SubnettingSubnetting

Quick Reference guide

SUBNETTING BASICS

• Subnetting is the process of logically dividing a large network address into smaller subnets

• A subnetwork, or subnet, is a distinctly addressed part of a single IP Network.part of a single IP Network.

• Advantages

–Can divide network in smaller parts

–Restrict Broadcast traffic

– Simplified Administration

SUBNETTING BASICS

192.168.2.0/2450 hosts

48 hosts

192.168.1.0/2650 hosts

48 hosts

192.168.1.0/24

Network without subnetting,

too many addresses are wasted

on each subnet

192.168.1.0/26

Network with subnetting,

Addresses are properly

utilized to avoid wastage

SUBNETTING BASICS

• The process of subnetting involves borrowing bits from the network part of a subnet mask and adding it to the host part, thus increasing the network bits and reducing the host bits.

• Subnetting results in prefix lengths greater than the default for any given class, and a subnet mask containing values other than “0” and “255”

192.168.10.0 = 11000000.10101000.00001010.00000001

255.255.255.128 = 11111111.11111111.11111111.10000000

Prefix length = /25

SUBNETTING BASICS• When dividing a network into smaller subnets, we must know

how many parts we are dividing it into.

• Each subnet contains a group of addresses so we’ll need to know which addresses belong to each subnet

• Each network has a network address, a subnet is a small part of a network and should have its own subnet address

• We are able to derive these value using the formulae below• We are able to derive these value using the formulae below– Number of subnets = 2x

Where X = number of bits borrowed from the host part of the subnet mask

– Number of Host addresses in each subnet = 2y-2

Where y = number of 0’s in the subnet mask

– Block Size = 256-Mask

Where Mask is the decimal equivalent of the octet from which bits are borrowed

SUBNETTING CLASS C ADDRESSES• 192 . 168 . 10 . 0 /25

• 11111111 . 11111111 . 11111111 . 10000000

• 255 . 255 . 255 . 128

Network part Host part

• Number of subnets = 2x = 21 = 2 subnets

• Number of hosts per subnet = 2y-2 = 27-2 = 126

• Block Size = 256-Mask = 256-128 = 128

Subnet address 192.168.10.0 192.168.10.128

First valid address 192.168.10.1 192.168.10.129

Last valid address 192.168.10.126 192.168.10.254

Broadcast address 192.168.10.127 192.168.10.255

SUBNETTING CLASS C ADDRESSES• 192 . 168 . 10 . 0 /26

• 11111111 . 11111111 . 11111111 . 11000000

• 255 . 255 . 255 . 192

Network part Host part

• Number of subnets = 2x = 22 = 4 subnets

• Number of hosts per subnet = 2y-2 = 26-2 = 62

• Block Size = 256-Mask = 256-128 = 64

Subnet address 192.168.10.0 192.168.10.64 192.168.10.128

1st valid address 192.168.10.1 192.168.10.65 192.168.10.129

Last valid address 192.168.10.62 192.168.10.126 192.168.10.190

Broadcast address 192.168.10.63 192.168.10.127 192.168.10.191

CIDR MAPPING FOR CLASS C

• 255.255.255.0 : /24 (default)

• 255.255.255.128 : /25

• 255.255.255.192 : /26

• 255.255.255.224 : /27• 255.255.255.224 : /27

• 255.255.255.240 : /28

• 255.255.255.248 : /29

• 255.255.255.252 : /30

CLASS C SUBNETTING SUMMARY

Prefix

notation

Mask Block Size Number of

subnets

Number of valid

Hosts per subnet

/25 128 128 2 126

/26 192 64 4 62

/27 224 32 8 30

/28 240 16 16 14

/29 248 8 32 6

/30 252 4 64 2

Network part Host part

SUBNETTING CLASS B ADDRESSES

• 172 . 12 . 0 . 0 /18

• 11111111 . 11111111 . 11000000 . 00000000

• 255 . 255 . 192 . 0• 255 . 255 . 192 . 0

• Number of subnets = 2x = 22 = 4 subnets

• Number of hosts per subnet = 2y-2 = 214-2 = 16384

• Block Size = 256-Mask = 256-192 = 64

CIDR MAPPING FOR CLASS B

• 255.255.0.0 : /16 (default)

• 255.255.128.0 : /17

• 255.255.192.0 : /18

• 255.255.224.0 : /19• 255.255.224.0 : /19

• 255.255.240.0 : /20

• 255.255.248.0 : /21

• 255.255.252.0 : /22

• 255.255.254.0 : /23

Network part Host part

SUBNETTING CLASS B ADDRESSES WITH CLASS C MASK

• 172 . 12 . 0 . 0 /26

• 11111111 . 11111111 . 11111111 . 11000000

• 255 . 255 . 255 . 192• 255 . 255 . 255 . 192

• Number of subnets = 2x = 210 = 1024 subnets

• Number of hosts per subnet = 2y-2 = 26-2 = 62

• Block Size = 256-Mask = 256-192 = 64

*remember to add the block size to the octet it was derived from*

Network part Host part

SUBNETTING CLASS A ADDRESSES

• 10 . 0 . 0 . 0 /14

• 11111111 . 11111100 . 00000000 . 00000000

• 255 . 252 . 0 . 0• 255 . 252 . 0 . 0

• Number of subnets = 2x = 26 = 64 subnets

• Number of hosts per subnet = 2y-2 = 218-2 = 262142

• Block Size = 256-Mask = 256-252 = 4

*remember to add the block size to the octet it was derived from*

Network part Host part

SUBNETTING CLASS A ADDRESSESWITH CLASS B MASK

• 10 . 0 . 0 . 0 /20

• 11111111 . 11111111 . 11110000 . 00000000

• 255 . 255 . 240 . 0• 255 . 255 . 240 . 0

• Number of subnets = 2x = 212 = 4096 subnets

• Number of hosts per subnet = 2y-2 = 212-2 = 4094

• Block Size = 256-Mask = 256-240 = 16

*remember to add the block size to the octet it was derived from*