Calculate broadcast address and valid host

Unanswered Question
May 6th, 2005

Write the subnet, broadcast address, and valid host range for each of the following:

1. 172.16.10.5/25

2. 172.16.10.33/27

3. 172.16.10.65/26

4. 172.16.10.17/30

5. 172.16.10.33/28

6. 192.168.100.25/30

7. 192.168.100.37/29

8. 192.168.100.66/27

9. 192.168.100.17/29

10. 10.10.10.5/20

Hi friends, refer to the above, i know how to subnet eg. 1. 172.16.10.5/25 (subnet will be 255.255.255.128) it's in my head but i don't know how to calculate broadcast address and valid host. Kindly advise. Thanks n regards

I have this problem too.
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tb@westend.com Fri, 05/06/2005 - 02:20

Hi,

if you have the Mask 255.255.255.128, you have 128 IP-Adresses.

Now you need to calculate 0 + 128 this is the next Subnet.(172.16.10.128)

The Broadcast of your Networt is 0+128-1.(172.16.10.127)

Your Network starts with 0, the "0" is your Network you can't use it.

So your valid range is from 1 to 126.

hope it helps.

cu thomas

mfreijser Tue, 05/10/2005 - 05:16

The exercise you are doing (writing down the subnets, broadcast address and valid host range) is a very important thing when working with networks. I guess almost everybody can agree with this. That's why it's terribly important to know it like the back of your hand.

http://www.idevelopment.info/data/Networking/Networking_Basics/ROUTERS_IP_Subnetting.shtml

This site gives a detailed explanation about IP subnetting, it definitely helped me with learning subnetting and maybe it can help you too!

Regards,

Michaël

pottsnet2 Thu, 07/07/2005 - 12:52

Hi Chialun,

This is a method that I learned from one of the Cisco Press books that I've been studying.

For 172.16.10.5/25...

Convert the VLSM notation to dotted decimal ("/25" = 255.255.255.128)

From the left, find the first non-255 octet value and subtract it from 256 (256-128=128). This is the network number increment.

Now, just find the multiple of the Network Increment that is closest to, but not greater than, the corresponding octet in the host IP address that you are working with and that will form the network number for that IP address.

For the subnet mask in this case, 255.255.255.128, the first network is x.x.x.0 and the second is x.x.x.128, so .5 is in the first network. As you probably know, the broadcast address is the last one before the next network address. So, the usable addresses are all of those between the network address and the broadcast address.

network#1 : 172.16.10.0

first Host: 172.16.10.1

last host: 172.16.10.126

Broadcast: 172.16.10.127

Network #2: 172.16.10.128

For the second one, 172.16.10.33/27;

Subnet Mask: 255.255.255.224

Network increment is 256-224=32

Network #1: 172.16.10.0

Network #2: 172.16.10.32 (.33 of the host IP address is greater than .32 and less than .64)

First Host: 172.16.10.33

Last Host: 172.16.10.62

Broadcast: 172.16.10.63

Network #3: 172.16.10.64

Hope this helps or at least was not too confusing.

ksriram29 Sun, 07/17/2005 - 01:22

Hi, pottsnet2

I have a doubt in subnet.

consider the address 192.168.10.1/26

everything is fine about the calculation.

Therefore,

total no. host bits: 6

total no. of hosts in a subnet: 2power6-2 = 62 hosts

total no. of subnet bits: 2

total no. subnets:2power2-2 = 2 subnets

the subnets will be

Network #1: 192.168.10.0

First Host: 192.168.10.1

Last Host: 192.168.10.62

Broadcast: 192.168.10.63

Network #2: 192.168.10.64

First Host: 192.168.10.65

Last Host: 192.168.10.126

Broadcast: 192.168.10.127

Network #3: 192.168.10.128

First Host: 192.168.10.129

Last Host: 192.168.10.190

Broadcast: 192.168.10.191

Network #4: 192.168.10.192

First Host: 192.168.10.193

Last Host: 192.168.10.254

Broadcase: 192.168.10.255

According to the formula to find the no. of subnets

2power(no. of subnet bits)-2 = 2power2-2 = 2 subnets.

But What we get here is 4 subnets. Do we have to ignore network #1 and network #2. If yes, will the ip address in that range cannot not be used.

Which is considered as the first subnet and the last subnet? Please explain.

Hope I have not confused anyone.

Thanks in advance.

Sriram K

nkanyiso-ndlovu Tue, 07/19/2005 - 01:38

255.255.255.252 is used for point to point networks like two routers connection each other. They only need two ip ip addresses. This helps not waste ip addresses.

The magic number is 4 therefore

0 - 3

4 - 7

8 - 11

12 - 15

16 - 19

.

.

.

.

256

Depending on what ip address you are given, you can determine the subnet and the broadcast address and still be left with 2 ip addresses everytime to use for point to point networks

pottsnet2 Tue, 07/19/2005 - 08:02

Hi ksriram29,

Sorry for the delayed response.

I'm not an expert but I think this is the issue;

If two subnets were to be excluded, they would be the first one (192.168.10.0) and the last one (192.168.10.192). They are referred to as Subnet Zero and the All-Ones Subnet. RFC 950, "Internet Standard Subnetting Procedure", recommends not using these subnets because it was thought to be confusing. This resulted in losing some otherwise usable addressed. Currently, Cisco routers will accept assignment of these subnets in their default configuration. Cisco Document ID: 13711 has a good description of the issue and a link to the actual RFC (http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml).

I would like to "Ask the Experts": for the purposes of Cisco certification exams, are the Zero Subnet and the All-Ones Subnet included in the count of available subnets?

Thanks!

scottmac Thu, 07/07/2005 - 18:23

Assuming you have (at least) a basic grasp of binary, it's easy if you remember it like this:

If the HOST portion of the address is all ZEROS, then it's the "Network Address," if the HOST portion of the address is all ONES, then it is the "Broadcast Address."

On the test, you may see addresses like "10.15.254.255 / 16" ... it's not a broadcast address because the host portion is not all ones.

By the same token, 10.15.254.0 / 16 (or 10.15.255.0, or 10.15.0.255) is neither a Broadcast or "network number"... because all of the host bits are not all ones, or all zeros.

It is extremely helpfull to draw a binary number line out before you begin your test, if you have any doubts, sketch it out in binary and look at the *host* portion of the address ... all ones = Broadcast, all zeros = "Network Number."

The bits will never lie or mislead you. Any questions you have will be resolved if you write it out in binary .. practice and get good at it, then start looking for shortcuts.

Shortcuts rarely help you understand WHY something is what it is.

Good Luck

Scott

pottsnet2 Fri, 07/08/2005 - 11:44

I think Scott is absolutely right... about the concept. A Network address is represented by the host portion being all zeros. However, that is represented by a decimal number that is a multiple of the network increment (or 0). And the broadcast address IS the last one before the next network.

Actually the book to which I referred, Cisco CCNA Certification Guide (Cisco Press), covers both the binary algorithm that Scott described and the decimal algorithm (not to be confused with a shortcut). I chose to share the decimal version because, to some, decimal math comes more naturally than binary math which is usually preceeded and/or followed with binary to decimal conversions.

On my first sitting for the CCNA exam, I wrote out each IP addressing question in binary. *FOR ME*, that took too long and I was rushed at the end.

I'm simply suggesting to use the best tool for the job for the person. If you choose the decimal route, it would be a very good idea to verify the answers using the binary method until you develop confidence in its use (or decide to ditch it).

Good Luck all

John

There are 10 kinds of people in the world; those who understand binary and those who don't :)

scottmac Fri, 07/08/2005 - 14:24

John:

I didn't mean to imply that you were presenting a shortcut, or that there's anything really worng with shortcutts (as long as you understand the basics and can revert in case something "doesn't look right").

You have some time before the actual start of the test ... whether I think I'll need it or not, I usually draw out a binary number line, fill it out for mask values (128-->255), write in host counts, etc. That whole process takes maybe a minute or two.

During the course of the test, if I need to verify a mask, a count, a subnet ... whatever, it's already there and ready to use. For the CCNA and / or the CCNP tests, it can save quite a bit of time.

Doing the binary thing is still necessary basic skill, IMHO, everyone should suffer through it until they understand it, then move on to other means.

I can guarantee that almost every answer is a "right answer" ... depending on how you miscalculate. If you draw it out in binary, there's no confusion, nothing ambiguous ... the bits don't lie.

If you create a "binary helper" up front and use it as a reference, overall, you save time and have a clear indication to the right answer.

Creating it at the beginning prevents the stress-induced vapor lock in the middle of the exam.

So again, I'm all for leaning & using faster methods, but not until the basic method is well understood.

FWIW

Scott

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