TODAY’S AGENDASUB NETTING

GROUP MEMBERS

WAQAS YOUSAF 10-TE-12

ZAEEM HASSAN 10-TE-98

WAQAS AKRAM 10-TE-64

IRFAN MUNAIR 10-TE-22

DEFINITION

In sub netting, a network is divided into smaller sub networks with each subnet having its own subnet address

Sub netting concept

Dividing the barrel of apples into small barrels or baskets does not give us any more apples…

3333

33

99 – 2 apples = 97 Usable Apples

Before sub netting:In any network (or subnet) we

can not use all the IP addresses for host addresses.

We lose two addresses for every network or subnet.

1. Network Address - One address is reserved to that of the network.

2. Broadcast Address – One address is reserved to address all hosts in that network or subnet.

97 Apples

(99– 2)

SUBBNETTING

Two parts of IP address1. Net ID2. Host ID To reach a host, we must first reach the

network using net id, then using host id to reach the host.

IP Address Classes

The 5 IP classes are split up based on the value in the 1st octet:

TWO LEVEL HIRERCHY

Sometimes two level hierarchy is not enough.

For example: An organization with a class B address.

The organization has two levels hierarchical addressing, but it cannot have more than one physical network.

• The host cannot be grouped, and all of the host has the same level

• Solution sub netting

Without subnetting

With subnetting

• In the above example, The rest of the internet is not aware that the

network is divided into three physical subnet. A packet destined for host 141.14.2.21 still

reaches router R1. The destination address of the IP datagram is still a class B where • 141.14 defines the net id• And 2.21 defines the host

However, when the datagram arrives a router R1, the interpretation of the IP address changes. • R1 knows the network 141.14 physically divided into

three subnets.• It knows that the last two octets define two things:

subnet id and host id ( 2.21 subnet id 2 and host id 21)

• Router uses first two octets as net id, the third subnet id, and the fourth host id

Three levels of hierachy

Subnetting class A

A class A address: Is made of a one-byte net id and a three-byte

host id Can have one single physical network with up

to 16,777,214 (224-2) users. If we want more physical networks, we can

divide this one range into several smaller ranges

Subnetting class B

A class B: Is made A two byte net id and two-byte host id Can have one single physical network and up

to (216-2)=65,534 hosts on the network. If we want more physical network, we can

divide this one big range into several smaller ranges.

Subnetting class C

A class C address: is made of a three byte net id and one-byte

host id Can have one single physical network and up

to 254 (28 – 2) host on that network If we want more physical network, we can

divide this one range into several smaller range.

Variable-length subnetting

The internet allows a site to use variable-length sub netting. For an example of when this may be desirable, consider a

site that is granted a class C address and needs to have five subnets with the following of host: 60,60,60,30,30 This site can not use a subnet mask with only two bits in the subnets

section because this allows only four subnet with 62 hosts ( 256/4 - 2 = 62).

Nor can the site use a subnet mask with three bits in the subnet section, because this allows 8 subnets with 30 hosts (256/8 -2 = 30)

Solution for the problem: variable length subnetting. The router uses two different masks, one applied after the other First mask uses the mask with 26 1s (11111111 11111111 11111111

1100000 or 255.255.255.255.192) to divide the network into four subnets Then it applies the second mask with 27 1s (255.255.255.224) to one of the

subnets to divide it into two smaller subnets

SUBNET MASK

An IP address has 2 parts: The Network identification. The Host identification.

Frequently, the Network & Host portions of the address need to be separately extracted.

In most cases, if you know the address class, it’s easy to separate the 2 portions.

DEFAULT STANDARD SUBNET MASKS

There are default standard subnet masks for Class A, B and C addresses