Introduction to TCP/IP

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Introduction to TCP/IP
Unit 1
TCP/IP Structure and Addressing
This unit introduces the Transmission Control Protocol/Internet
Protocol (TCP/IP) protocol suite. We begin with a review of the
Open Systems Interconnection (OSI) model. We then introduce
the TCP/IP Network, Transport, and Application Layer protocols.
Then we explore the different address types necessary to move
information from one user’s application to another’s across a
TCP/IP network. We review IP address structures and how they are
divided into logical and physical groups called subnets.
To end the unit, we review some of the most common network
devices responsible for moving data across networks.
1.OSI Model Review
2.Internet Protocol Suite
3.Internet Addressing
4.Internetworking Devices
100BaseFX—100BaseFX is a 100BaseT variant that runs over fiber
optic cabling. 100BaseFX is generally used for high-speed LAN
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100BaseT—100BaseT is based on 802.3 Ethernet and uses twisted
pair cabling, as does 10BaseT Ethernet. However, 100BaseT runs 10
times faster than 10Base2 and 10Base5, at 100 Mbps.
10Base2—10Base2 is one of several physical cabling standards for
802.3 Ethernet networks. 10Base2 uses thin RG-58A/U coaxial
cabling for connecting nodes on a bus topology, as does 10Base5.
It is also known as Thinnet.
10Base5—Also known as Thicknet, 10Base5 was the original
Ethernet cabling standard. It is called Thicknet because it uses a
coaxial cable approximately 0.5 inches thick.
10BaseT—10BaseT is one of several physical cabling standards for
802.3 Ethernet LANs. 10BaseT networks use twisted pair cabling
for connecting nodes in a star topology.
Address Resolution Protocol (ARP)—ARP is the protocol used by
IP (as in TCP/IP) for address resolution. Address resolution refers
to the ability of a station to resolve another station’s MAC
(hardware) address given its IP address.
AppleTalk—AppleTalk is Apple’s proprietary, seven-layer, peer-to-
peer network communications protocol for Macintosh networks.
AppleTalk runs on Apple network topologies, as well as over
Ethernet and Token Ring networks.
Asynchronous Transfer Mode (ATM)—ATM is a cell-switching
network that consists of multiple ATM switches that forward each
individual cell to its final destination. ATM can provide transport
services for audio, data, and video.
Bootstrap Protocol (BOOTP)—BOOTP, described in RFC 951, is a
method in which a diskless workstation or terminal can obtain
configuration information, such as its IP address and a configura-
tion file, from a server on which this information is stored.
bridge—A bridge is a hardware device that connects LANs. It can
be used to connect LANs of the same type, such as two Token
Ring segments, or LANs with different types of media, such as
Ethernet and Token Ring. A bridge operates at the Data Link Layer
of the OSI reference model.
bridge router (brouter)—A brouter is an internetworking device
that combines the functions of both a bridge and a router. See
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broadcast—The term broadcast is used in several different ways in
communications and networking. With respect to LANs, the term
refers to information (that is, frames) sent to all devices on the
physical segment. For example, a bus topology, in which a
common cable is used to connect devices, is considered a broadcast
technology. Another common use of the term broadcast relates to
frames. Broadcast frames contain a special destination address that
instructs all devices on the network to receive the frame.
checksum—Checksum is computed by the number of bits in a
transmitted unit of data. A checksum is appended to a data unit as a
simple error-detection method. The receiving node counts the data
bits and compares the result to the checksum, to see whether all bits
have arrived. If the numbers match, the transmission was likely
complete. TCP and UDP provide checksum count and verification.
collision—A collision occurs in an Ethernet network when two
frames are put onto the physical medium at the same time and
overlap fully or partially. When a collision occurs, the data on the
physical segment is no longer valid.
datagram—A datagram is a unit of information processed by the
Network Layer of the OSI reference model. The packet header
contains the logical (network) address of the destination node.
Intermediate nodes forward a packet until it reaches its
destination. A packet can contain an entire message generated by
higher OSI layers or a segment of a much larger message.
DECnet—DECnet is a proprietary network architecture created by
DEC (now Hewlett-Packard). The most recent iteration, DECnet
Phase V, specifies seven layers that correspond to the OSI reference
model, while maintaining compatibility with previous versions.
Domain Name System or Service (DNS)—In a TCP/IP network, a
user can communicate with another user by specifying a name,
such as TCP and IP require Internet
addresses for messages, thus one must be translated to the other.
This is the job of the DNS; given a name, it returns an Internet
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Dynamic Host Configuration Protocol (DHCP)—DHCP provides
configuration parameters to Internet hosts. It consists of two com-
ponents: a protocol for delivering host-specific configuration
parameters from a DHCP server to a host, and a mechanism for
allocation of network addresses to hosts. DHCP is built on a client/
server model, where designated DHCP server hosts allocate net-
work addresses and deliver configuration parameters to dynami-
cally configured hosts.
Ethernet—Ethernet technology, originally developed in the 1970s
by Xerox Corporation in conjunction with Intel and DEC, is now
the primary medium for LANs. The original Ethernet has 10-Mbps
throughput and uses the CSMA/CD method to access the physical
media. Fast Ethernet (100-Mbps Ethernet) and Gigabit Ethernet
(1,000-Mbps Ethernet) are also used.
expansion card—An expansion card is a device you plug in to a
PC backplane to add functionality, such as a modem, NIC, or disk
extranet—An extranet is a broader form of a private intranet. Extra-
nets are private TCP/IP networks that are shared between closely
aligned organizations, and are not available to the general public.
Fiber Distributed Data Interface (FDDI)—FDDI is a LAN standard
specifying a 100-Mbps token-passing network using fiber optic cable.
File Transfer Protocol (FTP)—FTP is a TCP/IP Application Layer
protocol used to transfer files between two computers.
hub—Also referred to as a wiring concentrator, a simple hub is a
repeater with multiple ports. A signal coming into one port is
repeated out the other ports.
Hypertext Transfer Protocol (HTTP)—HTTP is the Application
Layer protocol used to request and transmit documents by means
of the Web.
Institute of Electrical and Electronic Engineers (IEEE)—IEEE is a
professional organization composed of engineers, scientists, and
students. Founded in 1884, IEEE publishes computer and
electronics standards, including the 802 series that defines
shared-media networks such as Ethernet and Token Ring.
Internet—The term “Internet,” capitalized, refers to the global
internetwork of TCP/IP networks.
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Internet Control Message Protocol (ICMP)—ICMP is an integral
part of IP that handles error and control messages. Gateways and
hosts use ICMP to report problems about datagrams back to the
original source that sent the datagram. ICMP also includes an
echo request/reply used to test whether a destination is reachable
and responding.
Internet Corporation of Assigned Names and Numbers (ICANN)—
ICANN is a private, nonprofit organization responsible for
overseeing the domain name registration process, assigning IP
addresses, assigning protocol parameters, and managing the DNS
root servers. Learn more about ICANN at
Internet Engineering Task Force (IETF)—IETF is a large, open,
international community of network designers, operators,
vendors, and researchers concerned with the evolution of the
Internet architecture and smooth operation of the Internet.
Internet Group Management Protocol (IGMP)—IGMP is the
Internet standard by which hosts can communicate their
multicast group membership status to multicast routers. IGMP is
used to keep up-to-date information on which host is in which
multicast group.
Internet Packet Exchange/Sequenced Packet Exchange (IPX/
SPX)—IPX is NetWare’s proprietary Network Layer protocol. SPX is
the connection-oriented transport protocol concerned with
connection-oriented services such as sequencing packets and
guaranteeing their delivery, which provides reliability for IPX
Internet Protocol (IP)—IP is the TCP/IP standard protocol that
defines the IP datagram as the unit of information passed across
an internet. IP provides the basis for connectionless, best-effort
packet delivery service, and includes ICMP. The entire protocol
suite is often referred to as TCP/IP because TCP and IP are the two
most fundamental protocols.
internetwork—A complex network that may combine smaller
networks in different physical locations, based on different types
of network architectures, is referred to as an internetwork.
intranet—An intranet is an internal network. An intranet is
generally only accessible by corporate network users, but it runs
all the same protocols and services as does an Internet site.
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loopback—The TCP/IP loopback function allows a network
administrator to test IP software without concern for the
hardware or drivers. The loopback address is the
designated software loopback interface for the machine.
Medium Access Control (MAC)—MAC is one of the media-specific
IEEE 802 standards (802.3, 802.4, and 802.5) that defines the
protocol and frame formats for Ethernet, Token Bus, and Token
Ring. It is the lower sublayer of the Data Link Layer of the OSI
model used to transmit frames between NICs.
network interface card (NIC)—A NIC is an expansion board
inserted into a computer to enable the computer to be connected
to a network.
Network News Transfer Protocol (NNTP)—NNTP is the Internet
protocol used to transfer news messages between news servers and
clients. Users use news clients to post, distribute, and retrieve
news articles from a database stored on a central news server.
Open Systems Interconnection (OSI)—OSI began as a reference
model, that is, an abstract model for data communications.
However, now the OSI model has been implemented and is used
in some data communications applications. The seven-layer OSI
model falls logically into two parts. Layers 1 through 4, the
“lower” layers, are concerned with the communication of raw
data. Layers 5 through 7, the “higher” layers, are concerned with
the networking of applications.
PC card—Previously known as a PCMCIA card, a PC card is a
small, rectangular expansion card for portable devices, such as
laptop and palm computers. PC cards can be NICs, modems, and
hard drives, to name a few.
Post Office Protocol (POP)—POP is used to transfer information
from a mail server to a user’s computer so the information can be
read by a mail program at the user’s desk. POP3 is the latest
iteration of the protocol.
protocol data unit (PDU)—The concept of a PDU is used in the
OSI reference model. From the perspective of a protocol layer, a
PDU consists of information from the layer above plus the proto-
col information appended to the data by that layer. For example,
a frame is a PDU of the Data Link Layer, and a packet is a PDU of
the Network Layer.
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repeater—A repeater connects one cable segment of a LAN to other
segments, including connecting differing media. For example, a
repeater connects thin Ethernet cables to thick Ethernet cables. It
regenerates electrical signals from one segment of a cable onto all
other segments. Because a repeater reproduces exactly what it
receives, bit by bit, it also reproduces errors.
Request for Comment (RFC)—RFCs are the working documents of
the Internet research and development community. A document in
this series may be on essentially any topic related to computer
communication and may be anything from a meeting report to the
specification of a standard.
router—A router is a Layer 3 device with several ports that can
each connect to a network or another router. The router examines
the logical network address of each packet, then uses its internal
routing table to forward the packet to the routing port associated
with the best path to the packet’s destination. If the packet is
addressed to a network that is not connected to the router, the
router forwards the packet to another router that is closer to the
final destination. Each router, in turn, evaluates each packet and
then either delivers the packet or forwards it to another router.
Simple Network Management Protocol (SNMP)—SNMP is a TCP/
IP Application Layer protocol used to send and receive information
about the status of network resources on a TCP/IP network.
switch—A switch is a device that operates at the Data Link Layer
of the OSI reference model. It can connect LANs or segments of
the same media access type and dedicate its entire bandwidth to
each frame it switches.
Systems Network Architecture (SNA)—SNA is IBM’s architecture
for computer networking, which was designed for transaction
processing in mission-critical applications. SNA networks usually
involve a large number of terminals communicating with a
Telnet—Telnet is a TCP/IP Application Layer protocol that provides
a remote login capability to another computer on a network.
Token Ring—Token Ring is the IEEE 802.5-specified, ring-based,
token-passing LAN topology. Each node on the ring acts as a
repeater, passing a token from node to node as the token travels
around the entire ring. Each node must wait its turn to transmit
data, and may only transmit when it controls the token.
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Transmission Control Protocol (TCP)—TCP is the TCP/IP Transport
Layer protocol that provides reliable, full-duplex, stream service.
TCP allows a process on one computer to send data to a process on
another computer. TCP software implementations normally reside
in the OS and use IP to transmit information across the underlying
Trivial File Transfer Protocol (TFTP)—TFTP is the TCP/IP protocol
for file transfer with minimal capability and overhead. TFTP
depends on the unreliable, connectionless, datagram delivery ser-
vice UDP. TFTP is designed for use on diskless workstations that
keep such software in ROM.
unicast—A unicast is a transmission sent to a single network
address. This is in contrast to a broadcast, which is sent to all
network addresses simultaneously, and a multicast, which is sent
to several addresses at once.
Universal Serial Bus (USB) adapter—A USB adapter is a PC
expansion device that conforms to the USB standard. USB devices
support data transfer rates of up to 12 Mbps and up to 127 devices
on the same bus. USB device examples include NICs, modems,
scanners, and CD-ROM drives.
UNIX-to-UNIX Copy Program (UUCP)—UUCP is a standard UNIX
utility that copies files between UNIX systems. It can be used for
e-mail transfer.
User Datagram Protocol (UDP)—UDP is the TCP/IP protocol that
allows an application program on one computer to send a
datagram to an application program on another computer. UDP
uses IP to deliver datagrams. The difference between UDP
datagrams and IP datagrams is that UDP includes a protocol port
number, allowing the sender to distinguish among multiple
destinations (application programs) on the remote computer.
UDP also includes a checksum for the data being sent.
Lesson 1—OSI Model Review
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Lesson 1—OSI Model Review
This lesson reviews some of the OSI model’s key principles. The
OSI model is used as a frame of reference in almost all computer
networking texts and courses, and in this course, as well. The
information reviewed in this lesson is critical to understanding
computer networking.
At the end of this lesson you will be able to:
• Describe each OSI model layer’s primary function
• List the two main address types found in most computer
• Explain why three address types are used to move informa-
tion from source to destination
Primary Functions of the OSI Model Layers
The OSI Model Layers Table provides an overview of each OSI
model layer’s primary functions. It also presents the layer’s unit of
information and address type where appropriate.

There are two main types of computer addresses: physical
and logical.
OSI Model Layers
OSI Model
Layer Function
Unit of
Address Type
Application User functionality Program
Presentation Character representation
Characters and
Session Establishing, conduct-
ing, and ending sessions
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A physical address is also referred to as a:
• Hardware address
• Adapter address
• Network interface card (NIC) address
• Medium Access Control (MAC) address
A physical address is required for network devices to ultimately
deliver information to a given network node. The word “ulti-
mately” is used because information often starts out (at the
higher layers) addressed to some symbolic name, such as the host
name in the command “ftp serverhost.” The name “serverhost”
refers to the name of the target host computer the user is attempt-
ing to contact using the ftp (TCP/IP) application and protocol. For
the user to connect to this host, a physical address must somehow
be derived from the symbolic name, and then used in an agreed-
upon addressing scheme to reach the target. In this case, a name
service such as the Domain Name Service (DNS) derives a logical
software address from the symbolic name. The TCP/IP protocol
suite calls this logical address an IP address.
Transport Transmitting messages
from sending computer
process to receiving
computer process
Message Process to process
Network Transmitting individual
packets across a network
Packet Packet address
receiver’s network
and host location
Data Link Transmitting frames con-
taining a packet across a
link en route to final
Frame NIC (next node in
Physical Transmitting bits in the
form of signals across
physical media
OSI Model Layers (Continued)
OSI Model
Layer Function
Unit of
Address Type
Lesson 1—OSI Model Review
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The Physical Layer
You may want to associate a physical address with the OSI model
Physical Layer; indeed, this seems natural. However, the OSI
model Data Link Layer actually processes the physical address.
The Physical Layer only transmits bits to, and receives bits from,
the physical medium, and does not “see” the bits as organized
into meaningful patterns, such as an address. The Physical Layer
operates depending on the chosen network topology. For exam-
ple, Ethernet networks format, frame, and time bits in a different
manner than do Token Ring networks.
The Data Link
We can categorize physical addresses, for the purposes of net-
working, into two general types: local area network (LAN)
addresses and wide area network (WAN) addresses. A LAN address
is commonly found in an Ethernet or Token Ring LAN environ-
ment, while we use WAN addresses in High-Level Data Link Con-
trol (HDLC) or frame relay network protocol addressing.
Physical addresses support a flat network model. This means that
physical addresses are not routable; they only designate a host,
not both a network and host. To be routable, as in TCP/IP, a pro-
tocol must designate both a host and network address. Devices
communicating with a Layer 2 only addressing scheme cannot
communicate across network (Layer 3) boundaries.
A logical address differs from a physical address in that it is gener-
ally implemented as a software entity rather than a hardware
entity. There are two primary types of logical addresses, as follows:
• Network addresses, processed at the Network Layer
• Port or process addresses, processed at the Transport Layer
The Network Layer
One example of a logical address is an IP address (network), such
as The network administrator assigns each host on an
IP network a unique IP address. The IP address is a 32-bit address
that designates both a host portion and a network portion.
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Because layer protocols designate both a host and a network, they
allow us to build hierarchical networks. The Three-Layer Network
Hierarchy Diagram illustrates this concept.
Layer 3 addresses allow us to create multiple routed networks. We
can use the three-layer hierarchical model to isolate traffic to local
network segments, only passing traffic between layers when hosts
on one segment must communicate with hosts on other seg-
ments. Ideally, we would isolate traffic to the bottom two layers,
the access and distribution layer, and only pass WAN traffic to the
core layer. Routing and Layer 3 addresses make this possible.
Three-Layer Network Hierarchy
Lesson 1—OSI Model Review
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The Transport
Another logical address example is a port number (process
address), such as “23.” Devices use port addresses to pass informa-
tion to the higher layers. Devices use port numbers to keep track
of multiple simultaneous conversations.
Network applications use well-known port numbers to communi-
cate with their peer processes. For example, Web browsers request
Web services on the well-known TCP Port 80. The Well-Known
Port Numbers Table lists some of the more commonly used TCP
and User Datagram Protocol (UDP) addresses.
The Transport Layer is responsible not only for application
addressing, but also for providing reliable communications over
the best effort Layer 3 protocols. The Transport Layer provides
flow control, windowing, data sequencing, and recovery.
Layers 5
Through 7
The remaining OSI model layers work with the data itself. These
layers leave the end-to-end data transport issues to the lower
four layers.
Well-Known Port Numbers
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The Session Layer
The Session Layer is responsible for establishing, conducting, and
terminating conversations (sessions) between application entities.
The Session Layer provides the following services to the higher
layer protocols:
• Establish a session
• Conduct dialogs to prevent each conversing node from trans-
mitting data simultaneously
• Manage session activities by dividing each conversation into
• End the session gracefully
The Presentation
The Presentation Layer is responsible for data representation,
security, and compression. This layer ensures that the sender
transfers information to the receiver in a format that the receiving
application can understand. The Presentation Layer adds data
encryption and decryption protocols to protect data from unau-
thorized access and, by relying on data compression protocols,
ensures efficient use of the communications channel.
The Application
The Application Layer provides the user interface to the system.
Not all applications are user applications, however, with some
designed to provide services to other applications.
A network application identifies its peers, establishes communica-
tions, and enacts procedures for peer-to-peer data transfer. The
peer applications may also determine whether sufficient network
resources exist for successful communications.
Addressing Summary
The most important fact to remember concerning logical
addresses is that a logical address will not transmit information
“into the box.” Only the physical address, whether it is a broad-
cast, multicast (group), or single destination (unicast) address, can
accomplish this.
Lesson 1—OSI Model Review
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1.The OSI model Physical Layer serves which primary purpose?
a.It provides physical addresses for moving information
into the box.
b.It builds frames that carry higher layer PDU across net-
work segments.
c.It uses network and host addresses to route packets across
d.It transmits data in the form of bits across the physical
2.Which is the primary purpose of the OSI model Session Layer?
a.It builds frames that carry higher layer PDU across net-
work segments.
b.It builds a flat network model with unroutable physical
c.It establishes, manages, and terminates dialogs between
Presentation Layer entities.
d.It defines the format in which Application Layer informa-
tion is sent across the network.
3.Which is the primary purpose of the OSI model Presentation
a.It establishes, manages, and terminates sessions between
b.It uses logical addresses to identify higher layer applications.
c.It builds a flat network model with unroutable physical
d.It defines the format in which Application Layer informa-
tion is sent across the network.
4.The OSI model Data Link Layer serves which primary purpose?
a.It builds a flat network model with unroutable physical
b.It transmits data in the form of bits across the physical
c.It creates multiple routed networks with logical addresses.
d.It establishes, manages, and terminates sessions between
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5.The OSI model Application Layer serves which primary purpose?
a.It defines well-known logical port addresses for communi-
cations between applications.
b.It controls dialogs between two or more Presentation
Layer entities.
c.It establishes user application procedures for error recov-
ery and data integrity.
d.It creates a hierarchical addressing model for moving
information across networks.
Extended Activities
1.On a Windows PC, select Start, Find, Files, or Folders. In the
named box, type services. Select the drive on which the oper-
ating systems is installed (typically C:) in the Look In box.
In the Results window, double-click the Services file (there is
no extension). When prompted for a program with which to
open the file, choose Notepad. Notepad will open the file; this
file contains all the TCP and UDP well-known service ports as
defined by RFC 1060.
2.Locate in the Services file the following services and list their
port numbers:
e.Nbname (NetBIOS name)