subnetting not clear

Posted on 2008-10-27
Last Modified: 2012-05-05

the subject of subnetting is very un clear to me, i mean if we have many network segments and suppose we use class B IP addresses why we not just make the first netork id 135.120.00 as an example and the second 135.121.00 or
with network mask why we need to take from the 16 bits of the hosts to create subnets with masks such as ??!!

in my example if you know the class of ip address you will know the network id and subnet mask without needing to chack bits of it so whats the reason of complicating it ??!!!!!!!!
Question by:Pure_Heart
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Expert Comment

ID: 22810788
Basically, because the subnetting model gives you more flexibility and allows you to conserve IP addresses in a very limited and running-out IPv4 space.

Classes are fine, and they're easy, you're right.  But having to assign blocks of 256 (class C) or 65536 (class B) to subnets can be wasteful.  What if you have four different subnets you need to add, but you won't need over 1000 computers over any of them?

Well, you can't give them a class C space, because it's too small.  So you can give all four class B's -- and you've taken up 65536*4 = over 250,000 IP addresses when you only needed about 4000.

*Or* you could give them a /22 address within one class B space; each subnet gets 1024 IPs and is fine, and you save yourself three entire class B's plus a lot of space on the fourth.  As we're running out of IP addresses fast, that's a big deal.

But you can't do that with class references -- you have to have that "arbitrary" netmask.

So I guess the short answer to "why do we complicate things by using netmasks when we could just specify a class and be done with it?" is: because we don't *want* to use classes anymore.  They're very wasteful of a resource that's very limited, and we needed a better solution that conserved that space.

I hope that answers your question.

Author Comment

ID: 22810820
thanks for the replay, so this means for meduim business i can use classes without subnetting as we dont have prblem with IP adresses numbers, plus your solutions of subnetting is correct for internet not for local networks cuz i dont think there is an organisation that needs more adresses than any class capability for its local wan's and lan's ?

Author Comment

ID: 22810825
in short words also if we are not internet providers why we are worried about wasting ip adresses :)  ?

Expert Comment

ID: 22810843
Because companies grow, in short.  There are plenty of organizations that need subnetting flexibility.  You say "more addresses than any class capability" -- but what about less?  What if you have a class C assigned, and 256 IP addresses -- but you want your 100 or so computers in two separate subnets for security reasons?  Well, you can buy another C subnet and waste a whole lot of money.  Or you can just use two /25 subnets within your C and you're done.

I've been with a number of organizations for whom wasting IP addresses wasn't an option.  You pay for that waste.

If you don't feel like you need it, then don't use classless subnetting.  Make everything a /24 or /16, and call them Class C and Class B.  That's fine.  But the classless subnetting system has saved the IPv4 addressing system that was supposed to have died a decade ago for lack of IP addresses, and saved many companies innumerable amounts of money in the process.  It's been a definite success.  If your needs are simpler than that -- well, really, is there that much harm in having to specify a subnet mask? :-)
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Author Comment

ID: 22810860
you said "Well, you can buy another C subnet and waste a whole lot of money"  are you talking about public Ip adresses i mean if its not why they need to pay ? its distributed from the dhcp server for free :)

Accepted Solution

slinkygn earned 125 total points
ID: 22810943
Yes, I'm talking about public IP addresses.  Your examples of 135.120... are not private addresses. :-)

If you're talking about private IP addresses, then the point is moot.  The system is the way it is because it's required and useful for the public space, and it would be silly to have public and private IP addresses use two different systems.

I believe the original question has been answered, right?

Author Comment

ID: 22811042

ok thank you but i have a comment, i feel this is usflul only for internet providers, because why an organisation want to make all its computers IP addresses public ?
and if i got a public ip address and subneted it will all ip addresses that will be generated from that will be public also ?
and about the ip address range i mentioned in the example how come its public i mean it can be local i can assign it in the range for local network when installing the dhcp server right ?
thank you, i will post these questions in a new related qestion do yu can gain more points :)
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Expert Comment

by:Fred Marshall
ID: 22813025
An IP address has 4 octets in its address such as    So, if you get a public IP address or a private IP address you don't "subnet" the address.  It's already subnetted.

Public IP addresses are *defined* as being public.  This means that they are assigned specifically to someone - perhaps individually and perhaps in blocks / subnets.  If the addresses are public and you're using them in a private network then how are you going to keep them separate if you have internet access into the privately owned network?  That's why private address spaces are defined - just for that purpose.

Also, how is your private network going to know how to separate "your" addresses from actual public addresses?  Let's say you try to go to "" and DNS gives you which is already being used in your "private" network?  How will your computers get out to the internet if this is viewed as an internal LAN address?

So, yes, you can imagine creating a private network using public addresses but that would be a big mistake.  Use addresses from the private address blocks.  Subnet them however you want.  There are private address blocks of just about any size you might want including the network: /


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