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Choosing a Network Range

post #1 of 6
Thread Starter 
hey all ...

I am studying for my CCNA and have run into a puzzling thing while using Packet Tracer ...


I am practicing setting up networks by saying to myself ...


I have Company A - There are 6 departments in Company A and there are X amount of hosts in each department ...


The problem I am having is choosing a private network range for the departments ... ... I know they would be class C as B & A would have far to many hosts available meaning plenty wastage.


So how do you guys choose your class C network ... ... Is there a basic number you stick to for each octet or do you thumb suck any valid number for a class C

(ie - Class C = 192+ in the first octet so how do you choose between 192 or 216 ... etc)



All information would be greatly appreciated.
Regards.
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post #2 of 6
They teach you Classes in cisco O_o they havnt existeted in years! everyone used CIDR now

Well the way id do it is see the amount of departments, then give the adequte ips for huge growth

department a 192.168.0.0/20
deparment b 192.168.16.0/20

etc, plenty of room for gorwth as thats like 4k machines on the network,


On the security side of things however you would prfer to do something like
department a 192.168.0.0/26
giving only 62 valid Ip addresses to use,less growth

just depends on what you feel like doing


In provate networks its impossible to run out of addresses unless you are really silly and assign huge numbers to subnets / have lots of linking VPNs thumb.gif
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post #3 of 6
RFC 1918 tells us what the valid ranges are for private allocation. For Class C, the range is 192.168.0.0 through 192.168.255.255. Therefore, you could just step through the range in increments of '1' for the third octet if you wanted to keep it simple (192.168.0.0 /24, 192.168.1.0 /24, [...] 192.168.5.0 /24). Do not increment the first octet. Anything above 192.168.255.255 (e.g. 216.x.x.x) violates the RFC and is no longer within the private range.

Of course, you could breakup each /24 into smaller pieces like /25, /26, /27, etc. However, I believe this is beyond the scope of your original question.
 
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post #4 of 6
Quote:
Originally Posted by Ulquiorra View Post

They teach you Classes in cisco O_o they havnt existeted in years! everyone used CIDR now

Well the way id do it is see the amount of departments, then give the adequte ips for huge growth

department a 192.168.0.0/20
deparment b 192.168.16.0/20

etc, plenty of room for gorwth as thats like 4k machines on the network,


Your first /20 prefix would work and the second one would work if it were a /19 instead (8k+ addresses or 8k+ subnets). In routing, the /19 and /20 would most likely be used to summarize networks that fall within those ranges. Furthermore, unless one of those departments was going to do some serious subnetting, they should not be on a subnet that has room for thousands of machines. After a certain amount of devices, ARP alone will kill a network.
Edited by felladium - 3/15/13 at 5:34am
 
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post #5 of 6
Oh yeh i know you would never have a netowrk that size ... imagine the switches! THE HORROR xD twas just a example ^_^ most busniess wont ever need more than the /26 or 27 subnets if they are static IP'ing it

One thing though if the secind was a /19 wouldnt it engulf 0-15's subnet? the two 20's would allow 2 sepperate ones for clean-ness,
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post #6 of 6
Quote:
Originally Posted by Ulquiorra View Post

One thing though if the secind was a /19 wouldnt it engulf 0-15's subnet? the two 20's would allow 2 sepperate ones for clean-ness,

Yep. I did a little 'fire and forget' before working out the blocks. For some reason, I saw 16.0 /20 and it just didn't seem right at first.
 
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