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Posted on 2011-10-08

Hello Expert , I am doing A ccna course , and really intrest me , now we got to the subnetting chapter and I find it hard one , I have got a book , and there are many ways finding soluations to ne problem , my question is is there a simple way where u can answer the most common question like , what network certain IP address is on? what is the subnet mask , the broadcast address and the range , pls I gt exam on monday and I want to do all I can to understand and pass the exam , what confuse me the most is the no of bits that r needed to design a network , or when they say for example you got 37.0.0.0 and ur requirement is at least 500 hosts per network however as many subnets as possible without exceeding 1000 subnets , I need explaining pls and not throw a website and I need to deal with it , I need to understand more than anything else . thks alot

5 Comments

All about subnetting is related to understanding of binary representation of adresses and masks and calculating with with 2^x.

So you have 37.0.0.0. Mask for this network is 255.0.0.0 or /8 (255 is equal to eight 1 in binary representation of subnet mask).

To have 500 hosts in one subnet, you have to find smallest "x" in 2^x > 500

2^1 = 2

2^4 = 4

2^3 = 8

...

2^9 = 512

So subnet you need should have last 9 zeroes: 255.255.254.0 or /23 (maximum nuber of 1's is 32, so 32-9=23).

That is for hosts.

To calculate number of subnets which contains 1000 /23 networks, do the following:

- first find suitable supernet mask:

2^x >= 1000

By same system as above x=10, so you need to add 10 zeros to existing 9, to have 19 in total.

Converting it to mask: 255.248.0.0 or /13.

To calculate final number of networks with 1000 subnets consisted of 500 hosts each, you have to find how many /13 networks are in /8.

2^(13-8)=2^5=32

I hope this clears things a bit, and I hope I understood your question well. If not please post original question from your test.

Regards!

Easiest way is to memorize all 2^x, where x is 1 to 10. In converting that to subnets, x defines number of 0's at the end of subnet mask.

In my previous post, whole process is described in more detail, but in essence it is just finding closest bigger 2^x (example: for 200 hosts, first bigger is 2^8=256).

Regards!

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