subneting

Answered Question
May 31st, 2007

hello again, I need to master subneting. and I am having problem, I have a basic knowlege . class A. 255.0.0.0,B 255.255.0.0,and C. 255.255.255.0..I know how to get the binary part 0's and 1's . but I am having problem resolving the senarios

example. we have an ethernet port that was assigned 172.16.112.1/20 how many allowed hosts on this subnet.

Thanks

karim

I have this problem too.
0 votes
Correct Answer by mohammedmahmoud about 9 years 6 months ago

Hi,

Just to expand SSLIN's suggestion to expand your analysis:

2^8=256 (always remember that the highest octet value in an IP is 255 = 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1).

Thus 2^9 = 256 x 2 = 512, and 2^10 = 1024, and forward.

Anyway, by time you'll remember these days and laugh :)

HTH,

Mohammed Mahmoud.

Correct Answer by mohammedmahmoud about 9 years 6 months ago

Hi Karim,

If you see that the 3com document is not that easy as a start, no problem don't panic :) check the following links, but the 3com document contains nearly all the details, this is what makes it special, and eventually you need to be able to utilize its contents.

IP Addressing and Subnetting for New Users

http://www.cisco.com/warp/public/701/3.html

IP subnetting made easy

http://articles.techrepublic.com.com/5100-1035-6089187.html

IP Addressing and Subnetting

http://www.learntosubnet.com/

And anyway please do come back when you have any problems understanding any issue in any of these documents.

Take care and enjoy :)

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

Correct Answer by mohammedmahmoud about 9 years 6 months ago

Hi Karim,

Check the following link, it is one of the finest documents on subnetting, please do review it, and please do come back if you have further questions:

http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf

As for your example, you know that the subnet mask is 32 bits, accordingly a /20 subnet mask means that 12 bits are left for hosts, accordingly (2^12)-2 hosts can be defined, (2^12)-2=4094, the (-2) part is the all zeros IP (subnet ID: 172.16.112.0/20) and the all ones IP (subnet broadcast:172.168.127.255/20).

NOTE: All zeros and all ones, means that we have the host 12 bits all either zeros or ones.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

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Correct Answer
mohammedmahmoud Thu, 05/31/2007 - 15:07

Hi Karim,

Check the following link, it is one of the finest documents on subnetting, please do review it, and please do come back if you have further questions:

http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf

As for your example, you know that the subnet mask is 32 bits, accordingly a /20 subnet mask means that 12 bits are left for hosts, accordingly (2^12)-2 hosts can be defined, (2^12)-2=4094, the (-2) part is the all zeros IP (subnet ID: 172.16.112.0/20) and the all ones IP (subnet broadcast:172.168.127.255/20).

NOTE: All zeros and all ones, means that we have the host 12 bits all either zeros or ones.

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

cisco steps Thu, 05/31/2007 - 17:42

Mo~~

I am going to be honest here, I tried to follow the 3com document and I got lost. things are not even making since....

the way you explained the senario make much since then the 71 pages :-). is there any other docs that goes easy on the newbies.. lol..thanks

karim

Correct Answer
mohammedmahmoud Thu, 05/31/2007 - 17:54

Hi Karim,

If you see that the 3com document is not that easy as a start, no problem don't panic :) check the following links, but the 3com document contains nearly all the details, this is what makes it special, and eventually you need to be able to utilize its contents.

IP Addressing and Subnetting for New Users

http://www.cisco.com/warp/public/701/3.html

IP subnetting made easy

http://articles.techrepublic.com.com/5100-1035-6089187.html

IP Addressing and Subnetting

http://www.learntosubnet.com/

And anyway please do come back when you have any problems understanding any issue in any of these documents.

Take care and enjoy :)

HTH, please do rate all helpful replies,

Mohammed Mahmoud.

cisco steps Thu, 05/31/2007 - 17:57

Ok thanks Mo.. I will go head and take a look @ the ones you provided . Thanks again .

karim

chris.lepa Fri, 06/01/2007 - 10:55

Very good link Mohammed, I now know alot about IPV6 that I didn't know before :)

-Chris

guruprasadr Sat, 06/02/2007 - 20:14

HI Karim, [Pls Rate if Helps]

Online Subnet Calculator is a very good tool availaible online to calculate your Subnets, Wildcard etc., for Class A, B, C.

http://www.subnet-calculator.com/

PLS Rate / Vote if Helps

Best Regards,

Guru Prasad R

mohammedmahmoud Sun, 06/03/2007 - 03:11

Hi Karim,

The link provided by Guru is very helpful indeed, but i'll recommend that you do the addressing with your mind and use the subnet-calculator only for correcting your subnetting in order for you to master the subnetting issues.

HTH,

Mohammed Mahmoud.

scottmac Sun, 06/03/2007 - 05:57

Just a minor correction, but it's important to understand:

The subnet mask has *NOTHING* to do with determining the class of an IP address.

There are natural masks, as you describe. But it's very common to have a Class A or B address with extended masks (like 10.1.1.0 / 24). Even with a 24 bit mask, a "10." address will always be class A.

Class is determined ONLY by the leading bits of the leading octet (mask is not used to determine class).

Class A addresses begin with "0xxxxxxx" in the leading bit position (range is 0-127.255.255.255)

Class B addresses start with "10xxxxxx" (range is 128.0.0.0 - 191.255.255.255)

Class C addresses start with "110xxxxx" (range is 192.0.0.0 - 223.255.255.255)

Class D addresses start with "1110xxxx" (range is 224.0.0.0 239.255.255.255)

The rest of the numbers are "Class E" "experimental"

Again, the only thing that describes an address Class is the leading bits of the leading octet ... the mask can be almost anything for any class (or classless, of course, if classless routing is enabled on the device)

If an address has a Class, it's the front bits that describe which one it is.

Good Luck

Scott

cisco steps Tue, 06/12/2007 - 16:19

this question may DAM , but that is the only way I can learn.. now I am having trbl when I see that I have to calculate 2^12 or 2^19.. I do not want to write 2x2x2x2x....19 times to get the answer, is there any easy way .. let say you want to calculate 2^19 what would you do .. Thanks again , and forgive me if I did not do my math homework .. :-)

CSCO10892433 Tue, 06/12/2007 - 17:17

Hi

If you can calculate 2^10 and 2^20 easily and quickly, then you will not get trouble in calculating 2^12 and 2^19.

2^10 = 1k = 1024 (no calcaulator is needed here for me :)

2^20 = 1M =1024 *1024 = ... well, now I need a calculator :( Never mind, just remember it's 1M then it will be ok.

So, 2^12 = 2^2 * 2^10 = 4k = 4 *1024 = 4096 (no calculator, please)

and 2^19 = 2^20 / 2 = 1M/2 = 512k = 512 * 1024 = 5 hundred thousand and something

Believe me, you will never have to expand this mutiplication, just know it's 512k and it's quite enough for the CCNA exam (and for the real life, too)

HTH

SSLIN

Correct Answer
mohammedmahmoud Tue, 06/12/2007 - 23:19

Hi,

Just to expand SSLIN's suggestion to expand your analysis:

2^8=256 (always remember that the highest octet value in an IP is 255 = 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1).

Thus 2^9 = 256 x 2 = 512, and 2^10 = 1024, and forward.

Anyway, by time you'll remember these days and laugh :)

HTH,

Mohammed Mahmoud.

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