Subnetting


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# 1  
Linux Subnetting

Dear friends am confused with the logic behind subnetting. i am learning from video tutorial and quite didn't get it
so my question is as below

Quote:
Subnetting a class C : 255.255.255.128/25 bits
After subnetting class c address by borrowing one bit from host to network , it leaves host with 7 bit and network increases bit to 25 bits .

According to my video tutorial it stated that theoretically class C address has
one network and 255 hosts. And after subnetting 2 networks with 127 hosts. but i dont think so .... Smilie

i believe class c has around 2097150 network and 255 hosts. i didn't get that logic . please help me get that right in my head.
# 2  
Say you have a class C network of 192.168.1.0/24 which has 254 usable host addresses, because 192.168.1.0 and 192.168.1.255 are not usable (2^8-2).

By changing the CIDR suffix to 25 , you create two networks:
Code:
192.168.1.0/25

and
Code:
192.168.1.128/25

The first has 126 usable host addresses (192.168.1.{1..126}, 2^7-2)
The second has 126 usable host addresses (192.168.1.{129..254}), 2^7-2)


--
The subnet mask of the /24 network is 255.255.255.0
The subnet mask of the two /25 networks is 255.255.255.128
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# 3  
thanks a lot for wonderful explanation. Does that means
Code:
192.168.1.0/24

has 254 usable host address and that 254 host has to share only one network address. ie i can have only one network and within that network i cannot have more than 254 computers.

And using that subnetting i get two network with each network having 126 usable host address.

can you please check what i quoted below make sense or its wrong

Quote:
SmilieSmilielogically every public address without subnetting has only one network address no matter its class A,B or C. With difference class it makes only difference with how much Host it will have .. is that correct SmilieSmilie

Last edited by lobsang; 03-11-2017 at 07:00 AM..
# 4  
Hi,

To put it as simply as possible, in any given network range the very first address and the very last address are not usable for hosts. The first address is the network address, and the last address is the broadcast address.

So for 192.168.1.0/24 you'd have:

192.168.1.0 - Network address (NOT usable for hosts)
192.168.1.1 through 192.168.1.254 - Free IPs (Usable for hosts)
192.168.1.255 - Broadcast address (NOT usable for hosts)

And for /25 sub-nets (and all others besides) it'd be the same: the first address in the range and the last address in the range are not usable for hosts.
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# 5  
i just came to learn that my public ip address through whatsmyip

Code:
46.126.40.51

since its class A IP address so default subnet mask is
Code:
46.126.40.51/8  ie 255.0.0.0

does that means apart from from 46.0.0.0 and 46.255.255.255. i can use all the rest for host. And i believe its possible within one network

And when some one bought a class A static IP address for personal use. does that means i can have around 16777214 host within a single network.

Last edited by lobsang; 03-11-2017 at 08:47 AM..
# 6  
Hi,

That's almost certainly not correct, no. Your Internet provider is extremely unlikely to have the whole of 46./8 allocated to them. That'd be over sixteen million usable IPs, and a fairly large chunk of the Internet. In all likelihood the network range is 46.126.40/24, but it could be anything smaller or a bit larger than that.

My advice in understanding this whole thing would be: forget about all this Class A, B, C stuff. The important thing is the netmask. In day-to-day life you'll seldom encounter any external Internet-live IP ranges that are larger than a /24. In private network ranges it's not unusual for /8 or /16 to be seen, certainly.

I think at this point it would also be helpful to explain what it is you're trying to do, and why you think you need a huge chunk of the 46/8 part of the Internet to do it.

Edited to add: also, there's no way you or anyone anywhere can buy a whole Class A ! The IPv4 address space is almost entirely exhausted worldwide at this point. What almost everyone on domestic or small office broadband does is use an internal private network range, like 10.x.x.x or 192.168.x.x, and then use NAT to talk to the outside world on a single IP or far smaller number of live IPs.
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# 7  
No. You ARE one, amongst many other clients, of the hosts that your provider hosts in their subnet. I'd bet they'd be not amused if you tried to use another IP in that range - if you were capable to do it at all.
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