SUBNETTING

 

Understanding IP Addressing

The overall phenomenon of logical addressing works on the Layer-3 of the OSI reference model and the network components like routers and switches are the host devices that are most popularly used.

An IP Address is a 32-bit logical address that distinctively classifies a host of the network. The host can be a computer, Laptop or any other device (including Mobile)

The 32 bits binary IP address is made up of two distinctive parts i.e. The Network address and the Host address.

It also has 4 octets as each octet is having 8 bits. This octet is converted into decimal and is separated by a format i.e. dot. Thus it is represented in a dotted-decimal format. The range of an octet in binary is from 00000000 to 11111111 and in decimal from 0 to 255.

Example of an IP Address format:

192.168.1.64 (in decimal)

11000000.10101000.00000001.01000000 (in binary).

 

The binary one is difficult to memorize thus, in general, the dotted decimal format is used worldwide for representation of the logical addressing.

 

Let’s understand in detail how the binary octet values are converted into decimal values:

There are 8 bits and each bit has the value of 2 to the power n (2^n). The rightmost have the value 2^0 and left most have the value 2^7.

So the value of each bit is as follows:

2^7 2^6 2^5 2^4 2^3 2^2 2^1 2^0 (^ denotes the power)

Thus the result would be:

128+ 64+ 32+ 16+ 8+ 4+ 2+ 1

When all the bits are 1 then the values come out to be 255 (128+64+32+16+8+4+2+1= 255).

Suppose all the bits of an octet is not 1. Then see how we can calculate the IP address:

1 0 0 1 0 0 0 1, 128+0+0+16+0+0+0+1= 145.

By combining the bits of the octets in different combinations according to the need, we can derive the overall IP address of the desired network. As per the requirement, these are divided into various classes of a network called as class A, class B, class C, class D, and class E.

Most popularly class A, B and C are used for commercial purposes and class D and E have reserved rights.

Network Classes And Subnet Mask

The organization which governs the internet has divided the IP addresses into different classes of the network.

Each class is identified by its subnet mask. By the categorization of a default subnet mask, we can easily identify the class of an IP address of the network. The first octet of an IP address identifies the particular class of an IP address.

The classification is shown with the help of the below table and figure.

Class	Ist octet Decimal Range	Network/Host ID	Default subnet mask
A	1 to 126	N.H.H.H	255.0.0.0
B	128 to 191	N.N.H.H	255.255.0.0
C	192 to 223	N.N.N.H	255.255.255.0
D	224 to 239	Reserved for Multicasting
E	240 to 254	Experimental

 

 

• The class ‘A’ Address ranging from 127.0.0.0 to 127.255.255.255 cannot be used and is reserved for loopback and diagnostic functions. The numbers of hosts which can be connected to this network are greater than 65536 hosts.
• The number of hosts connected within the class B networks is from 256 to 65534 hosts.
• The number of hosts connected within the class C network is less than 254 hosts. Therefore the class C network mask is perfect for the minor networks which are known as subnetworks. We utilize the bits from the last octet of class C for constructing mask. Thus we need to rearrange and optimize the subnet depending upon the availability of the bits.

 

 

 

 

Subnet Mask	Last octet binary Value	No. of hosts connected
255.255.255.128	10000000	126
255.255.255.192	11000000	62
255.255.255.224	11100000	30
255.255.255.240	11110000	14
255.255.255.248	11111000	6
255.255.255.252	11111100	2

 

 

Let us assume the case of a class A IP address:

For Example, take a pair of IP address and subnet mask 10.20.12.2 255.0.0.0

#1) Convert this Combination into a binary value:

#2) The bits corresponding to the subnet mask with all 1’s represent the network ID as it is a class A network and the first octet represents the network ID. The bits corresponding to all 0’s of the subnet mask is the host ID. Thus the network ID is 10 and the host ID is 20.12.2

#3) From the given subnet, we can also calculate the IP range of a particular network. If the IP is 10.68.37.128 (assuming class A case)

Subnet mask: 255.255.255.224
IP range =256-224= 32.
Out of 32 IP’s, ideally one is used for the gateway, second is for the network IP and the third is for broadcast IP.
Thus total usable IP’s are 32-3= 29 IP’s.

The IP range will be 10.68.27.129 to 10.68.27.158.

Subnetting

Subnetting allows us to create various sub-networks or logical networks within one network of a particular class of the network. Without subnetting, it is almost unrealistic to create big networks.

For constructing a big networking system, every link must have a unique IP address with every device on that linked network which is being the participant of that network.

With the help of a subnetting technique, we can split the large networks of a particular class (A, B or C) into smaller subnetworks for inter-connection between each node which are situated at different locations.

Each node on the network would have a distinctive IP and subnet mask IP. Any switch, router or gateway that connects n networks has n unique Network ID and one subnet mask for each of the network it interconnects with.

The formulae of subnetting is as follows:

2^n >= requirement.

The formulae of a number of hosts per subnet is as follows:

2^n -2

Now let’s understand the overall process with the help of an Example:

We have taken an example of Class C network ID with a default subnet mask.

Suppose Network ID/IP address is: 192.168.1.0

Default Subnet mask: 255.255.255.0 (in decimal)

Default Subnet mask: 11111111.11111111.11111111.00000000 (in binary)

Thus the number of bits are 8+8+8+0= 24 bits. As mentioned earlier, for subnetting in class C network, we will borrow bits from the host portion of the subnet mask.

Therefore, to customize the subnet as per requirement:

We take a subnet mask of 255.255.255.248 (in decimal)

11111111.11111111.11111111.11111000 (in binary).

From the above binary notation, we can see that the last 3 bits of the last octet can be used for host ID addressing purpose.

Thus the number of subnets= 2^n = 2^3= 8 subnets (n=3).

Number of hosts per subnet= 2^n -2= 2^3 -2= 8-2= 6 Subnets i.e. usable Host IP.

Now the IP addressing scheme is as follows:

Network IP	First Usable IP	Last Usable IP	Broadcast IP
192.168.1.0	192.168.1.1	192.168.1.6	192.168.1.7
192.168.1.8	192.168.1.9	192.168.1.14	192.168.1.15
192.168.1.16	192.168.1.17	192.168.1.22	192.168.1.23
192.168.1.24	192.168.1.25	192.168.1.30	192.168.1.31
192.168.1.32	192.168.1.33	192.168.1.38	192.168.1.39
192.168.1.40	192.168.1.41	192.168.1.46	192.168.1.47
192.168.1.48	192.168.1.49	192.168.1.54	192.168.1.55
192.168.1.56	192.168.1.57	192.168.1.62	192.168.1.63

The subnet mask for all the above IP’s in the table is common i.e. 255.255.255.248.

With the help of the above example, we can clearly see, how subnetting helps us to construct inter-networking between various links and nodes of the same subnetwork. All these above IP’s can be used for inter-networking the devices within the overall network.

Note: Subnet mask is most widely used everywhere in a computer networking system. Hence, there is one more method to represent the subnet mask of a particular network which is chosen and standardized as it is easy to denote and memorize.

Subnet mask– 255.255.255.248 (binary)

11111111.11111111.11111111.11111000 (decimal notation)

From the decimal notation we can calculate the number of bits having 1 in each octet:

8+8+8+5= 29

Thus the Subnet mask can be denoted as /29.

With Network ID it can be denoted as 192.168.1.9/29.

From the above notation, anyone who knows the standard notation and formulae of subnetting can understand that the IP is using a subnet mask of 255.255.255.248 or /29.

The different Subnetting scheme in binary and decimal notation is shown below:

Subnet Mask	Notation in decimal	Notation in Binary	Number of Usable IP
/24	255.255.255.0	11111111.11111111.11111111.00000000	254
/25	255.255.255.128	11111111.11111111.11111111.10000000	126
/26	255.255.255.192	11111111.11111111.11111111.11000000	62
/27	255.255.255.224	11111111.11111111.11111111.11100000	30
/28	255.255.255.240	11111111.11111111.11111111.11110000	14
/29	255.255.255.248	11111111.11111111.11111111.11111000	6
/30	255.255.255.252	11111111.11111111.11111111.11111100	2

The ‘/’ notation method of the subnet mask is most widely used as it is easy to memorize and the binary notation and decimal are very lengthy in size.

As we are denoting the mask scheme while interconnecting the network components through the figure, if we use the decimal and binary method then the overall diagram will become very complex and difficult to understand.

There are so many IP’s on the platform to be shown and it becomes difficult to memorize as well. Thus generally, people who are familiar with routing and IP addressing scheme use short notation methods in figures and diagrams.

Example 1:

 

Understanding Subnetting with an Example of Interconnection of Network Devices:

The above figure shows how subnetting is used for interconnection of subnetworks. Firstly, as per our need for the number of hosts required to be connected and meet the other requirements of the network, we customize the subnet mask and network ID accordingly and assign to the devices thereafter.

The above network is using class C network mask and /29 subnet mask means network IP can be divided into 8 subnets. Each router has a unique IP address for each linked subnetwork.

There is an important point to be noticed that the more the bits we carry from the subnet mask for host ID then the more will be the subnets obtainable for the network.

Example 2:

Class B Network:

Subnet mask	Notation in binary	Number of Usable IP	Number of Subnets
255.255.128.0	11111111.11111111.10000000.00000000	32766	2
255.255.192.0	11111111.11111111.11000000.00000000	16382	4
255.255.224.0	11111111.11111111.11100000.00000000	8190	8
255.255.240.0	11111111.11111111.11110000.00000000	4094	16
255.255.248.0	11111111.11111111.11111000.00000000	2046	32
255.255.252.0	11111111.11111111.11111100.00000000	1022	64
255.255.254.0	11111111.11111111.11111110.00000000	510	128
255.255.255.0	11111111.11111111.11111111.00000000	254	256
255.255.255.128	11111111.11111111.11111111.10000000	126	512
255.255.255.192	11111111.11111111.11111111.11000000	62	1024
255.255.255.224	11111111.11111111.11111111.11100000	30	2048
255.255.255.240	11111111.11111111.11111111.11110000	14	4096
255.255.255.248	11111111.11111111.11111111.11111000	6	8192
255.255.255.252	11111111.11111111.11111111.11111100	2	16384

The above table shows the details of the number of subnets and hosts that can be connected per subnet mask by using Class B subnetting Scheme.

For connecting a host in big quantity and WAN communication systems, the Class B subnetting is very effective as it gives a wide range of IP’s for configuration.