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TOPIC: Why Subetting!!??

Why Subetting!!?? 13 years 3 weeks ago #1779

Hi! ppl

:? I know this is really a stupid question to ask but i want to get everything off my mind before i step into a wonderful palce called Cisco Land so here it does. what is the basic concept of subnetting :?: why do we need to do it and where is it applied applied :?: :oops: :wink:
mess with the best and die like the rest
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Re: Why Subetting!!?? 13 years 3 weeks ago #1781

  • tfs
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Basically, it is to segment your network for various reasons, such as security and limit broadcast. For example, you might have 3 departments at work: marketing, sales and accounting. You may want to segment your network into 3 subnets, one for each department.

This will make your network more manageable and efficient, especially, if you have a lot of hosts. Segmenting by department or use is a common way of doing this.

There is a good discussion of subnetting under the Networking/General/Subnetting menu.

Hope this is what you are looking for.
Thanks,

Tom
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another clereficationwanted 13 years 3 weeks ago #1783

ok cool
another thing i wanted to know was, what is the maximum and the minimum number of subnets and can be accomidated in Class C and the maximum and minimum number of hosts in the above said subnetwork? thanx
mess with the best and die like the rest
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Re: Why Subetting!!?? 13 years 3 weeks ago #1786

  • sahirh
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Ok lets jump straight in !
Well first off, possible class C subnet masks for the 4th octet are listed below
(example 255.255.255.192 or 255.255.255.254)

MASK BITS
192 = 2
224 = 3
240 = 4
248 = 5
252 = 6
254 = 7

Now the formula for number of subnets is
(2 ^ n) - 2 where n is the BITS column above
so if we take 224 then

(2 ^ 3) - 2 = 6 subnets

now you get the number of hosts by subtracting the number of BITS from the total bits in the octet which is 8 so subtract 8 -2 = 6
you have 6 bits for the host ID.. the formula for number of hosts is the same as above, just change the value of n to 6.. so..

(2 ^ 5) - 2 = 30 hosts

you can calculate those values for all the above masks

If you dint get it, don't worry, I explained it assuming you'd already got the basics down.. go hit the subnetting section from the Networking / Protocols link at the top of the page, its explained very easily.
Sahir Hidayatullah.
Firewall.cx Staff - Associate Editor & Security Advisor
tftfotw.blogspot.com
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Re: Why Subetting!!?? 13 years 3 weeks ago #1793

  • bildad
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In the example above, wouldn't the number of host bits be 5 for mask 224?
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Re: Why Subetting!!?? 13 years 3 weeks ago #1794

  • tfs
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Yes.

224 is 11100000. To figure out the number of networks (bits on the right), just take the number of bits on (111) and calculate that (be sure to add 1 for 0). So the number is 7 + 1 = 8, take out the 2 illegal numbers (000 and 111) and you have 6.

To get the hosts, just reverse this and make all the 0 bits = 1 and 1 bits = 0 or 00011111. Calculate that and you have 11111 = 31 + 1 = 32. Take out the illegal number (00000 and 11111) and you have 30 hosts.

This is just another way to look at it. There is another way to do it that I have used. If you look at the "Certifications, Books & More" forum and at the "Subnetting tips and tricks" at my tip, it says the same thing in a different way.

Everyone has there own way of remembering it, you just need to find out what works for you.

I don't know if you've looked at the subnetting topic on this site, but it is a good place to start.
Thanks,

Tom
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