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IPv4 Subnetting

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Subnetting

Subnetting allows you to take one larger network and break it into a bunch of smaller networks.

Advantages of Subnetting

  1. 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.
  2. Optimized Network Performance: This is a result of reduced network traffic.
  3. Simplified Management: It’s easier to identify and isolate network problem in a group of smaller connected networks than within one gigantic network.
  4. Facilitated Spanning of Large Geographical Distances: A single large network that spans long distances can create problem in every area. Connecting multiple smaller networks makes the system more efficient.

How To Create Subnets?

To create Sub Networks, we need to take bits from the host portion of the IP address and reserve them to define the subnet address.

Host & Network Bits Config in IP Classes
Host & Network Bits Config in IP Classes

Subnet Mask

A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address.

Understanding The Power of 2

2 power of 1 = 2
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.

Quick Reference Subnet Mask Table
Quick Reference Subnet Mask Table
Class Start End #Host
A 10.0.0.0 10.255.255.255 16,777,216
B 172.16.0.0 172.31.255.255 65,534
C 192.168.0.0 192.168.255.255 256

 

/8 255.0.0.0 16,777,214
/9 255.128.0.0 8,388,352
/10 255.192.0.0 4,194,176
/11 255.224.0.0 2,097,088
/12 255.240.0.0 1,048,544
/13 255.248.0.0 524,272
/14 255.252.0.0 262,136
/15 255.254.0.0 131,068
/16 255.255.0.0 65,024
/17 255.255.128.0 32,512
/18 255.255.192.0 16,256
/19 255.255.224.0 8,128
/20 255.255.240.0 4,064
/21 255.255.248.0 2,032
/22 255.255.252.0 1,016
/23 255.255.254.0 508
/24 255.255.255.0 254
/25 255.255.255.128 124
/26 255.255.255.192 62
/27 255.255.255.224 30
/28 255.255.255.240 14
/29 255.255.255.248 6
/30 255.255.255.252 2

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. Easy, huh? Yes

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 Subnetting

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.

What are the Broadcast address = for 0 subnet 127, for 128 subnet 255.

Subnet 0 128
First Host 1 129
Last Host 126 254
Broadcast Host 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:

Subnet 0 64 127 192
First Host 1 65 129 193
Last Host 62 126 190 254
Broadcast 63 127 191 255

Example 3: 192.168.10.0/27

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.

Subnet Address 0 32 64 96 128 160 192 224
First Valid Address 1 33 65 97 129 161 193 225
Last Valid Address 30 62 94 126 158 190 222 254
Broadcast Address 31 63 95 127 159 191 223 255

Example 4: 192.168.10.0/28

Subnet Mask – 255.255.255.240
Subnets = 24 =16
Hosts = 24 -2 = 14
Valid Subnets = 256-240 = 16

Subnet 0 16 240
First Host 1 17 241
Last Host 14 30 254
Broadcast 15 31 255

Example 5: 192.168.10.0/29

Subnet Mask = 255.255.255.248
Subnets = 25 = 32
Hosts = 23 -2 = 6
Valid Subnets = 256-248 = 8

Subnet 0 8 248
First Host 1 9 249
Last Host 6 14 254
Broadcast 7 15 255

Example 6: 192.168.10.0/30

Subnet Mask = 255.255.255.252
Subnets = 26 = 64
Hosts = 22 -2 = 2
Valid Subnets = 256-252 = 4

Subnet 0 4 252
First Host 1 5 253
Last Host 2 6 254
Broadcast 3 7 255
  • Class B Subnetting

Example 1: 172.16.0.0/17

Subnet Mask = 255.255.255.128.0
Subnets = 21 =2
Hosts = 215 -2 = 32766
Valid Subnets = 256-128 = 128

Subnet 0.0 128.0
First Host 0.1 128.1
Last Host 127.254 255.254
Broadcast 127.255 255.255

Remember that Subnetting is performed in the third octet, so the subnet numbers are really 0.0, and 128.0, as shown on the above table.

Example 2: 172.16.0.0/18

Subnet Mask = 255.255.192.0
Subnets =22 = 4
Host = 214 -2 =16382
Valid subnets = 256-192 = 64

Subnet 0.0 64.0 128.0 192.0
First Host 0.1 64.1 128.1 192.1
Last Host 63.254 127.254 191.254 255.254
Broadcast 63.255 127.255 191.255 255.255

Example 3: 172.16.0.0/20

Subnet Mask = 255.255.240.0
Subnets 24 = 16
Hosts 212 -2 = 4096
Valid Subnets = 256-240 = 16

Subnet 0.0 16.0 32.0 240.0
First Host 0.1 16.1 32.1 240.1
Last Host 15.254 31.254 47.254 255.254
Broadcast 15.255 31.255 47.255 255.255

Example 4: 172.16.0.0/23

Subnet Mask = 255.255.254.0
Subnets 27 = 128
Hosts 29 -2 = 510
Valid subnets 256-254 = 0, 2,4,6,8 up to 254

Subnet 0.0 2.0 4.0 254.0
First Host 0.1 2.1 4.1 254.1
Last Host 1.254 3.254 5.254 255.254
Broadcast 1.255 3.255 5.255 255.255

Example 5: 172.16.0.0/24

Subnet Mask = 255.255.255.0
Subnets = 28 =256
Hosts =28 -2 = 254
Valid subnets = 256-255 = 1, 0,1,2,3,4, all the way to 255.

Subnet 0.0 1.0 2.0 254.0 255.0
First Host 0.1 1.1 2.1 254.1 255.1
Last Host 0.254 1.254 2.254 254.254 255.254
Broadcast 0.255 1.255 2.255 254.255 255.255

Example 6: 172.16.0.0 /25

Subnet mask = 255.255.255.128
Subnets = 29 = 512
Hosts = 27 – 2 = 126
Valid subnets? Okay, now for the tricky part. 256-255 = 1. 0,1,2,3,4 etc.

But you can’t forget the one subnet bit used in the fourth octate.

Subnet 0.0 0.128 1.0 1.128 2.0 255.128
First Host 0.1 0.129 1.1 1.129 2.1 255.129
Last Host 0.126 0.254 1.126 1.254 2.126 255.254
Broadcast 0.127 0.255 1.127 1.255 2.127 255.255

Example 7: 172.16.0.0/26

Subnet Mask = 255.255.255.192
Subnets =210 = 1024
Hosts = 26 -2 = 62
Valid Subnets = 256-192 = 64

Subnet 0.0 0.64 0.128 1.0 255.192
First Host 0.1 0.65 0.129 1.1 255.193
Last Host 0.62 0.126 0.190 1.62 255.254
Broadcast 0.63 0.127 0.191 1.63 255.255

Example 8: 172.16.0.0 /27

Subnet mask = 255.255.255.224
Subnets =211 = 2048
Hosts = 25 -2 = 30
Valid subnets 256 -224 = 32. 0, 32, 64, 96,128….224

Subnet 0.0 0.32 0.224 255.0 255.224
First Host 0.1 0.33 0.225 255.1 255.255
Last Host 0.30 0.62 0.254 255.30 255.254
Broadcast 0.31 0.63 0.255 255.31 255.255
  • Class A Subnetting

Example: 1: 10.0.0.0/16

Subnets = 28 = 256
Hosts = 216 – 2 = 65,534
Valid Subnets 256-255 = 1. 0, 1, 2, 3, 4 etc.

Subnet 10.0.0.0 10.1.0.0 10.254.0.0 10.255.255.0
First Host 10.0.0.1 10.1.0.1 10.254.0.1 10.255.0.1
Last Host 10.0.255.254 10.1.255.254 10.254.255.254 10.255.255.254
Broadcast 10.0.255.255 10.1.255.255 10.254.255.255 10.255.255.255

Exmaple 2: 10.0.0.0 /20

Subnet Mask = 255.255.240.0
Subnets = 212 = 4096
Hosts =212 -2 4094
Valid subnets = 256-240 =16

Subnet 10.0.0.0 10.0.16.0 10.0.32.0 10.255.240.0
First Host 10.0.0.1 10.0.16.1 10.0.32.1 10.255.240.1
Last Host 10.0.15.254 10.0.31.254 10.0.47.254 10.255.255.254
Broadcast 10.0.15.255 10.0.31.255 10.0.47.255 10.255.255.255

Example 3: 10.0.0.0/26

Subnet mask = 255.255.255.192
Subnets 218 = 262,144
Hosts = 26 -2 = 62
Valid Subnets? In the second and third octet, the block size is 1, and in
the fourth octate, the block size is 64.

Subnet 10.0.0.0 10.0.0.64 10.0.0.128
First Host 10.0.0.1 10.0.0.65 10.0.0.129
Last Host 10.0.0.62 10.0.0.126 10.0.0.190
Broadcast 10.0.0.63 10.0.0.127 10.0.0.191
Subnet 10.255.255.0 10.255.255.192
First Host 10.255.255.1 10.255.255.193
Last Host 10.255.255.62 10.255.255.254
Broadcast 10.255.255.63 10.255.255.255

VLSM

VLSM (Variable Length Subnet Mask), a way to take one network and create many networks using subnet masks of different lengths on different types of network designs.

Summarization

Summarization, also called route aggregation, allows routing protocols to advertise many networks as one address. The purpose of this is to reduce the size of routing tables on routers to save memory.

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