COMPUTER NETWORKS CS610 Lecture-26 Hammad Khalid Khan

Review Lecture 25  Computing The Class of An Address  Dotted Decimal Notation  Division of Address Space  A Classful Addressing Example

Subnets and Classless Addressing  As the Internet grew, the original classful addressing scheme became a limitation. – The IP address space was being exhausted. – Because all networks had to choose one of three possible sizes, many addresses were unused.

Subnets and Classless Addressing  Two new mechanisms were invented to overcome the limitations: – Subnet addressing – Classless addressing

Subnets and Classless Addressing  Instead of having three distinct address classes, allow the division between prefix and suffix to occur on an arbitrary boundary.  The classless addressing scheme solves the problem by allowing an ISP to assign a prefix that is 28 bits long (allowing the host to have up to 14 hosts).

Subnet/Address Mask  D = Destination Address  (A,M) = (32-bit IP Address,32-bit Address Mask) – A == (D & M) – Example:- 32-bit mask – – Denoted in dotted decimal as

Subnet and Classless Addresses

Subnet/Address Mask – 32-bit network prefix Denoted in dotted decimal value as bit destination address Binary equivalent:

Subnet/Address Mask A logical ‘and’ between D and M produces the binary result Which is equal to prefix

Subnet/Address Mask  How can an IP address be divided at an arbitrary boundary?  To use a classless or subnet address, table inside host and routers that contain address must keep two pieces of information with each address: the 32-bit address itself and another 32-bit value that specifies the boundary.  Known as the Address Mask or Subnet Mask.

CIDR Notation  Inside a computer, each address mask is stored as a 32-bit value.  When we enter a prefix and an address mask they use a modified form of dotted decimal addressing called CIDR addressing.  Known as CIDR notation /16

CIDR Notation  As an example of how CIDR adds flexibility, suppose a single class B prefix (e.g ) i.e host addresses  16-bit CIDR mask denoted as: / 16 That is, by making CIDR mask correspond exactly to the old classful interpretation.

CIDR Notation  It will be fine if 2 16 hosts are attached. If it does have two customers with only twelve computers each, the ISP can use CIDR to partition the address into three pieces, – Two of them each big enough for one of the customers. – Remainder available for future customers.

CIDR Notation  For example:- – One customer can be assigned /28 – And the other customer can be assigned /28 Both customers have same mask size, the prefix differs. i.e. each customer has a unique prefix.

CIDR Host Addresses

Summary Of Special IP Addresses

Summary  Subnet and Classless Addressing  Subnet/Address Mask  CIDR Notation  Special IP Addresses