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TOPIC: IP Subnetting - Number of Hosts

IP Subnetting - Number of Hosts 10 years 2 months ago #15830

I have a basic doubt regarding the IP subnetting. I have gone through the IP subnetting pages available in Firewall.cx.

I understand the purpose of subnetting is to split a Network into smaller networks using the subnet masks. My question is whether we can increase the number of hosts if we change the default subnet mask.

I found the following line from the page "IP Subnetting - The Basic Concepts"

"... see how we can achieve a Class C network with more than 254 hosts."

This is what which is confusing me. Can we increase the number of hosts for a Class C network from 254 ?

Please help me understand this !!
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Re: IP Subnetting - Number of Hosts 10 years 2 months ago #15837

  • Rockape
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Hi There,

Don't get too caught up on the class idea. Things are changing and Cisco kit (I believe) works with classful and classless networks. But to answer your question. Yes, you can increase the number of hosts on a network by subnetting. There is a document here on the site that explains it very well. It is quite a long document, but well worth the read. When I first came across subnetting on a course, it blew my mind. But after a few days I finally got the idea. So, take the time to read the document, and then throw any questions out to everyone else. You will get some good advice.
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Subnetting Idea 10 years 2 months ago #15841

  • ashish_hcl
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Hi ajathsatru,


Let me give you a brief idea and I believe this may help you

Class No of Possible Networks No of Host in that network grp
Class A (2 pwr 7 ) -2 (2 pwr 24) -2
Class B (2 pwr 14) -2 (2 pwr 16) -2
Class C (2 pwr 21 ) - 2 (2 pwr 8 ) -2 = 254


the above information is directly informs abt the network portion and host potion of an Ip address in a particular class.
However relating it to your question let me tell you that subnetting would always have less no of host then the host mentioned above for full class A, B or C network so I am sure that answers your question
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Re: IP Subnetting - Number of Hosts 10 years 2 months ago #15884

  • Rockape
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Hi there,

Right, I've had a think about this, and had a quick chat with the bishop to confirm them, so here goes.

You don't really increase the number of hosts on your network (but you do :shock: )

What you do is "steal" "unused" parts of your LAN, and create another IP range. This allows you to "add" host to your network.

For example, if you have an IP range that allows for 30 hosts, but you only use10 (for example). You can "steal" the other 20 host allocations, and create another "mini" LAN and assign some more hosts to that.

Myself and the bishop discussed this, and believe in most real life environments you wouldn't do this. However, there may be certain situations where you need to (but we won't go into that).

So, yes it is possible to get more, but normally you wouldn't need to.

Not sure if that has answered your question. So, if you have a specific problem you are trying to solve, let me know.

Cheers
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Re: IP Subnetting - Number of Hosts 10 years 2 months ago #15889

  • d_jabsd
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What you do is "steal" "unused" parts of your LAN, and create another IP range. This allows you to "add" host to your network.

For example, if you have an IP range that allows for 30 hosts, but you only use10 (for example). You can "steal" the other 20 host allocations, and create another "mini" LAN and assign some more hosts to that.

this is correct, but I want to clarify it for those that do not understand the ins and outs of subnetting, by using real numbers.

using the above example as a base, well start with a subnet that allows 30 hosts. I am also going to assume that one of the hosts, is the gateway.

30 hosts is a /27 (in CIDR notation) -netmask is 255.255.255.224 with a total address space of 32 addresses (2 burned on network and broadcast)

If you are only using 10 of these and you want to steal part of it for another smaller subnet, you can't take all 20, but you can split the subnet in half to give yourself 2 smaller subnets.
This will give you 2 subnets that can support 14 hosts each

14 hosts = /28 -netmask is 255.255.255.240 with a total address space of 16 addresses (2 burned on network and broadcast)

When splitting a larger subnet into smaller subnets, you cannot arbitrarily pick a block and call it subnet. They must begin and end on proper boundries.

The easiest way to do this is to use halving.

a /24 can be split in half to give 2x /25
254 hosts -> 2x 126 hosts

a /25 can be split in half to give 2x /26
126 hosts -> 2x 62 hosts

a /26 can be split in half to give 2x /27
62 hosts -> 2x 30 hosts

a /27 can be split in half to give 2x /28
30 hosts -> 2x 14 hosts

a /28 can be split in half to give 2x /29
14 hosts -> 2x 6 hosts

a /29 can be split in half to give 2x /30
6 hosts -> 2x 2 hosts

a /30 can be split in half to give 2x /31
2 hosts -> 2x 1 host (legal subnet for point to point links where broadcast address is unnecessary the other address is the network)

a /31 can be split in half to give 2x /32
a /32 is a host only address used quite a bit for loopback addresses or for assigning mulitple addresses from the same subnet to a single interface.


Hopefully this doesn't confuse people even more, but with all the discussion on subnetting lately, i thought I would share my method of quickly determining block size and their respective boundries.
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Re: IP Subnetting - Number of Hosts 10 years 2 months ago #15897

  • Rockape
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d_jabsd,

Couldn't have put it better myself.

Nice reply. :)
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