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how to subnet - DOC by 08zTHnc


									how to subnet: by John Moore CCNA

The key thing with subnetting is using the shortcut methods. Also practice, practice, practice!

Try once you are able to answer the questions using the shortcuts it comes
to you a lot easier. Practice converting to binary to decimal and over and over.


To find the range or increment number use 1 or both of the following:

If you have a subnet mask of convert the "interesting octet" this case 192 to
1100 0000

The last 1 in binary from LEFT to RIGHT is the increment, which is 64.

Or the second shortcut is subtracting 256 from the interesting octet:
256 - 192 = 64

Now that you have the range you can start from 0 then increment by the "increment" number.




255 (broadcast for 192 subnet)

Note: "Never go above the 255"

Finding the range is easy once you have the increments.
Start with the subnet number and add 1
Then go to the next subnet number and subtract 2 for your last usable:
Your broadcast is 63, and your range is 1-62 usable.

If your IP's on your interfaces fall within this range you know that it is a usable IP, otherwise it is
a broadcast or a subnet id.
Sometimes with IP's in the high number range it is easier to convert the "interesting octet" to
binary and do the AND operation against its mask. This way you find the subnet ID faster if you
are working with several subnets.

For example

You have an ip of

mask of

If you did the shortcut you would come up with an increment of 4 in the third octet.
Note always be mindful of what octet you are working in.





but instead of figuring out all the increments it's easier (imo) to convert to binary and do the AND
We are working in the interesting octet, 3rd octet.

200 = 1100 1000

Mask = 1111 1100

1100 1000
1111 1100
1100 1000
subnet id =

We know our increment of 4
First usable
Last usable =
Broadcast =
Next subnet

ip-subnet zero

Routers today have this turned on by default but if it is not turned on the question will reflect this
on the exam. As a rule this is turned on by default unless otherwise stated in the question.
What does it mean if it is not turned on?
That the 0 subnet and the last broadcast subnet cannot be used.
For example, and
Our equation for the amount of subnets equals 2^S-2=Subnets
If it is turned on then it is 2^S=Subnets

S=amount of bits that are turned 1 or "on".


Variable Length Subnet Mask

This is used to subnet into multiple smaller networks to allow for efficient use of IP addresses,
increments will be different depending on the number of hosts needed per network.

For example, you are assigned network but you need a certain amount of hosts with
different networks.

Given the above assigned network provide sufficient addresses for the proposed scenario without
waisting IP's.

2 leased lines connecting 3 routers. Router A with direct link to Router B and Router C (router's b
and c do not connect to each other)
network with 10 hosts
network with 25 hosts
network with 50 hosts

With the above we know that we have 2 point-to-point links between Router A and Router B and
Router A and Router C.
We start with network

We need 2 ip addresses for RouterA and RouterB
We need to identify how many bits in this address we need:
our mask is
for 2 addresses we need 2 bits
2^2-2 = 2
or in binary 0000 00 00 last 2 bits are needed for the hosts
Now we can turn the rest of the bits into 1's to give us our mask
(last octet shown in binary) 1111 1100

What are our usable IP's? Well let's look at our "increment".
256-252 = 4

Network starts with 0 and our next network is 4 = subnet id or Network = first usable address = last usable address = broadcast (not usable) = The NEXT subnet id or network

So routera would have IP of and routerb would have IP of both with mask of

Now lets work on the point-to-point link from routera to routerc
We already have our next network of from above lets increment again by 4   = second network   = first usable address   = last usable address   = broadcast (not usable)   = The NEXT subnet ID or network

Router A other interface IP will be routerc point-to-point interface will be
both with mask of

Now that we have our point-to-point interfaces we need to figure out what are networks attached
to each router LAN interfaces should be.

First we should identify the largest network then go down from there. 50 is the largest amount so
lets start here.

We need to count how many bits do we need to achieve at least 50 hosts.

We need 6 bits to achieve our desired amount of hosts:
1100 0000
Where does the last 1 read from left to right fall into? Hopefully you said the 64 place. Or if you
use the other shortcut of 256-192 = 64.

Now starting from our last network we have a subnet mask of what?

Well we need 6 bits for the hosts so now how many bits did we have left over?
we had 2 left over in which equals or /26
so our network needing 50 hosts looks like this: = subnet ID = First usable address = Last usable = Broadcast address (not usable) = Next Network

We achieved the next network by adding the 64 increment (our increment changed as above) to the
subnet ID.

Our next highest number of hosts is 25 hosts.
How many bits do we need for the hosts portion?

2^H-2 = hosts

Answer is 5 bits
2^5-2 = 30 hosts

Our next network above is since we only took 5 bits we are left over with 3 bits for
the subnet mask:
1110 0000

This equals 224 or or /27

Our increment is 32 so our next network is?

72 + 32 = 104 = Subnet id for 25 hosts = first usable address = last usable address = broadcast address (not usable) = Next usable network
Lastly we need 10 hosts.

Can you figure out the rest?
How many bits do we need for 10 hosts?
What is our increment?
What is the subnet mask for this network?

192.168.1.???   = Subnet ID for 10 hosts
192.168.1.???   = first usable address
192.168.1.???   = last usable address
192.168.1.???   = broadcast address
192.168.1.???   = next network

How many bits do we need for 10 hosts? 4
What is our increment? 16
What is the subnet mask for this network? or /28 = Subnet ID for 10 hosts = first usable address = last usable address = broadcast address = next network
So what happens to the rest of the IP’s ranging from to They are used
for future use and the above allows you to utilize all available IP addresses with no wasted

Note: only routing protocols that can utilize VLSM is RIPv2 (not 1), OSPF, and EIGRP.

Route Summarization

Route Summarization is used to allow routers to summarize smaller networks in to larger networks to
allow for less router updates and lower route tables, so less CPU and memory utilization on the router.
For example let’s say we have networks:
All on routerb

All on routerc

On routera

To summarize we convert the IP’s into binary:
1011 0001.0001 0000.0000 0001.0000 0000
1011 0001.0001 0000.0000 0010.0000 0000
1011 0001.0001 0000.0000 0011.0000 0000

Starting from left to right we mark all the “like” bits until we find the bits that do not match.
In this example we identify that in the 3rd octet we have all a match up to the 22nd bit. Now turn all of
these to 1’s and everything to the right to 0’s.
1111 1111.1111 1111.1111 1100.0000 0000
Our mask is now and all networks with would match each so our
advertisements to router A or C would only need to be the network.

See if you can figure out the other networks route summarization.

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