KHURASAN UNIVERSITY خــــراســــــــان پوهـنـتـــــــــون
SUBNETTING
By: Irfanullah Azimi
SUBNETTING
What is a Subnet?
➢ A subnet, or subnetwork is a logically subdivision of IP network.
What is IP Subnetting?
➢ IP Subnetting is a process of dividing a large IP network in smaller IP networks. In
Subnetting we create multiple small manageable networks from a single large IP
network.
Why Subnetting?
➢ Shortage of IP Addresses
➢ Classless IP Addressing
SUBNETTING BENEFITS
Greater number of networks.
Improves Network Performance and Speed
Benefits Reduces Network Congestion
Greater administrative control of each network
Boosts Network Security.
NETWORK PORTION AND HOST PORTION
➢ Identifying network portion and host portion in an IP address is the
first step of Subnetting.
➢ Subnetting can only be done in host portion.
➢ Subnet mask is a 32 bits long address used to distinguish the
network portion from host portion in an IP address.
IP Address 144 45 11 1
Subnet Mask 255 255 0 0
Network portion Host portion
CLASSLESS IP ADDRESS
➢ No exact bits for network address and for host address
➢ Extends IPv4 Life
20 bits
16 bits 12 bits
16 bits
Classless – how? After Subnetting
Before Subnetting
➢ Bits borrowed from host field
network
network network
network host
subnet host
NETWORK ADDRESS AND BROADCAST
ADDRESS
➢ In each network there are two special addresses
➢ Network Address
➢ Broadcast Address
➢ Since each subnet represents an individual network, it also uses
these two addresses.
VALID HOST ADDRESSES
➢ All addresses between Network address and Broadcast address are
known as valid host addresses.
➢ Only valid host addresses can be assigned to the devices in a
network.
Block Size
Network Address Valid Host Addresses Broadcast Address
Block Size
POWER OF 2
➢ Let’s consider
➢ H is the number of host bits
➢ N is the number of host bits used as network bits
➢ Number of new networks resulting from the subnetting is: 2N
➢ Number of hosts per network is: 2H - 2
IP Address 144 45 11 1 /24
Subnet Mask 11111111 11111111 11111111 00000000
IP Address 144 45 11 1 /25
Subnet Mask 11111111 11111111 11111111 10000000 Subnet Mask: 255.255.255.128
SUBNETTING DIRECTION
➢ Subnetting always flows in single direction (left to right) without skipping
any bit.
➢ Let’s take an example.
➢ A class C network is subnetted in 4 subnets. Find the number of host bits used in
Subnetting and their location in address space.
➢ To create 4 subnets we need to 2 (22 = 4) Subnetting eligible host bits.
➢ Since in class C network space Subnetting eligible bits starts from 25 and Subnetting
always goes from left to right without skipping any bit, the bits used in this network
are 25 and 26.
CLASSLESS INTER-DOMAIN ROUTING (CIDR)
➢A method for allocating IP addresses
➢The Internet Engineering Task Force introduced CIDR in 1993
➢To replace the previous addressing architecture (Class full)
➢CIDR is a slash notation of subnet mask for classless addresses. CIDR tells us
number of on bits in a network address
SLASH NOTATION (CIDR NOTATION)
➢It’s a compact representation of Subnet mask. In this notation a slash (/) sign and
total number of the on bits in subnet mask are written with IP address instead of
full Subnet mask.
In Slash notation In binary notation In decimal notation
10.10.10.10/8 00001010.00001010.00001010.00001010 10.10.10.10
11111111.00000000.00000000.00000000 255.0.0.0
172.168.1.1/16 10101100.10101000.00000001.00000001 172.168.1.1
11111111.11111111.00000000.00000000 255.255.0.
192.168.1.1/24 11000000.10101000.00000001.00000001 192.168.1.1
11111111.11111111.11111111.00000000 255.255.255.0
192.168.1.1/28 11000000.10101000.00000001.00000001 192.168.1.1
11111111.11111111.11111111.11110000 255.255.255.240
SUBNETTING EXAMPLE
SUBNETTING EXAMPLE
