IPv4 SubNetting
It allows you to take one larger network and break it into a bunch of smaller networks.
Subnetting
Advantages of Subnetting:
Reduced network traffic – Routers create broadcast domains. The more broadcast domains you create, the smaller the broadcast domains and the less network traffic on each network segment.
Optimized network performance – This is a result of reduced network traffic.
Simplified management – It’s easier to identify and isolate network problem in a group of
smaller connected networks than within one gigantic network.
2 power of 2 = 4
2 power of 3 = 8
2 power of 4 = 16
2 power of 5 = 32
2 power of 6 = 64
2 power of 7 = 128
2 power of 8 = 256
2 power of 9 = 512
2 power of 10 = 1,024
2 power of 11 = 2,048
2 power of 12 = 4,096
2 power of 13 = 8,192
2 power of 14 = 16,384
Default Subnet mask
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
CIDR (Classless Inter-Domain Routing)
CIDR is a method for allocating IP addresses. The Internet Engineering Task Force introduced CIDR in 1993 to replace the previous addressing architecture of class full network design in the Internet. Their goal was to slow the growth of routing tables on routers across the Internet, and to help slow the rapid exhaustion of IPv4 addresses.
Subnetting of class C Address –
In a class C address, only8 bits are available for defining the hosts. And rest of the bits is defined for the Networks. Remember that subnet bits start at the left and go the right, without skipping bits. This means that the only class c subnet masks can be following.
Binary Decimal CIDR (Classless Inter-Domain Routing) --------------------------------------------------------- 10000000 = 128 /25
11000000 = 192 /26
11100000 = 224 /27 11110000 = 240 /28 11111000 = 248 /29 11111100 = 252 /30
We can’t use a /31 or /32 because we have to have at least 2 host bits for assigning IP address to hosts
step 1: How many subnets in the network?
Number of subnets = 2x 11000000
22 = 4
Step 2: How many hosts per subnet? Number of host = 2y - 2
26 – 2 = 62
Step 3: What are the valid subnets?
256 – Subnet mask = block size
An example would be 256 – 192 = 64. The block size of a 192 mask is always 64. Start counting at zero in blocks of 64 until you reach the subnet mask value and these are your subnets.
0, 64, 128, 192.
Step 4: What’s the broadcast address for each subnet?
The broadcast address is always the number right before the next subnet. The 0 subnet has a broadcast address of 63 because the next subnet is 64. The 64 subnet has a broadcast address of 127 because the next subnet is 128, etc.
Step 5: What are the valid hosts?
Valid hosts are the numbers between the subnets, omitting all the 0s and all 1s. For example, if 64 are the subnet number and 127 is the broadcast address, then 65–126 is the valid host range—it’s always the numbers between the subnet address and the broadcast address.
Subnetting Practice Examples:
Class C Addresses Example 1: 192.168.10.0 /25
Subnet mask = 255.255.255.128 How many subnets = 21 =2 How many hosts per subnets = 27 – 2 = 126 hosts
What are the valid subnets = 256-128.
Our subnets are 0 and 128.
whtat are the Broadcast address = for 0 subnet 127, for 128 subnet 255.
Subnet First Host Last Host Broadcast
0 128
1 129
126 254
127 255
Example 2: 192.168.10.0 /26
Subnet mask = 255.255.255.192
How many subnets? Since 192 are 2 bits on (11000000), the answer would be 22 = 4
How many hosts per subnet? We have 6 host bits off (11000000), so the equation would be 26 – 2 = 62 hosts.
What are the valid subnets? 256 – 192 = 64. Remember, we start at zero and count in our block size, so our subnets are 0, 64, 128, and 192.
What’s the broadcast address for each subnet? The number right before the value of the next subnet is all host bits turned on and equals the broadcast address. 63,127,191,255.
What are the valid hosts? These are the numbers between the subnet and broadcast address. The easiest way to find the hosts is to write out the subnet address and the broadcast address. This way, the valid hosts are obvious. The following table shows the 0, 64, 128, and 192 subnets, the valid host ranges of each, and the broadcast address of each subnet:
IPv4 SubNetting
Subnet 0 First Host 1 Last Host 62 Broadcast 63
Example 3: 192.168.10.0/27
64 127 192 65 129 193 126 190 254 127 191 255
Subnet Mask 255.255.255.224
How many subnets? 224 are 11100000, so our equation would be 23=8. How many hosts? 25 – 2 = 30.
What are the valid subnets? 256 – 224 = 32. We just start at zero and count to the subnet mask value in blocks (increments) of 32: 0, 32, 64, 96, 128, 160, 192, and 224.
