Sub Netting

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

SUBNETTING FOR BEGINNERS v 2.0 J.T ´s QUICK REFERENCE ABOUT THE SUBNETTING PROCESS

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

SUB NETWORKS FOR BEGINNERS During the CISCO CCNA 1 at ITCR I have found that one of the most difficult to understand topics seems to be the subnetting, since it has to be done manually meaning no calculator allowed. Many people, including myself usually get frustrated because of this matter and neither the book nor the online course help that much. There might be better documents out there, even though I’ve done this myself just for fun and learning, I’ve spend a few hours trying to understand this subject and creating this step by step, of course it may content many errors, which I would really appreciate if whomever can let me know about them in order to improve the documentation. Thanks to my CCNA instructor Fabian Montenegro for his valuable help. As far as I understood from our instructor and the materials IPV4 is done, meaning that IP addresses are not enough this days due to the fast speed growing of the internet. While the world of networking moves towards the IPV6 there might be a temporary solution this solution is the subnetting by the way one among a few others. For someone to be able to understand this document these are the basic requirements: • • • • • •

-Basic knowledge of networking terminology, like IP classes, hosts, mask. -Understanding of English since I am presenting this document in English. -Paper. -Pen or pencil. -Well understanding of conversions from decimal to binary and vice versa. -Basic knowledge of math.(You don’t have to be a genius to get this easy).

Subnetting allows the network administrators to create subnets taking the host part of the IP, how?, well I will explain. I don’t pretend to teach networking and all that sort of stuff, just the subnetting process in a short, easy way, so I will develop an example as I write, adding notes, just like I would do in a regular notebook.

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

IP classes. First thing you outta know is that IP addresses are divided in classes. Classes are structured as follows: CLASS NAME A B C

BEGINS WITH 0 10 110

RANGE 0-127 128-191 192-223

IP addresses are usually presented using this format: 192.168.1.0 An IP address is divided in four different octets, for this matter we are going to pay special attention to the first octet, since it defines the default mask for the class. CLASS NAME A B C

RANGE 0-127 128-191 192-223

DEFAULT MASK N.H.H.H N.N.H.H N.N.N.H

As you can see in the mask there are network fields defined with N and host fields defined by H, subnetting can actually be defined as using the host fields of the IP to create subnets, the default mask can actually tell us what is for the network portion and what is for the host portion, but the actual field from which we are going to create the sub nets belongs to the IP. This will be explained in detail later on, for now you just have to understand this scheme after putting all together: CLASS C IP: 192.168.1.0, MASK 255.255.255.0 Default mask: N.N.N.H N

H

Up to this point, we have an IP Class C, with a N.N.N.H mask and we have to take a certain part to create subnets and the rest to keep it as the host portion of the address.

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

Let’s suppose that we have an IP address class C named 192.168.1.0, this address is a whole address that would allow you to create a single network with 256 hosts. 28 =256 ====èwith a mask = N.N.N.H or 255.255.255.0, for instance. Let’s say that we need smaller LAN’s with at least 12 hosts on each network, we can think about this like dividing a hotel in smaller rooms. Each room has the same size. There are two very easy formulas that we got to use in order to calculate this correctly, but first the big question we got to ask ourselves would be 2 powered to what is equals or higher than 12?? or: 2 ? > = 12 The answer is very easy and would be found using the following formulas: Total of hosts= 2h Total of networks = 2n We must learn two more concepts here before the development of an example in order to understand all how this works: Every network and sub network have a network address, you may understand this as the address that represents the subnet. Also every network and sub network have a broadcast address, we may understand this as the address that every computer inside of a network uses in order to send a communication for it to be listened by all other devices. For those who care about details this is a key: When all the bits of the host fields are ‘0’ you are looking at the network address, when all the bits of the host fields are in “1” you are looking at the broadcast address. Imagine networks an sub networks like this, you may understand where broadcast and network addresses are: Subnet o network address subnet 1

host 0 (network) host 1

subnet 2

host 2 subnet 3 subnet 4 subnet 5 broadcast address

host 3 host 4 host 5 (broadcast)

Now that we have defined all the theory needed to understand how subnetting works, I will develop an example step by step using the same class c.

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

Step 1: What is the problem? Let’s say that we have the class c address 192.168.1.0 and the company needs to have smaller networks than the one that they can create using the whole class C., the given mask is 255.255.255.0. They request every network to have 12 hosts. Step 2: How many hosts and how many networks can I create? Easy to calculate: 2 powered to what is equals or higher than 12?? or: Hosts: 2 h > = 12 2 4 = 16 Networks: 2 N > = 12 2 4 = 16 Graphically our instructor made us to figure out something like this: 128 N 1

64 N 1

32 N 1

16 N 1

8 H 0

4 H 0

2 H 0

1 H 0

1 àalways means network fields 0 àalways means host fields Seems like we will have 16 networks and 16 hosts for every network, but we have to remember that for every network will take 2 addresses for broadcast and network and every sub network will take two host addresses as broadcast and network addresses. So it will be this way: Usable Hosts: 2 h -2 2 h > = 12 2 4 = 16 -2=14 Usable Networks: 2 N -2 2 N > = 12 2 4 = 16 -2=14 Finally we are getting the number of sub networks and the number of hosts, which are 14 in each case.

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

Step 3: As we explained before the default mask follows this pattern, N.N.N.H or 255.255.255.0, in order for us to build the new mask please remember that a 1 always means a network field and a 0 always means a host field.

255

255

255

0

1

1

1

1

0

0

0

0

N 128

N 64

N 32

N 16

H 8

H 4

H 2

H 1

Taking those 4 that you can see in black and with a ‘’1’’ assigned now we can determine the new mask which is: 255 8 bits

255 8 bits

255 8 bits

240

The 240 comes out from the sum of the values according the positions of the bits of the network portion of the octet (128+64+32+16)= 240. Another common way to represent the sub network would be 255.255.255.240 /28, you may read this as ‘’slash 28’’ and comes from (8+8+8+4), in other words, 24 bits from the original first 3 octets of the mask plus 4 bits already taken from the last octet in order to create sub networks. Up to this point the procedure has been explained and the last thing remaining here is to create a table with the sub networks and the location of the hosts within the network, which would be pretty easy, disregarding if they are 2 or 2000.

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

#sr

Address

Host 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000

Network address 0000 (0) 0001 (1) 0010 (2) 0011 (3) 0100 (4) 0101 (5) 0110 (6) 0111 (7) 1000 (8) 1001 (9) 1010 (10) 1011 (11) 1100 (12) 1101 (13) 1110 (14) 1111 (15)

Sn O

192.168.1.X

192.168.1.0

SR1

192.168.1.X

192.168.1.16

SR2

192.168.1.X

192.168.1.32

SR3

192.168.1.X

192.168.1.48

SR4

192.168.1.X

192.168.1.64

SR5

192.168.1.X

192.168.1.80

SR6

192.168.1.X

192.168.1.96

SR7

192.168.1.X

192.168.1.112

SR8

192.168.1.X

192.168.1.128

SR9

192.168.1.X

192.168.1.144

Usable IP From 192.168.1.1 to 192.168.1.14

Broadcast

192.168.1.15

From 192.168.1.17 to 192.168.1.30 From 192.168.1.33 to 192.168.1.46

192.168.1.31

From 192.168.1.49 to 192.168.1.62 From 192.168.1.65 to 192.168.1.78 From 192.168.1.81 to 192.168.1.94 From 192.168.1.97 to 192.168.1.110 From 192.168.1.113 to 192.168.1.126 From 192.168.1.129 to 192.168.1.142 From 192.168.1.145 to 192.168.1.158

192.168.1.63

192.168.1.47

192.168.1.79

192.168.1.95

192.168.1.111

192.168.1.127

192.168.1.143

192.168.1.159

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

SR10

192.168.1.X

192.168.1.160

SR11

192.168.1.X

192.168.1.176

SR12

192.168.1.X

192.168.1.192

SR13

192.168.1.X

192.168.1.208

SR14

192.168.1.X

192.168.1.224

SR15

192.168.1.X

1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111 1111

0000 (0) 0001 (1) 0010 (2) 0011 (3) 0100 (4) 0101 (5) 0110 (6) 0111 (7) 1000 (8) 1001 (9) 1010 (10) 1011 (11) 1100 (12) 1101 (13) 1110 (14) 1111 (15)

192.168.1.240

From 192.168.1.161 to 192.168.1.174 From 192.168.1.177 to 192.168.1.190 From 192.168.1.193 to 192.168.1.206 From 192.168.1.209 to 192.168.1.222 From 192.168.1.225 to 192.168.1.238

192.168.1.175

From 192.168.1.241 to 192.168.1.254

192.168.1.255

192.168.1.191

192.168.1.207

192.168.1.223

192.168.1.239

The table shows in blue the network and the broadcast address of the class. Then there are 14 sub networks remaining. Each one contains 16 host names or IP addresses. The first one of every subnet is the network address and the last one is the broadcast address. The SR# field is the sub net number, starting always from subnet 0 to subnet 15 (16 in total).The address is the actual address class that the sub networks are going to have. The host field of the table shows the host names, they are 16, minus the broadcast and the network addresses, which are highlighted, becoming 14 available host names.

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT

Cisco CCNA, Level 1 Costa Rica ITCR, 2005

To calculate a certain network number without calculating all networks and hosts, just do the following: Example, Calculate Subnet 14, which is represented 11100000 in binary code. As a ´´tip´´ if you want to find the network 14 you have to ‘’assign the number 14’’ on the network side:

1

1

1

0

0

0

0

0

N 8

N 4

N 2

N 1

H

H

H

H

And then you might want to calculate the value for the last octet on that network address.

1

1

1

0

0

0

0

0

N 128

N 64

N 32

N 16

H 8

H 4

H 2

H 1

(128+64+32)= 224 192.168.1.224 Knowing the network address already it will be easy to calculate the specific hosts in a network or fully usable addresses as well as the broadcast address. 11100000 (192.168.1.224) net

SN14:

all range of 14 addresses here

11101111 (192.168.1.239) broad

Written by Jose Tijerino. Cisco CCNA, ITCR Send comments to: [email protected] No copyrighted material other than the logo has been used, free distribution authorized by JT