Help with this configuration.

lazystyle · 2008/10/09

Hi everyone, could someone help me work out what is wrong with the following configuration, or point me in the direction of how I can solve it. It is a question from a job application.

Host IP address: 172.16.55.36
Subnet mask: 255.255.255.240
Default Gateway: 172.16.55.1
DNS servers: 172.18.1.1, 172.18.1.2

Thanks for looking.

Scott Smith · 2008/10/09

That host cant see the gateway or DNS server with that subnet mask.
.240 gives you 16 addresses.

From Port forward .com

Okay I'm going to change our subnet from 255.255.255.0 to 255.255.255.240. Lets say the ip address of our computer is 192.168.1.132. How many ip addresses do we have? Well lets convert the subnet to binary. You should get the following binary subnet.

11111111.11111111.11111111.11110000

Okay, we have four zeros. So take 2^4 which equals 16. Alternatively 2*2*2*2 equals 16 as well. We have 16 ip addresses in our range. Well lets draw out our table. We can't draw our ip address table without knowing where the range starts. This is how you figure that out. We take our subnet and AND it to our ip address converted into binary. Go ahead and conver our ip address into binary. You should get the following.

11000000.10101000.00000001.10000100 = 192.168.1.132

Now we AND that with our subnet. It is easy to think of ANDing as finding the truth of two numbers. Every 1 is true and every 0 is false.

True and true is true! 1 and 1 is 1.
True and false is false. 1 and 0 is 0.
False and true is false. 0 and 1 is 0.
False and false is false. 0 and 0 is 0.

So lets line up our subnet and our ip address.

11000000.10101000.00000001.10000100 = 192.168.1.132
11111111.11111111.11111111.11110000 = 255.255.255.240
-----------------------------------
11000000.10101000.00000001.10000000

So look at the first column. We have a 1 and a 1. 1(true) and 1(true) = 1(true) The next column is the same thing. The thrid column is a 0 and a 1. 0(false) and 1(true) = 0(false). Continue doing that for the whole number. Now that we have our result lets take it and convert it back to base 10, so we can get the first ip address in our range.

11000000.10101000.00000001.10000000 = 192.168.1.128

Great! Now that we have our first ip address and we know that we have 16 ip addresses, we can make our table.

192.168.1.128 - Subnet Address
192.168.1.129 - Gateway
192.168.1.130 -
192.168.1.131 -
192.168.1.132 - Our computer!
192.168.1.133 -
192.168.1.134 -
192.168.1.135 -
192.168.1.136 -
192.168.1.137 -
192.168.1.138 -
192.168.1.139 -
192.168.1.140 -
192.168.1.141 -
192.168.1.142 -
192.168.1.143 - Broadcast Address

Well that gives you a pretty good idea of how computers figure out subnetting. You should have a good idea what you network should look like now, and what subnetting is. Let me know if you found this useful by using our Comments page!
Click to expand...

ReggieB · 2008/10/09

I wrote this tool a couple of years ago:

http://www.nicholshayes.co.uk/Node2Node.html

You can use it to play with pairs of IP addresses to see if they will talk together. If you put the computer's IP address in as PC1 and the gateway router address as PC2, you can have a play around to see what addresses work together.

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Help with this configuration.

lazystyle Inactive Thread Starter

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Help with this configuration.

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