Introduction to Internetworking

2 The IP Addressing Scheme (IPv4) (psu.edu) Dotted Decimal Notation: A notation more convenient for humans – Dividing 32 bits into four 8-bit sections (4 bytes) – Value range of a byte: 0 (all 0) (all 1)

3 The IP Addressing Scheme (IPv4) IP address is divided into two parts – A prefix: network identifier Coordinated globally. – Internet Corporation for Assigned Names and Numbers (ICANN) – A suffix: host identifier Assigned locally (within each network)

4 Original Classes of IP Addresses The original classful IP addressing divided the IP address space into three primary classes – Determined by the first four bits. – Each class has a different size prefix and suffix

5 Division of the Address Space The address space has unequal sizes – Class A contains half of all addresses Major ISPs, large organizations, government, MIT, etc. – Each network in Class C only has 256 available addresses. Small organizations space/ space/

Exercise Which class does each of these addresses belong to? 1st byte Class A B C D E

7 Subnet and Classless Addressing As the Internet grew, the original classful addressing scheme became a limitation – Everyone wants a class A or class B address – Address are often wasted

8 Subnet and Classless Addressing Allow the division between prefix and suffix to occur on an arbitrary bit boundary – initially used within large organizations – Classless addressing extended the approach to all Internet

9 Specifying Network Boundary: Address Masks A 32-bit value that specifies the exact boundary between the network prefix and the host suffix – All 1s for network prefix and all 0s for host suffix. – Examples: ( ) ( ) ( )

10 Address Masks When forwarding an IP packet, hosts and routers need to determine if an IP address (D) belongs to a specific network with prefix (N) and mask (M): N == (D & M) NetworkMaskNext Hop Router 2 ……

11 Address Masks Example: – Destination IP address: NetworkMaskNext Hop Router 2 …… = = & = = N == (D & M)

12 Exercise: Masks with Arbitrary Boundary Given the address mask: = Which are the network prefix (in dotted decimal notation) of the following destination addresses? a) = b) =

13 CIDR Notation Classless Inter-Domain Routing (CIDR) – An easy way to specify a mask ddd.ddd.ddd.ddd/m – ddd.ddd.ddd.ddd: IP address. – m: the number of one bits in the mask /28 – A mask of 28 bits

14 CIDR Prefix and Host Addresses Once an ISP assigns a customer a CIDR prefix, the customer can assign host addresses for its network users. – /28

15 Special (Reserved) IP Addresses (1) Network address: host address 0 – A network address should never appear as the destination address in a packet

16 Special (Reserved) IP Addresses (2) Directed broadcast address: – Adding a suffix that consists of all 1 bits to the network prefix

CIDR Exercise 1 Is the CIDR prefix /29 valid? Why or why not?

CIDR Exercise 2 Suppose you are an ISP that owns the /24 (C class) IPv4 address block. Show the CIDR allocation you would use to allocate address blocks to three customers who need addresses for 100, 60, and 60 computers respectively.