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TOPIC: Please clear my Doubt

Please clear my Doubt 5 years 8 months ago #36091

  • rizin
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Hi,

I would like to know Whether the Below statement is correct.

Is 128 Subnets is right ????

According to my analyze from Website articles, the answer will be 128 Subnets and Possible network will be 126. if im wrong please consider this post as my learning purpose.

Following are the statement from One site.

You have the following Network ID: 131.112.0.0. You need at least 500 hosts per network. How many networks can you create? What subnet mask will you use?
Answer: If you need 500 users then 2^9th would give you 512 (remember the first and last are network and broadcast), 510 usable. So of your 32 bits you would turn the last 9 off for host and that would give you give you a 255.255.254.0 subnet mask (11111111.11111111.11111110.00000000). Now that we know that we can see that you have the first 7 of your third octet turned on so to figure out how many subnets you have us the formula 2^7th= 128. So you can have 128 subnets with 500 people on them.

Rizin
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Re: Please clear my Doubt 5 years 8 months ago #36119

  • S0lo
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Yes, you are absolutely correct. 7 bits are left in the 3rd octet,(counting from the left). This is 2^7 = 128

It's worth mentioning here that this is a class B address (since 131 in the first octet) which has a classfull subnet mask of 255.255.0.0. Thats why we can not change any bits in the first and second octet, hence it does not count in sub-networks.

Had it been a class A or class C, the answer will be different.
Studying CCNP...

Ammar Muqaddas
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Re: Please clear my Doubt 5 years 8 months ago #36120

  • rizin
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Hi Ammar,

Thanks, lol.. i thought no one will answer it for such a silly questions, however Ammar my purpose here to bring notice to note the Number 126 Networks.

Following comments of mine can be ignored if it looks awkward :D

Since i got it from one of the Website & to confirm myself as well as pointing as kind of error to intrude in the minds of subnetting newbies.

That gentleman from that website Done a Good Job for the interview goer preparation, but it will be nightmare for anyone want to just copy and paste it in there mind. While they came across interviewer intervention to prove them in details about 128 subnets and 500 hosts instead of 126 Networks with 500 Hosts.

Note: I did one error writing Site despite of Website

Thanks,

Rizin.
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Re: Please clear my Doubt 5 years 8 months ago #36124

  • S0lo
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Thanks, lol.. i thought no one will answer it for such a silly questions, however Ammar my purpose here to bring notice to note the Number 126 Networks.

Your welcome, :)
By default, new Cisco routers will allow 128 subnets, NOT only 126. Because both subnet ZERO and the LAST subnet can be used as valid subnets. But in very older routers (like 2500 series), subnet zero was not enabled by default, the admin had to allow it, otherwise the total number of subnets in those old routers will be 127 by default.

Thats as far as I know.
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Ammar Muqaddas
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Re: Please clear my Doubt 5 years 8 months ago #36127

  • rizin
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Thanks again Ammar,

Well that sounds good that i learned something new today subnet Zero, let me work on it & see how it is being applied in networks.

Cheers buddy !

Rizin
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