Subnetting in 3rd octet of Class C private address Scheme??

[Rabnawaz Anwar]

Hi,

My friend has told me that we can use subnetting in class C i.e we can have 192.168.0.0/20 network.I am not not convinced as Class C have three octets of network and one octet for hosts.My friend says that class C private range is from 192.168.0.0 to 192.168.255.255 and hence we can subnet in third octet as well.I am not convinced please clarify my confusion whether we can subnet in 3rd octet of class C private address scheme or can we have 192.168.0.0/20,192.168.0.0/22,192.168.0.0/18 etc network scheme.

[Jon Marshall]

You are right when you say a class C address uses the first three octets for the network and the last octet for the hosts. So 192.168.0.0/20 is not a subnet but you can use it and it is generally called a supernet or a summarised network range. So -

192.168.0.0/20   - subnet mask = 255.255.240.0 which means that this address covers 16 class C addresses ie.

192.168.0.0/24 - > 192.168.15.0/24

so it is not a subnet address because that is when you take a classful network and use more of the host part for the network part but it is still a valid address range.

Jon

[Rabnawaz Anwar]

If its valid range i.e 192.168.0.0/20 then why cant we have these dhcp scopes for clients that will assign addresses in the range??

192.168.0.1/20 to 192.168.15.254/20 = 4096 clients

192.168.16.1/20 to 192.168.31.254/20 = 4096 clients

192.168.32.1/20 to 192.168.47.254/20 = 4096 clients

to

192.168.240.1/20 to 192.168.255.254/20= 4096 clients

[Jon Marshall]

Not sure i fully understand your question. You can have a scope that would do that if you want although you wouldn't because that many clients in one subnet (vlan) would be not be a good design. Imagine a broadcast which has to be sent to every single host in that subnet.

Whether or not the DHCP server will allow you to configure a scope using the above addresses i don't know, it has been a long time since i setup any DHCP scopes but i can't see why it wouldn't. But even if it didn't it is a perfectly valid range.

Jon

[Rabnawaz Anwar]

I am designing network for university envrironment where there  about 600 network nodes for faculty members,400 nodes for  administration staff and about 400 nodes for  students.I am planning to have three Vlans using Class C private address scheme.Please guide me is it possible or I have move to Class A or B.

[Jon Marshall]

You don't have to use a class B, you can just use multiple class C networks if you want. If you have three vlans then you can have three class C networks.

At the last place i worked we used /25s ie. half a class C per vlan but a full class C is usually considered a good size for a vlan.

You set your DHCP scopes using a 255.255.255.0 mask for each scope.

I still feel there is some confusion here or i am not answering your real question. If so please clarify ?

Jon

[Rabnawaz Anwar]

How will I use class C multiple networks to have three Vlans that meet my ip range i.e Faculty=600,administration=400 and students = 400.As if we use 255.255.255.0 mask for each scope then only 254 address available per scope that does not meet my requirement.Plz list the three networks with subnet mask in class C.

[Jon Marshall]

How will I use class C multiple networks to have three Vlans that meet my ip range i.e Faculty=600,administration=400 and students = 400.As if we use 255.255.255.0 mask for each scope then only 254 address available per scope that does not meet my requirement.Plz list the three networks with subnet mask in class C.

It is generally recommended to use a class C per vlan but you are saying you only want to use 3 vlans which would mean a lot more than a class C per vlan.

Is there any reason you specifically only want to use three vlans ?

eg if faculty need 600 IPs then 3 class C networks would 254 x 3 = 762 IPs which would be enough for any growth. But using the recommendation of a class C per vlan that would also mean 3 vlans.

You don't have to follow the recommendation you can have only one vlan for faculty and if you wanted to do this then you would use -

192.168.0.0/22  which would give you 1022 IPs. You can't any lower because the next one down is 192.168.0.0/23 which only gives you 510 IPs.

But again, is there any specific reason you only want three vlans ?

Jon

[devils_advocate]

As Jon has said already, you are not subnetting but its perfectly fine to use 192.168.0.0 as a /20 although its a fairly big subnet so you need to make sure there is a reason you are using a subnet this big.

A /22 is probably the biggest I would go to be honest.

[Joseph W. Doherty]

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Posting

Subnetting is dividing a network, so to subnet class C, you would work subdivide the 4 octet.  In CIDR notation, a subnet of a class C would be a /25..31.

Combining multiple class C networks, i.e. in CIDR notation larger than a class C's /24, would be (as Jon mentioned) supernetting.

Where this becomes confusing, I can subnet a class B (or class A) in its 3rd octet because that's subdividing a /16 (or /8).

[abpsoft]

Hi,

my gosh - it's 2014. That means classful addressing just celebrates its second decade of being DEAD. And that's only the interdomain scope, RFC950 subnetting was obliterated even earlier (by VLSM, in 1987), even though that passed by most every vendor for decades, too.

That being said, I'm fully aware that classful addressing - and even worse assumptions roughly based on that by folks misunderstanding what it was about - is still wired into a lot of stuff. When supernetting an ex-Class-C like you are planning (192.168.0.0/20 etc), be aware that some operating systems from a certain software manufacturer in Redmond somehow sport the misconception that for example 192.168.1.255 is a forbidden address to which no connections should ever be made. There's even a KB entry about it and AFAIR it was fixed starting with Vista, but 2003 and XP still have it to date. Their "solution"? Avoid using those addresses. So consider that when designing your DHCP scopes...

HTH,

Andre.