SUB NETTING WITH EXAMPLES

Subnetting is a process of dividing large network into the smaller networks based on layer 3 IP address. Every computer on network has an IP address that represent its location on network. Two version of IP addresses are available IPv4 and IPv6. In this article we will perform sub netting on IPv4.

IPv4

IP addresses are displayed in dotted decimal notation, and appear as four numbers separated by dots. Each number of an IP address is made from eight individual bits known as octet. Each octet can create number value from 0 to 255. An IP address would be 32 bits long in binary divided into the two components, network component and host component. Network component is used to identify the network that the packet is intend for, and host component is used to identify the individual host on network.
IP addresses are broken into the two components:

Network component :- Defines network segment of device.
Host component :- Defines the specific device on a particular network segment

IP Classes in decimal notation

    Class A addresses range from 1-126

    Class B addresses range from 128-191

    Class C addresses range from 192-223

    Class D addresses range from 224-239

    Class E addresses range from 240-254

• 0 [Zero] is reserved and represents all IP addresses.
• 127 is a reserved address and is used for testing, like a loop back on an interface.
• 255 is a reserved address and is used for broadcasting purposes.

Subnet mask

Subnet mask is a 32 bits long address used to distinguish between network address and host address in IP address. Subnet mask is always used with IP address. Subnet mask has only one purpose, to indentify which part of an IP address is network address and which part is host address.
For example how will we figure out network partition and host partition from IP address 192.168.1.10 ? Here we need subnet mask to get details about network address and host address.

• In decimal notation subnet mask value 1 to 255 represent network address and value 0 [Zero] represent host address.
• In binary notation subnet mask on bit [ 1] represent network address while off bit[0] represent host address.

In decimal notation

    IP address       192.168.1.10

    Subnet mask      255.255.255.0

Network address is 192.168.1 and host address is 10.
In binary notation

    IP address 11000000.10101000.00000001.00001010

    Subnet mask 11111111.11111111.11111111.00000000

Network address is 11000000.10101000.00000001 and host address is 00001010

Network ID

First address of subnet is called network ID. This address is used to identify one segment or broadcast domain from all the other segments in the network.

Block Size

Block size is the size of subnet including network address, hosts addresses and broadcast address.

Broadcast ID

There are two types of broadcast, direct broadcast and full broadcast.
Direct broadcast or local broadcast is the last address of subnet and can be hear by all hosts in subnet.
Full broadcast is the last address of IP classes and can be hear by all IP hosts in network. Full broadcast address is 255.255.255.255
The main difference between direct broadcast and full broadcast is that routers will not propagate local broadcasts between segments, but they will propagate directed broadcasts.

Host Addresses

All address between the network address and the directed broadcast address is called host address for the subnet. You can assign host addresses to any IP devices such as PCs, servers, routers, and switches.

Subnetting

Subnetting is a process of breaking large network in small networks known as subnets. Subnetting happens when we extend default boundary of subnet mask. Basically we borrow host bits to create networks. Let’s take a example

Being a network administrator you are asked to create two networks, each will host 30 systems.

Single class C IP range can fulfill this requirement, still you have to purchase 2 class C IP range, one for each. Single class C range provides 256 total addresses and we need only 30 addresses, this will waste 226 addresses. These unused addresses would make additional route advertisements slowing down the network.
With subnetting you only need to purchase single range of class C. You can configure router to take first 26 bits instead of default 24 bits as network bits. In this case we would extend default boundary of subnet mask and borrow 2 host bits to create networks. By taking two bits from the host range and counting them as network bits, we can create two new subnets, and assign hosts them. As long as the two new network bits match in the address, then the address belongs to the same network. You can change either of the two bits, and you would be in a new subnet.

Advantage of Subnetting

• Subnetting breaks large network in smaller networks and smaller networks are easier to manage.
• Subnetting reduces network traffic by removing collision and broadcast traffic, that overall improve performance.
• Subnetting allows you to apply network security polices at the interconnection between subnets.
• Subnetting allows you to save money by reducing requirement for IP range.

Sub netting math

Subnetting process involves binary math calculation. Computers communicate with each others in binary language. To succeed in any kind of networking career, you might be fluent in binary math calculation. Subnetting needs two type of calculation, convert decimal to binary and convert binary to decimal.
Binary system works exactly same as decimal system, except the base number. Base number is 2 in binary system and 10 in decimal system. To calculate decimal equivalent value of a binary number, you have to replace base value 10 with 2. Binary numbers are displayed in columns and each position in binary system has double value than the position in right. From earlier section of this article you know that each number of an IP address is made from eight individual bits known as octet. So you should remember at least eight decimal equivalent value from binary position.

Base position	27	26	25	24	23	22	21	20
Decimal value	128	64	32	16	8	4	2	1

Convert decimal to binary
To convert a decimal number in binary we would use addition till number method. In this method we start adding from left to get target value. If after adding right position value, sum is lower than target number, keep adding, or if sum is greater than target number skip the position value. Only the value of on bit [1] will be added in sum. Off bit [0] has zero value. For example, convert decimal number 117 in binary.
Target decimal number 117
Move direction From Left ===========================> to Right

Base position	27	26	25	24	23	22	21	20
Decimal value	128	64	32	16	8	4	2	1
Bit status	0	1	1	1	0	1	0	1
Decimal value in addition	0	64	32	16	0	4	0	1

Binary value of 117 is 01110101.

Decimal calculation	Bit in binary
128 is greater than 117	off the bit
0+64 = 64 is less than 117	on the bit
0+64+32 = 96 is less than 117	on the bit
0+64+32+16 = 112 is less than 117	on the bit
0+64+32+16+8 = 120 is greater than 117	off the bit
0+64+32+16+0+4 = 116 is less than 117	on the bit
0+64+32+16+0+4+2 = 118 is greater than 117	off the bit
0+64+32+16+0+4+0+1 = 117 is equivalent to 117	on the bit

Convert binary in decimal
To convert a binary in decimal we will follow above method in reverse mode. We will find the decimal value of on binary bit position and add them. For example convert 10101010 binary number in decimal.
Target binary number 10101010
Move direction From Left ===========================> to Right

Base position	27	26	25	24	23	22	21	20
Decimal value	128	64	32	16	8	4	2	1
Bit status	1	0	1	0	1	0	1	0
Decimal value in addition	128	0	32	0	8	0	2	0

Decimal value of 10101010 is 170 [ 128+0+32+0+8+0+2+0 ]

Binary bit	Decimal value
1 On bit	128
0 Off bit	0
1 On bit	64
0 Off bit	0
1 On bit	32
0 Off bit	0
1 On bit	8
0 Off bit	0
1 On bit	2
0 Off bit	0

Review above method and do as much practice of following as you can.

• Pick any number from 0 – 255 and convert it in binary.
• Pick any combination from 00000000 – 11111111 and convert it in decimal.

Better you are with this conversion, the better you will do on the sub netting.

Combination provided by binary position

Now you know the process of converting binary in decimal and decimal in binary. Our next step is to figure out the combination provided by a binary position.
First bit provide two combination 0 or 1. If we take two bits than we have four combinations 00,01,10,11. IP address has 32 bits, so you should be able to find the number of combinations provided by each bit position until position number 32.

Number of bit[s]	Number of combinations	Number of bits	Number of combinations
1	2	17	131072
2	4	18	262144
3	8	19	524288
4	16	20	1048576
5	32	21	2097152
6	64	22	4194304
7	128	23	8388608
8	256	24	16777216
9	512	25	33554432
10	1024	26	67108864
11	2048	27	134217728
12	4096	28	268435456
13	8192	29	536870912
14	16384	30	1073741824
15	32768	31	2147483648
16	65536	32	4294967296

Default subnet mask

Class	Subnet Mask	Format
A	255.0.0.0	Network.Host.Host.Host
B	255.255.0.0	Network.Network.Host.Host
C	255.255.255.0	Network.Network.Network.Host

Key terms to remembers

• A subnet is a smaller portion of large network treated as its own separate network. To create subnet we borrow bits from host portion and assign them as network bits. This mean more networks, fewer hosts.
• If the network bits on two addresses do not match, then the two packets are intended for two separate networks.
• On a 32 bits IP address at least eight bits must belong to the network portion and at least 2 bits must belong to the host portion.
• Each IP address has a predefined IP class and that cannot be changed.
• Each class has a predefined default subnet mask that tell us the octets, which are already part of the network portion, as well as how many bits we have available to work with.
• Whatever network class is it, we cannot change those bits that are already assigned.
• We cannot assign the network ID and the broadcast address to a host.
• Regardless how many bits are left in the host field, network ID and the broadcast address must be reserved.
• Subnet bits start at the left and go to the right, without skipping bits.

CIDR [ Classless Inter Domain Routing]

CIDR is a slash notation of subnet mask. CIDR tells us number of on bits in a network address.

• Class A has default subnet mask 255.0.0.0. that means first octet of the subnet mask has all on bits. In slash notation it would be written as /8, means address has 8 bits on.
• Class B has default subnet mask 255.255.0.0. that means first two octets of the subnet mask have all on bits. In slash notation it would be written as /16, means address has 16 bits on.
• Class C has default subnet mask 255.255.255.0. that means first three octets of the subnet mask have all on bits. In slash notation it would be written as /24, means address has 24 bits on.