1. Chapter 18. IP: Internet Protocol Addresses
Jing Wang
Towson University

2. 18.1. Introduction
Addressing scheme used by the Internet Protocol (IP)
How IP addressing scheme divided addresses into classes
Subnet addressing and classless addressing

3. 18.2. Addresses For The Virtual Internet
To provide uniform addressing in an internet, protocol software defines an abstract addressing scheme that assigns each host a unique protocol address.
Users, application programs, and higher layers of protocol software use the abstract protocol addresses to communicate.

4. 18.3. The IP Addressing Scheme
Internet Protocol address (IP address or Internet address)
An Internet Address (IP address) is a unique 32-bit binary number assigned to a host and used for communication with the host

5. 18.4. The IP Address Hierarchy
Each 32-bit IP address is divided into
A prefix – a unique value known as network number
A suffix – a unique address on a given physical network

6. 18.4. The IP Address Hierarchy
Properties of IP address hierarchy
Each computer is assigned a unique address
Although network number assignments must be coordinated globally, suffixes can be assigned locally without global coordination

7. 18.5. Original Classes Of IP Addresses
Figure The five classes of IP addresses in the original classful scheme. The address assigned to a host is either class A, B, or C; the prefix identifies a network, and the suffix is unique to a host on that network.

8. 18.5. Original Classes Of IP Addresses
Classful IP addressing
The original IP addressing scheme divides host addresses into three primary classes. The class of an address determines the boundary between the network prefix and host suffix.

9. 18.6. Computing The Class of An Address
Figure The mapping between the first four bits of an IP address and the class of the address. The mapping was used with the original classful scheme.

10. 18.7. Dotted Decimal Notation
Dotted decimal notation is a syntactic form that IP software uses to express 32-bit binary values when interacting with humans.
Dotted decimal represents each octet in decimal and uses a dot to separate octets.
Dotted decimal addresses range from through
– 0
– 255

11. 18.7. Dotted Decimal Notation
Figure Examples of 32-bit binary numbers and their equivalent in dotted decimal notation. Each octet is written in decimal with periods (dots) used to separate octets.

12. 18.8. Classes And Dotted Decimal Notation
Figure The range of decimal values found in the first octet of each address class.

13. 18.9. Division Of The Address Space
Figure The number of networks and hosts per network in each of the three primary IP address classes.

14. 18.10. Authority For Addresses
An organization obtains network numbers from ISPs
Internet Service Providers (ISPs)
the communication company that supplies Internet connections
ISPs coordinate with Internet Assigned Number Authority
To ensure that each network prefix is unique throughout the entire Internet

15. 18.11. A Classful Addressing Example
Figure An example private internet with IP addresses assigned to hosts. The size of the cloud used to denote a physical network corresponds to the number of hosts expected on the network; the size of a network determines the class of address assigned.

16. 18.12. Subnet And Classless Addressing
Limitation of the original classful addressing scheme
IP address space being exhausted
Because all networks had to choose one of three possible sizes, many addresses unused
New mechanism
Subnet addressing
Classless addressing

17. 18.12. Subnet And Classless Addressing
Instead of having three distinct address classes, allow the division between prefix and suffix to occur on an arbitrary bit boundary
Example
A network contains 9 hosts
Classless addressing subdivide a single class C address into 16 address that each have a 28-bit prefix and a 4-bit suffix
Created 16 networks that each have up to 14 hosts.

18. Address Masks
To use classless or subnet masking, tables inside hosts and routers that contain addresses must keep two pieces of information with each address:
The 32-bit address itself
Another 32-bit value that specifies the boundary between network prefix and suffix
Address mask or subnet mask
1 bits mark the network prefix
0 bits mark the host portion
Subnet addressing was in use for a decade before the idea was extended to classless addressing

19. Address Masks
How can an IP address be divided at an arbitrary boundary?
Suppose a router is given a destination address D, and a pair (A, M) that represents a 32-bit IP address and a 32-bit address mask
To make a comparison, the router tests the condition A== (D&M)
The router uses the mask with a “logical and” operation to set the host bits of address D to zero, and then compares the result with the network prefix A

20. 18.13. Address Masks Example 32-bit mask 255.255.0.0
32-bit network prefix
Consider a destination address
A logical “and” between destination address and the address mask produces the binary result
Which is equal to the prefix

21. CIDR Notation
Classless Inter-Domain Routing (CIDR) and IP subnetting techniques each use a 32-bit address mask to denote the boundary between the network prefix and host suffix.
Software that interacts with humans either uses the slash notation that was developed for CIDR or dotted decimal notation instead of binary notation

22. 18.14. CIDR Notation Classless Inter-Domain Routing (CIDR)
Known as CIDR notation, the new form specifies the mask associated with an address by appending a slash and the size of the mask in decimal (slash notation)
Example
Classful address consists of 16-bit network prefix and a 16-bit host suffix
In CIDR notation, the address can be written
/16
Appendix 3

23. 18.15. A CIDR Address Block Example
Suppose an ISP begins with a single class B prefix (e.g., )
Classful addressing
The ISP can only assign the prefix to one customer with up to 216 host addresses
Can not have 2 customers with 12 computers each
Under CIDR
/16 correspond to classful
/28 and /28 for each of the 2 customers, same mask size but prefixes differ
ISP retains most of the original addresses

24. CIDR Host Addresses
Figure Illustration of CIDR addressing for a /28 prefix. Note that because bits are numbered starting at zero, the prefix covers bits 0 through 27. Thus, bits 28 through 31 correspond to the host suffix.

25. 18.17. Special IP Addresses Network Address
Address /16 denotes a network that has been assigned the prefix
Directed Broadcast Address
IP defines a directed broadcast address for each physical network
Limited Broadcast Address
Limited broadcast is used during system startup by a computer that does not know the network number
IP will broadcast any packet sent to the all-ones address across the local network
This Computer Address
The TCP/IP protocol suite contains protocols a computer can use to obtain its IP address automatically when the computer boots.
When using such startup protocols to use IP to communicate, the computer can not supply a correct IP source address
Loopback Address
IP defines a loopback address used to test network applications.
When one application sends data to another, data travels down the protocol stack to the IP software, which forwards it back up through the protocol stack to the second program
IP reserves the network prefix 127/8 for use with loopback. Most popular:

26. 18.18. Summary Of Special IP Addresses
Figure Summary of the special IP address forms.

27. 18.20. Routers And The IP Addressing Principle
An IP address does not identify a specific computer. Instead, each IP address identifies a connection between a computer and a network.
A computer with multiple network connections (e.g., a router) must be assigned one IP address for each connection

28. 18.20. Routers And The IP Addressing Principle
Figure An example of IP addresses assigned to two routers. Each interface is assigned an address that contains the prefix of the network to which the interface connects.

29. Multi-Homed Hosts
A computer that connects to multiple networks is called multi-homed
Increase reliability, performance
Like a router, a multi-homed host has multiple protocol addresses, one for each network connection.

30. 18.22. Summary Addressing scheme CIDR
IP divides each internet address into a two-level hierarchy
An IP address is a 32 bit number.
Originally, an address was placed in one of five classes which can be determined by the values of the first four bits
CIDR
Stores a 32-bit mask along with each address
Each IP address identifies a connection between a computer and a network.