Thought I understood subnetting.!? Guess not!! 2

[butchrecon]

Well for the most part I understand subnetting. However I am a bit confused on the figuring out available subnets and total nodes for a given address.
Lets take the address 192.168.1.21 with a mask of 255.255.255.0. How can I get the total number of subnets and total nodes from that. I am having trouble understanding the formula (2^n -2) formula. Can anyone help? James Collins
Field Service Engineer
A+, MCP

email: butchrecon@skyenet.net

Please let us (Tek-tips members) know if the solutions we provide are helpful to you. Not only do they help you but they may help others.

 

[jimbopalmer]

I can help with the -2 part. the addresses 0 and 255 are considered unwise to use as the Local portion of an IP address ( in binary, all zeros and all ones, in case you were not subneting using a 255.255.255.0 mask)

so instead of 256 addresses, you only get 254

as for the 2^n part

2^8 is 256 as we have 8 zeros when 255.255.255.0 is converted to binary

hope it helps some, or generates corrections I tried to remain child-like, all I acheived was childish.

 

[CCIEWANNABE]

IP Address: 192.168.1.21
Address Class: C
Network Address: 192.168.1.0

Subnet Address: 192.168.1.0
Subnet Mask: 255.255.255.0
Subnet bit mask: 110nnnnn.nnnnnnnn.nnnnnnnn. hhhhhhhh
Subnet Bits: 24
Host Bits: 8
Possible Number of Subnets: 1
Hosts per Subnet: 254

Formula for hosts =
{2^ Host Bits: 8} – 2 = 254 hosts per subnet bits

Formula for subnets = after network portion applied to bits.
Class C uses three octets 110nnnnn.nnnnnnnn.nnnnnnnn. apply to mask gives one subnet value for last octet as 255.255.255.hhhhhhhh

Selected Subnet Address: 192.168.1.0 255.255.255.0
Usable Addresses : 254
Host range : 192.168.1.1 to 192.168.1.254
Broadcast : 192.168.1.255

Notice that the broadcast 192.168.1.255 use one of the bit values hence only one-subnet bit is used

Route once; switch many

 

[CCIEWANNABE]

Notice that the broadcast 192.168.1.255 use one of the bit values hence only one-subnet bit is used should be as

Notice that the broadcast 192.168.1.255 use one of the bit values hence only one-host bit is used. Route once; switch many

 

[butchrecon]

CCIEWANNABE
Here is one. If I have an IP of 131.13.124.12
How would I figure out the Subnet mask of this? James Collins
Field Service Engineer
A+, MCP

email: butchrecon@skyenet.net

Please let us (Tek-tips members) know if the solutions we provide are helpful to you. Not only do they help you but they may help others.

 

[TheOldMan346]

Wow! After the exhaustive treatment you gave the question in thread581-313899, I thought "This guy knows his sh...er, stuff!"

Actually, it comes down to the technical definition of subnetting. Subnetting allows you to create multiple logical networks that exist within a single Class A, B, or C network. As you know, the subnet mask determines the network and host portion of the IP address. Since the case you originally give is a class C and you are using a /24 subnet mask, there is only 1 subnet as CCIEWANNABE stated.

Hee is some info from Cisco on subnetting.

http://www.cisco.com/warp/public/701/3.html

As for the second question, you can't derive the subnet mask given a single IP address. You need to know the mask. That address would be a valid host with any of the following subnet masks.

Mask___________Subnets________Hosts
255.255.0.0 0 65534
255.255.128.0 2 32766
255.255.192.0 4 16382
255.255.224.0 8 8190
255.255.240.0 16 4094
255.255.248.0 32 2046
255.255.252.0 64 1022
255.255.254.0 128 510
255.255.255.0 256 254
255.255.255.128 512 126
255.255.255.192 1024 62
255.255.255.224 2048 30
255.255.255.240 4096 14
255.255.255.248 8192 6
255.255.255.252 16384 2

Does that help?
The Old Man

 

[butchrecon]

Dont get my question wrong. I know subnetting. There are a few things that are still fuzzy to me. This thread being one of them. I was not sure if you could derive a subnet mask form jsut an IP address. Guess not. The formula (2^n - 2) was a formula I had issues with. It has been cleared up. Thanks for the assistance. Those were my issues. James Collins
Field Service Engineer
A+, MCP

email: butchrecon@skyenet.net

Please let us (Tek-tips members) know if the solutions we provide are helpful to you. Not only do they help you but they may help others.

 

[jimbopalmer]

"How would I figure out the Subnet mask of this?"

So far as I know we are given a subnet mask, it is not derived from the address. you can do some reverse engineering to find a subnet mask with trace route, when does a trace wake up a router, but masks are not easy to get out of addresses.

I would love to be wrong on this one, as I would learn something new for sure. I tried to remain child-like, all I acheived was childish.

 

[CCIEWANNABE]

IP Address : 131.13.124.12
Address Class : B
Network Address : 131.13.0.0

Subnet Address : 131.13.0.0
Subnet Mask : 255.255.0.0
Subnet bit mask : 10nnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh
Subnet Bits : 16
Host Bits : 16
Possible Number of Subnets : 1
Hosts per Subnet : 65534

Route once; switch many

 

[CCIEWANNABE]

There is a classless and class full IP address design issue to consider. If it were classless it would have a subnet short hand extension. For example /18 or 255.255.192.0 If a router was using RIP AKA RIP v 1 it would know that this was a class full "B" address 10nnnnnn.nnnnnnnn..
Yet, if a router is using Rip v 2 protocol then we can get communication confusion. Rip v 2 protocol supports the use of VLSM.

If I were asked this question; I would respond as I posted as a B class address on subnet 131.13.0.0 because of the address restrictions going on today. There is a lack of valid IP address assignments open.

If the question was posted on a test just compare the answers. Classless can have any shorthand assignments. In example /18 /19 /20 /27 etc., or a variable sized Octet Value!
That’s the clue to look for in a question and answer session. So we assume this is a class full address and start with the first subnet design available. That’s good design practice.

Visit these Links:

http://webdesign.about.com/gi/dynam...ttp://support.wrq.com/tutorials/tutorial.html

Excellent, excellent source requires PDF Reader Adobe free download

http://www.3com.com/other/pdfs/infra/corpinfo/en_US/501302.pdf

Route once; switch many