COMPUTER NETWORKS CS 610 Lecture26 Hammad Khalid Khan

COMPUTER NETWORKS CS 610 Lecture-26 Hammad Khalid Khan

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

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

Subnets and Classless Addressing w Instead of having three distinct address classes, allow the division between prefix and suffix to occur on an arbitrary boundary. w 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 w D = Destination Address w (A, M) = (32 -bit IP Address, 32 -bit Address Mask) – A == (D & M) – Example: - 32 -bit mask – 11111111 00000000 – Denoted in dotted decimal as 255. 0. 0

Subnet and Classless Addresses

Subnet/Address Mask – 32 -bit network prefix 100000001010 00000000 Denoted in dotted decimal value as 128. 10. 0. 0 32 -bit destination address 128. 10. 2. 3 Binary equivalent: 100000001010 00000011

Subnet/Address Mask A logical ‘and’ between D and M produces the binary result 100000001010 00000000 Which is equal to prefix 128. 10. 0. 0

Subnet/Address Mask w How can an IP address be divided at an arbitrary boundary? w 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. w Known as the Address Mask or Subnet Mask.

CIDR Notation w Inside a computer, each address mask is stored as a 32 -bit value. w When we enter a prefix and an address mask they use a modified form of dotted decimal addressing called CIDR addressing. w Known as CIDR notation 128. 10. 0. 0/16

CIDR Notation w As an example of how CIDR adds flexibility, suppose a single class B prefix (e. g. 128. 211. 0. 0) i. e. 216 host addresses w 16 -bit CIDR mask denoted as: 128. 211. 0. 0 / 16 That is, by making CIDR mask correspond exactly to the old classful interpretation.

CIDR Notation w It will be fine if 216 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 w For example: – One customer can be assigned 128. 211. 0. 16 /28 – And the other customer can be assigned 128. 211. 0. 32 /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 w Subnet and Classless Addressing w Subnet/Address Mask w CIDR Notation w Special IP Addresses