VLSM maximum subnets calculation - why this amount ?

[SJ K]

Hi all,

I chance upon this website http://www.ip-calc.com/ , enter the following

ip address - 192.168.1.0
subnet mask - 255.255.255.252  (cidr 30).

The results is that I can have 4 maximum subnets and a maximum of 2 addresses.

Q1) Can some gurus advise on how the calculation of the 4 maximum subnets is done ?  I have use 30bits for network ID, and only left with 2 bits for hosts = which give me a total of 4 IP addressees, in which 1 is the network address and the another is the broadcast address.

Please advise.
Regards,
Noob

[Jon Marshall]

You just had the bad luck to use a site with a faulty calculator.

Try this site instead -

http://www.subnet-calculator.com/

Jon

[SJ K]

Hi Jon,

Its the same if i click on the CIDR option on the website you have given
4 maximum subnets for 192.168.1.0/30.

 

Regards,
Noob

[Jon Marshall]

Okay, I understand.

If you use the CIDR option it assumes one IP subnet.

The "Maximum subnets" bit is just the number of bits you are using for the host part.

If you want to see how may subnets you can have with a network and subnet mask use the "Subnets" option.

Jon

[SJ K]

Hi Jon,

Thanks for the reply.

What is "one IP subnet" ? Can a subnet have only 1 IP ?

Don't really understand the maximum subnet portion in the CIDR option.
What is it actually implying ?

Hope you can shed further light into it.

Thanks

Regards,
Noob

[Jon Marshall]

Don't think of it as a subnet.

What that field is showing in the CIDR option is the number of bits you can use for hosts.

Jon

[SJ K]

Hi Jon,
 

192.168.1.0/30
Maximum subnet = 4
Maximum address = 2

You mention in the previous thread that the field "Maximum subnet"  is the number of bits you can use for hosts

Do you mean bits or IP addresses ?

Regards,
Noob

[Jon Marshall · Accepted solution]

Do you mean bits or IP addresses ?

Another good question :-)

It's not the number of bits or useable IP addresses.

It seems to be an addition of the subnet address itself, plus the useable IP addresses in that subnet, plus the broadcast address for the subnet.

Apologies for the confusion.

Jon

[SJ K]

Thanks Jon,

Sorry for troubling you over such minor questions.

Regards,
Noob

[Jon Marshall]

No trouble at all.

They aren't minor questions and I didn't help because I should have looked more carefully at what it was showing.

It was my fault it wasn't clear.

Jon

[SJ K]

Hi Jon,

Sorry to message you here, I have posted a discussion since yesterday, but I just cant get it to show in the Discussions

https://supportforums.cisco.com/discussion/12458376/uplink-ports-and-actual-sfp-port-switches-please-help

Can you let me know if you can see it ?

Regards,
Noob

[Jon Marshall]

No I can't see it.

When I click on the link it tells me I am no authorised to view the page.

Not sure how you have managed to do that :-)

Jon

[pmanolak]

Its pretty cleared. It is said the maximum subnet you can divide the 192.168.1.0/30 and the number of /30 subnets that you have from the classfull ip 192.168.1.0/24, which is actually 128 maximum subnets.

The confusion I assume has to do that the response says for maximum 4 subnets and maximum of 2 hosts. But this is not happens at the same time.

If we subnet from /30 to /31 we have 2 subnets and 4 hosts (2 hosts per subnet).

If we sunbet from /30 to /32 we have 4 subnets with again 4 hosts (1 host per subnet. used usually for loopack).

here are the results:

$ ipcalc 192.168.0.1 /30 /31
Address: 192.168.0.1 11000000.10101000.00000000.000000 01
Netmask: 255.255.255.252 = 30 11111111.11111111.11111111.111111 00
Wildcard: 0.0.0.3 00000000.00000000.00000000.000000 11
=>
Network: 192.168.0.0/30 11000000.10101000.00000000.000000 00
HostMin: 192.168.0.1 11000000.10101000.00000000.000000 01
HostMax: 192.168.0.2 11000000.10101000.00000000.000000 10
Broadcast: 192.168.0.3 11000000.10101000.00000000.000000 11
Hosts/Net: 2 Class C, Private Internet

Subnets after transition from /30 to /31

Netmask: 255.255.255.254 = 31 11111111.11111111.11111111.1111111 0
Wildcard: 0.0.0.1 00000000.00000000.00000000.0000000 1

1.
Network: 192.168.0.0/31 11000000.10101000.00000000.0000000 0
HostMin: 192.168.0.0 11000000.10101000.00000000.0000000 0
HostMax: 192.168.0.1 11000000.10101000.00000000.0000000 1
Hosts/Net: 2 Class C, Private Internet, PtP Link RFC 3021

2.
Network: 192.168.0.2/31 11000000.10101000.00000000.0000001 0
HostMin: 192.168.0.2 11000000.10101000.00000000.0000001 0
HostMax: 192.168.0.3 11000000.10101000.00000000.0000001 1
Hosts/Net: 2 Class C, Private Internet, PtP Link RFC 302

--------------------------------------

$ ipcalc 192.168.0.1 /30 /32
Address: 192.168.0.1 11000000.10101000.00000000.000000 01
Netmask: 255.255.255.252 = 30 11111111.11111111.11111111.111111 00
Wildcard: 0.0.0.3 00000000.00000000.00000000.000000 11
=>
Network: 192.168.0.0/30 11000000.10101000.00000000.000000 00
HostMin: 192.168.0.1 11000000.10101000.00000000.000000 01
HostMax: 192.168.0.2 11000000.10101000.00000000.000000 10
Broadcast: 192.168.0.3 11000000.10101000.00000000.000000 11
Hosts/Net: 2 Class C, Private Internet

Subnets after transition from /30 to /32

Netmask: 255.255.255.255 = 32 11111111.11111111.11111111.11111111
Wildcard: 0.0.0.0 00000000.00000000.00000000.00000000

1.
Hostroute: 192.168.0.0 11000000.10101000.00000000.00000000
Hosts/Net: 1 Class C, Private Internet

2.
Hostroute: 192.168.0.1 11000000.10101000.00000000.00000001
Hosts/Net: 1 Class C, Private Internet

3.
Hostroute: 192.168.0.2 11000000.10101000.00000000.00000010
Hosts/Net: 1 Class C, Private Internet

4.
Hostroute: 192.168.0.3 11000000.10101000.00000000.00000011
Hosts/Net: 1 Class C, Private Internet

Hosts: 4

I suppose the confusion is by the way the info presented.
Due to this, I prefer the terminal app ipcal and non of the online calculators.

I hope the above help and not mess the things more that previously!!

[Joseph W. Doherty]

---

@pmanolak wrote:

If we sunbet from /30 to /31 we have 4 subnets with again 4 hosts (1 host per subnet. used usually for loopack).

I suspect you meant:

If we subnet from /30 to /32 we have 4 subnets with again 4 hosts (1 host per subnet. used usually for loopback).

[pmanolak]

Indeed Joseph. I corrected.

Thanks for mentioning. Typo error!