IPv4 Subnetting

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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

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

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                     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.

The subnet address                 0          32        64        96        128      160      192      224
The first valid host                 1          33        65         97       129      161      193      225
The last valid host                  30        62        94        126      158      190      222      254
The 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 octet.

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               255.225
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                   10.255.255.0.0
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 octet, 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

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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