1. The Concept of Universal Service
The chief problem with multiple networks is obvious:
A computer attached to a given network can only communicate with other computers attached to the same network
The problem became evident in the 1970s as large organizations began to acquire multiple networks
Each network in the organization formed an island
In many early installations
each computer attached to a single network
and employees had to choose a computer appropriate for each task
an employee was given access to multiple screens and keyboards
the employee was forced to move from one computer to another to send a message across the appropriate network
Users are neither satisfied nor productive when they must use a separate computer for each network
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

2. Internetworking
Despite the incompatibilities among network technologies
researchers have devised a scheme that provides universal service among heterogeneous networks, called internetworking
The scheme uses both hardware and software
Additional systems are used to interconnect a set of networks
Software on the attached computers provides universal service
The resulting system of connected physical networks is known as an internetwork or internet
An internet is not restricted in size
internets exist that contain a few networks
the global Internet contains tens of thousands of networks
the number of computers attached to each network can vary
some networks have no computers attached
while others have hundreds
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

3. Physical Network Connection with Routers
The basic component used to connect heterogeneous networks is a router
Physically a router is
an independent hardware system dedicated to the task of interconnecting networks
contains a processor and memory as well as a separate I/O interface for each network to which it connects
Figure 20.1 illustrates that the physical connection of networks with a router is straightforward
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

4. Internet Architecture
Figure 20.2 shows each router with exactly two connections
commercial routers can connect more than two networks
a single router could connect all four networks in the example
An organization seldom uses a single router to connect all of its networks
There are reasons for multiple connections:
Load-balancing and speed
the processor in a given router is insufficient to handle the traffic passing among an arbitrary number of networks
Redundancy improves internet reliability
To avoid a single point of failure
The protocol software continuously monitors internet connections
It instructs routers to send traffic along alternative paths when a network or router fails
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

5. Internet Architecture
An organization must choose a design that meets the organization's need for
Reliability
Capacity
Cost
The exact details of internet topology to be chosen often depend on the following
bandwidth of the physical networks
expected traffic
organization's reliability requirements
cost
performance of available router hardware
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

6. A Virtual Network
Internet provides the appearance of a single seamless communication system
a combination of hardware and software provides the illusion of a uniform network system
Internet software hides the details of
physical network connections
physical addresses
routing information
Users/application programs are not supposed to be aware of the underlying physical networks or the routers that connect
We say that an internet is a virtual network system
because the communication system is an abstraction
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

7. Review of TCP/IP Layering
Recall from Chapter 1 that the Internet protocols use a five-layer reference model as Figure 20.4 illustrates
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

8. Review of TCP/IP Layering
Physical Layer (Layer 1)
Specify details about the underlying transmission medium and hardware.
All specifications related to electrical properties, radio frequencies, and signals belong to layer 1
Network Interface Layer (Layer 2)
Specify details about communication between higher layers of protocols (implemented in SW) and the underlying network (implemented in hardware).
Specifications are about network addresses, maximum packet size, and hardware addressing E-services, and video teleconferencing
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

9. Review of TCP/IP Layering
Internet Layer (Layer 3)
Specify communication across the Internet (spanning multiple interconnected networks)
Layer 3 (IP) specifies the format of packets sent across the Internet
Also specifies mechanisms used to forward packets
Transport Layer (Layer 4)
Provide for communication from an application program on one computer to an application program on another.
Includes specifications on controlling the maximum rate a receiver can accept data,
Provides procedures that are used to insure reliable transfer
mechanisms to avoid network congestion, and
techniques to insure that all data is received in the correct order
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

10. Review of TCP/IP Layering
Application Layer (Layer 5)
Specify how a pair of applications interacts when they communicate.
Specify details about the format and the meaning of messages that applications can exchange.
Examples of network applications in layer 5 are: exchange, file transfer, web browsing
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

11. Addresses for the Virtual Internet
IP address is divided into two parts:
A prefix
identifies the physical network to which the host is attached
Each network in the Internet is assigned a unique network number
A suffix
identifies a specific computer (host/node) on the network
Each computer on a given network is assigned a unique suffix
IP address scheme guarantees the following properties:
Each computer is assigned a unique address
(i.e., a single address is never assigned to more than one computer)
Network number (prefix) assignments must be coordinated globally
Suffixes are assigned locally without global coordination
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

12. Dotted Decimal Notation
A notation more convenient for humans to understand is used
Notation that has been accepted is
express each 8-bit section of a 32-bit number as a decimal value
use periods to separate the sections
The scheme is known as dotted decimal notation
Figure 21.2 illustrates examples of binary numbers and the equivalent dotted decimal notation
Dotted decimal treats each octet (byte) as an unsigned binary integer
the smallest value, 0
occurs when all bits of an octet are zero (0)
the largest value, 255
occurs when all bits of an octet are one (1)
dotted decimal addresses range
through
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.