 /24
b. What subnet mask is required to supply the required number of nodes per network?
c. How many new networks are created?
d. What are the first and last 2 networks created?
e. What is broadcast address of the first and last network created?

A. A. There are three default masks, based on the Class the IP address lies in. Look up the ranges for Class A, B, and C, and never forget them. You never need a /# in the subnet mask, because the purpose of that number is for shorthand so writing out the mask is not necessary. You will see IP addresses that include the CIDR notation (the /#) as a shortcut.

B. You need to determine the number of bits to borrow that give you the ability to have 100 hosts. By default, a Class C address provides 1 network and 254 hosts. You can use 1 more bit towards the subnet to create two networks that have 126 hosts. Borrow too many bits, and you'll get several networks, but less than 100 IP addresses per subnet.

C. See above

D. Depends. If you borrow 1 bit, the mask will end in 128 and that means 256-128 = 128, this will give you the range of addresses.

E. The broadcast is the last IP address of the subnet. If the subnet goes from .0 to .127, .0 is the ID and .127 is the broadcast. .1-.126 would be the range of usable IP addresses.

Read the links people posted. They help.

prdemon

I used this guy to clear up alot of those "how many subnet's and how many host's" question's
IP Addresses and Subnetting - YouTube
My math suck's big time, so subnetting was torcher for me. I use alot of memorization for calculating CIDR so I need to practice everyday. Good luck and keep practicing!

adam220891

If you write down the 'two to the tables' and a small binary chart, no memorization is ever needed.

After working with subnetting for a while, you will not need the chart. Example: You see a /19. You know it's /16 + 3 which means the interesting octet is 11100000 and that's 224.

Shdwmage

I used this: TrainSignal Webinar: Networking Fundamentals: Mastering IP Addressing in Under 1 Hour - YouTube
It was fantastic. It's kind of long, but I had a solid grasp.

mady0009

Learn To Subnet: A Free, Lecture-Based Presentation on IP Addressing and Subnetting

Gngogh

After i watch this video subnet became very, very easy and simple.

Subnetting Cisco CCNA -Part 1 The Magic Number - YouTube