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[heavenly]

My assigned IP address is 206.35.130.0. Of course, my default subnet mask is 255.255.255.0. I'm a network admin, and required to implement a networking plan that will allow for 10 subnets.

What would be the custom subnet mask that would allow for the required amount of subnets?

Would it be 255.255.255.240?

I'm kind of confused. Can anyone help me out?

 

[GamerShu]

u are a network admin, and u need help with this????

 

[heavenly]

No! It's a A+ Cert question. I used to know this stuff but it's been about 2 years since the last time I studied it.

 

[blahness]

My assigned IP address is 206.35.130.0. Of course, my default subnet mask is 255.255.255.0. I'm a network admin, and required to implement a networking plan that will allow for 10 subnets.

What would be the custom subnet mask that would allow for the required amount of subnets?

Would it be 255.255.255.240?

I'm kind of confused. Can anyone help me out?

for 10 subnets you would need to use 4 host bits so yes your new mask would be 255.255.255.240

 

[heavenly]

Thanks.

Can you elaborate on how you came to that conclusion, if it's not much trouble?

 

[borghe]

240 gives you 16 addresses per subnet (including network address and boradcast)
224 gives you 32 addresses

if you need 10 subnets, 240 will give you the most addresses per subnet while giving you at least 10 subnets (16 actually). 224 only allows for 8 subnets.

 

[heavenly]

240 gives you 16 addresses per subnet (including network address and boradcast)
224 gives you 32 addresses

if you need 10 subnets, 240 will give you the most addresses per subnet while giving you at least 10 subnets (16 actually). 224 only allows for 8 subnets.

Thanks, again.

Did you use that equation (2^N - 2) to figure that out? Or did you use a table? If the former, what value does the N equate to in this particular scenario I presented?

 

[Deleted member 1235]

i'll post a bit soon. I have just passed the exam and I think I can explain pretty well, got to shoot out for an hour or so now though.

in short

(2^N - 2) will give you the number of hosts per subnet from the subnet mask. N is the number of 0s in the subnet mask when in binary.

eg

255.255.255.0
is
11111111.11111111.11111111.00000000

2^8-2 = 254

figure out how many hosts you NEED per subnet first , then determine your subnets. will explain better later.

 

[borghe]

catfish is right. there is no easy math formula to determine the subnet mask based on number of subnets needed. basically just remember 1 2 4 8 16 32 64. Those are the number of subnets per subnet mask bit from left to right (so no bits in the last octet gives you 1 subnet, one bit gives you 2, two bits gives you 4, etc). it is kind of goofy to keep track of because as your subnet mask grows, the number of subnets grows also but not at a similar rate, and the number of hosts per subnet shrinks but again not at a similar rate. subnet masking is annoying, and I've been dealing with it for over 10 years. It never gets any better (though a little easier the more you use it).

 

[heavenly]

catfish is right. there is no easy math formula to determine the subnet mask based on number of subnets needed. basically just remember 1 2 4 8 16 32 64. Those are the max number of subnets per subnet mask bit from left to right (so no bits in the last octet gives you 1 subnet, one bit gives you 2, two bits gives you 4, etc)

Okay. That made alot of sense and cleared up alot of things. I thought there was an easy equation to determined/calculate the subnet mask ip based on only the amount of subnets needed.

So, I'm assuming most network admins (newbies) use a table to expedite the decision making process (subnetting), right?

 

[borghe]

Okay. That made alot of sense and cleared up alot of things. I thought there was an easy equation to determined/calculate the subnet mask ip based on only the amount of subnets needed.

So, I'm assuming most network admins (newbies) use a table to expedite the decision making process (subnetting), right?

or you just memorize it after doing it so much.

 

[Deleted member 1235]

example

put the binary number column values on your work sheet

128 64 32 16 8 4 2 1

this often helps you out when adding until it becomes ingrained anyway.

somebody tells you you need to have 3 networks with at least 62 hosts available each subnet. and say we are using the standard 192.168.0.0 network address

so how many bits are you going to need for host address?

2^8-2 is 254. thats a few to many
2^7-2 is 126. still quite a lot
2^6-2 is 62 Bingo so that means you need 6 bits (or 0's in subnet mask) for host adresses.

put that in binary

11111111.11111111.11111111.11000000
or
255.255.255.192
or
/26 (this one is just a count of the number of 1's from the left)
is your subnet mask for that network

applying that to

192.168.0.0
lets you have the first subnet
192.168.0.1 - 192.168.0.62 (valid host range) 192.168.0.63 will be the broadcast address for that subnet
192.168.0.64 - 192.168.0.126 (valid host range) 192.168.0.127 is broadcast again

I think that covers it pretty much. I hope all my numbers are right, I'm pretty jetlagged and need a coffee, but if you have any questions I'll be happy to answer them. this stuff is kind of a bitch and you do need to go over it a few times. I've been doing networking nearly 5 years, and I had to relearn it, because once your subnets are in place, you don't often need to refigure out this stuff.

 

[alejob]

I took the A+ in April and I didn't get this kind of questions. I took the Net+ and I still didn't get a subnetting Q. I took the CCNA and I still didn't get this kind of Q, offcourse if you don't know subnetting by then you are screwed.

u are a network admin, and u need help with this????

:lol I think that's just part of the question.

 

[Mashing]

While I haven't taken an A+ test I find it incredulous that something like this would be included on the test. Subnetting is one of the more difficult aspects of networking (in my opinion) and really has no relevancy for a A+ test. What does subnetting have to do with fixing computers? It belongs on a MCSE or CCNA test of course, but on an A+ test it is ridiculous.

 

[heavenly]

Catfish,

That info was thorough. Thanks.

I do have another subnet question. In my MCSE Windows 2000 Professional book, it talks about the pattern key once you've figured out the subnet mask. I guess this is for deciphering the valid host ip ranges.

For example, a subnet mask for a class B ip address (145.15.0.0) needing 31 subnets is 255.255.248.0, which has a pattern key of 8? Then they use the pattern key "8" to break down the subnets in intervals of 8:

So, the first subnet would be 145.15.8.0; the next would be 145.15.16.0; therefore making the first valid host ip range on the 1st subnet to be 145.15.8.1 - 145.15.15.254, and the next, 145.15.16.1 - 145.15.23.254, and so forth.

In short, what is this pattern key? I see that you didn't use a pattern key in your example, and just use intervals of 62, if I'm not mistaken.

Other than that I think you pretty much nailed it down for me. Thanks alot.

 

[borghe]

for the record, I had two subnet math questions on my networking essentials test. that was almost ten years ago though so I don't remember what they were.. I just remember that I was drawing in the air with my finger to do some quick math.