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Subnet query

Posted on 2006-06-09
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Last Modified: 2010-03-19
Hello all,

In my building for a large company the PCs (on a domain) all have an IP address starting with 10.x.x.x. Now this to me is a class A address and the subnet mask should be 255.0.0.0. however the subnet mask on all the Pcs in the office is
255.255.255.0 suggesting a class C address. It has always been this way and I wondered if anyone knew why this is so?
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Question by:btglobal
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LVL 11

Expert Comment

by:rvthost
ID: 16869462
btglobal - Check out the following which discusses classful versus classless:

http://www.experts-exchange.com/Networking/Q_21782082.html

With subnetting, it really doesn't matter what you do.  Like your example above, you could use a 172.16.x.x with a 255.255.255.0 mask as well.  
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Expert Comment

by:Rob Williams
ID: 16869608
True classfull subneting would state that 10.0.0.0 255.0.0.0 would be a class A subnet but it is not as common to do that anymore. Classless addressing is used allowing more options for addressing schemes. Therefore 10.1.1.0 255.255.255.0 would allow a range of IP's from 10.1.1.1 to 10.1.1.254 with a network ID of 10.1.1.0 and a broadcast address of 10.1.1.255  
As a result more diversified ranges can be chosen such as:
10.1.1.0  255.255.252.0 which results in 10.1.1.1 to 10.1.3.254 with a network ID of 10.1.1.0 and a broadcast address of 10.1.3.255
or
10.1.1.0  255.255.255.240 which results in 10.1.1.1 to 10.1.1.14 with a network ID of 10.1.1.0 and a broadcast address of 10.1.1.15
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pseudocyber earned 750 total points
ID: 16869620
You are correct, with "old" thinking and notation, a 10.0.0.0 address is a "Class A" with a "natural mask" of 255.0.0.0.

However, with "new" thinking and notation - specifically Variable Length Subnet Masking (VLSM) - one can take the 10.0.0.0/8 network and subnet it down as small as they like.  In your case, it's been subnetted down to so that the network is 24 bits and you have 8 bits of hosts.

We do a lot of this, because we like the fact that 10 offers us 24 bits to play with - so we have different subnets for servers, users, future VOIP, dmz's, floors, etc.  

So, 10.0.1.0/24 might be the first floor at headquarters, 10.0.2.0/24 might be the second floor, and so on.

With good planning, the entire Headquarters site could be summarized into 10.0.0.0/16 even though it has smaller subnets inside it.

Hope this helps.
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LVL 31

Expert Comment

by:masnrock
ID: 16869700
If you were going with the classful approach to subnetting, such would be true. But if you're using CIDR (Classless Internet Domain Routing, which is used by most), then the subnet mask you gave would not necessarily apply.

Why 255.255.255.0 for the subnet mask? Because they figured that 256 IP addresses would suit the needs of the particular area or office to which those addresses are assigned. By not taking a classful approach to subnetting, it's a heck of a lot easier to give an office about as many IP addresses as needed without worrying about overkill. Plus you reduce the amount of machines affected by broadcasts. Imagine if you had an office needing 1000 IP addresses (assume growth is built into this number), and you were given a class B network. Whereas if you were given a chunk of IPs where you had a subnet mask of 22 bits, you'd have 1024 addresses. The other approach would be obviously to assign 4 class C networks, but why bother when you can just have one network?

Hope this helps.
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