Chapter 18. IP: Internet Protocol
Addresses
Jing Wang
Towson University
18.1. Introduction
• Addressing scheme used by the Internet
Protocol (IP)
• How IP addressing scheme divided
addresses into classes
• Subnet addressing and classless
addressing
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.
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
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
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
18.5. Original Classes Of IP
Addresses
Figure 18.1. 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.
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.
18.6. Computing The Class of An Address
Figure 18.2. 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.
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 0.0.0.0
through 255.255.255.255
• 00000000 – 0
• 11111111 – 255
18.7. Dotted Decimal Notation
Figure 18.3. 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.
18.8. Classes And Dotted Decimal
Notation
Figure 18.4. The range of decimal values found in the first
octet of each address class.
18.9. Division Of The Address
Space
Figure 18.5. The number of networks and hosts per
network in each of the three primary IP address classes.
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
18.11. A Classful Addressing
Example
Figure 18.6. 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.
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
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.
18.13. 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
18.13. 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
18.13. Address Masks
• Example
–
–
–
–
–
–
–
32-bit mask 255.255.0.0
11111111 11111111 00000000 00000000
32-bit network prefix 128.10.0.0
10000000 00001010 00000000 00000000
Consider a destination address 128.10.2.3
10000000 00001010 00000010 00000011
A logical “and” between destination address and the
address mask produces the binary result
– 10000000 00001010 00000000 00000000
– Which is equal to the prefix 128.10.0.0
18.14. 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
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 128.10.0.0 consists of 16-bit
network prefix and a 16-bit host suffix
– In CIDR notation, the address can be written
128.10.0.0/16
• Appendix 3
18.15. A CIDR Address Block
Example
• Suppose an ISP begins with a single class B
prefix (e.g., 128.211.0.0)
• 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
– 128.211.0.0/16 correspond to classful
– 128.211.9.16/28 and 128.211.0.32/28 for each of the
2 customers, same mask size but prefixes differ
– ISP retains most of the original addresses
18.16. CIDR Host Addresses
Figure 18.7. 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.
18.17. Special IP Addresses
•
•
•
•
•
Network Address
– Address 128.211.0.0/16 denotes a network that has been assigned the prefix
128.211
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:
127.0.0.1
18.18. Summary Of Special IP
Addresses
Figure 18.8. Summary of the special IP address
forms.
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
18.20. Routers And The IP
Addressing Principle
Figure 18.9. 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.
18.21. 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.
18.22. Summary
• Addressing scheme
– 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.