#vlsm

Subnetting - Proper Method

Procedure:

1. Determine the classful network mask if the prefix is not define. 2. Determine how many subnet bits needed for the required number of networks. Use this formula  2 raise to subnetid = number of required networks. And look for the new network mask (called subnet mask) 3. Determine the Delta (sometimes called Increment); where Delta = to the decimal value of the right most binary 1. 4. Plot the network, usable and broadcast address and its subnet mask

Example1:

Given: 192.168.35.0 How many networks do I need for above diagram? The Answer is 4. (Remember in CCNA each router interface is 1 broadcast domain) What are the IP address I can assign to each router interface and to hosts?

Let us use the procedure above.

1. Determine the classful network mask if the prefix is not define. Answer is 255.255.255.0 (since the given address is part of Class C)

2.Determine how many subnet bits needed for the required number of networks. Use this formula  2 raise to subnetid = number of required networks. And look for the new network mask (called subnet mask)

Needed networks = 4

We are after really about subnet bits.

Therefore;

3. Determine the Delta (sometimes called Increment); where Delta = to the decimal value of the right most binary 1.

The right most  binary 1 is located on the Octet 4. Using 128  64  32  16  8  4  2  1  value, Delta = 64

Subnet mask = 255.255.255.192 (/26)

4. Plot the network, usable and broadcast address using value of Delta and its subnet mask

4.a. plot the Network address;  Delta = 64

4.b. plot the Broadcast address. The last address before the next Network address is  a broadcast address.

4.c. plot the Usable address. These are the adress range between Network Address and Broadcast Address.

Please NOTE that we are not assigning Network and broadcast address to any devices.

Let us assign the IP address to each network device interfaces.

Subnetting - Shortcut method 

( Aha! so may shortcut pala! ) Actually you just need to memorize the procedure, formula and by practice.

Procedure (Looking for numberof network address)

1. Using our hand to compute the subnet bits. Using this Formula 2 4 8 16 32 64 128 256 512 1024 and so on...

2. Look Delta and subnet mask

3. Plot the network address

Example:

Given 192.168.20.0  : Required 16 network.

1. Using our hand to compute the subnet bits. Using this Formula 2 4 8 16 32 64 128 256 512 1024 and so on...

The formula I'm using is counting this on using my starting from  2  4  8  16  32 64 128 256 512 1024 and so on... since I need 16 network, using the formula above how many finger I used.

Above information - means that we need 4 subnet bits to create 16 network.

2. Look Delta and subnet mask

Delta = 16

Subnet Mask = 255.255.255.240 (/28)

Remember that Delta is the right most binary bit 1 (in decimal form) of a new netmask which is 16

For Subnet Mask - 128+64+32+16 = 240

3. Plot the network address

The increment of each subnet is 16 as we seen above. I only look for the NETID (network address) because from there we can identify the usable address and broadcast address.

Network Address for network1            = 192.168.20.0 /28 Usable address for network1           = 192.168.20.(1 - 14) /28 Broadcast address of network1       = 192.168.20.15 /28

Another example: Given: 172.16.0.0 /16 : Required network is 32 Formula: 2 4 8 16 32 64 128 256 512 1024 Answer: Subnet bits = 5

Procedure (Looking for number of host address)

1. Counting again by hand and using this formula again 2 4 8 16 32 64 128 256 512 1024..so on (minus 2 )

2. Plot the host portion

3. Look Delta and subnet mask

4. Plot the network address

Example: 192.168.0.0 ; required 16 hosts per subnet.

1. Counting again by hand and using this formula again 2 4 8 16 32 64 128 256 512 1024..so on (minus 2 )

Using this formula again 2 4 8 16 32 64 128 256 512 1024..so on (minus 2 ) Here, we should focus on the host portion. Means how many host bits (0) do we need to cater 16 hosts per subnet?

This example is tricky! Is it correct that we only need 4 host bits? Answer is NO. Remember, IP address block should have network and broadcast address. That is the reason why we need to subtract 2. because 16 -2  = 14 host only. We need to add another host bit.

And the correct answer should be below.

Host bits = 5 which is 32 -2 = 30 hosts per subnet.

2. Plot the host portion

OK! Now I know the host bits. How will i know the subnet mask ,   and NET ID for each subnet.

Check the given again. 192.168.0.0 is part of class C. Means that the default nework mask is /24 (or 255.255.255.0) Meaning we cannot modify the first 3 octet. Because subnetting is borrowing subnet bits from the host portion. In Class C network. the host portion is Octet4. Procedure here is to plot the "0s" from right to left. The rest make "1s".

3. Look Delta and subnet mask

From above table, we can now determine the other subnetting requirements.

4. Plot the network address

Another example: (This time try to use your mind to get the answer) Given: 10.16.0.0 /8; Required 100 users per subnet.

VLSM (Variable Length Subnet Mask)

This subnetting technique is commonly used by networks today to maximize the use of IP  address block by creating different subnet size. If you will notice, subnetting can divide classful network. But they still found out some waste of usable address. For example we have a IP block with a prefix of /25 - meaning 126 address are the usable address and for point-to-pint connection only 2 address are needed and the rest will be unused. And most of the time engineers are focus on usable address for each block when doing this.

Giving an example:

NOTE:

For VLSM - always create the subnet the largest first to smallest network requirement. The rest are the same with subnetting procedure. Let us start by defining the required network from largetst to smallest.

So we finish the VLSM table. Let us know assign these addresses to our network design above.

In practical world, VLSM is commonly use but you need to give allowance on your computation for your company future expansion. That is the reason, why large company uses class A (10.X.X.X) for their internal network communication.

Try to follow my example until you become familiar with the computation.

Supernetting / CIDR / Route summarization

I would say Supernetting is most commonly use by engineers for route summarization. It is a summarization of a multiple subnets under a common network prefix - also known as CIDR (Classles Inter Domain Routing). Procedure: 1. Plot the network address  in binary value. 2. On all of the network address, look for the significant bit which is common to all network address bits. Count the number of common bits 3. Find the summarize network address prefix and subnetmask

Example: On the diagram below, R1 wants to see a summarize network for each company. In order to provide the requirement. R2 and R3 needs to advertise their summarize network towards R1.

1. Plot the network address  in binary value.

2. On all of the network address, look for the significant bits which is common to all network address bits. Count the number of common bits

3. Find the summarize network address prefix and subnet mask.

Using the value we got above, we can now use it for R2 and R3 to advertise their summarize attached networks towards R1

In R1 routing table, it should be like this 192.168.0.0 / 19 via R2 directly connected address 172.16.0.0 /17 via R3 directly connected address