Chapter 11

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Chapter 11

Depth TCP/IP Networking

Network+ Guide to Networks, Fourth Edition


Understand methods of network design unique to
TCP/IP networks, including subnetting, CIDR, NAT
and ICS

Explain the differences between public and private

Describe protocols used between mail clients and
mail servers, including SMTP, POP3, and IMAP4

Employ multiple TCP/IP utilities for network
discovery and troubleshooting

Designing TCP/IP
Based Networks

Review of some TCP/IP fundamentals:

IP is a routable protocol

On a network using TCP/IP, each interface associated
with unique IP address

Some nodes may use multiple IP addresses

IP addresses consist of four 8
bit octets

Many networks assign IP addresses and host names
dynamically, using DHCP

Every IP address can be associated with a network class


Separates network into multiple, logically defined
segments (subnets)

Each subnet’s traffic separated from every other
subnet’s traffic

Enhances security

Subnetworks must be connected via routers or other Layer 3

Improves performance

Data is selectively retransmitted

Simplifies troubleshooting

Classful Addressing

Adheres to network class distinctions

Only Class A, B, and C addresses are recognized

Network ID limited to first 8 bits in Class A, first

16 bits in Class B, and first 24 bits in Class C

Fixed network ID size ultimately limits number of
hosts a network can include

Classful Addressing (continued)

Figure 11
Example IP addresses with classful addressing

Subnet Masks

Subnetting depends on subnet masks to identify how
a network is subdivided

Indicates where network information is located in an IP

“1” bits indicate corresponding bits in IP address contain
network information

“0” bits indicate corresponding bits in IP address contain
host information

To calculate host’s network ID given IP address and
subnet mask, perform ANDing

Subnet Masks (continued)

Table 11
1: Default subnet masks

Subnet Masks (continued)

Figure 11
Example of calculating a host’s network ID

Table 11

Reserved Addresses

Certain types of IP addresses reserved for special

In network IDs, bits for host information set to 0

In broadcast addresses, octet(s) representing host
information set to all 1s (255 in decimal notation)

Subnetting Techniques

Subnetting breaks rules of classful addressing

Some bits that in classful addressing would represent host
information changed to represent network information

Reduce number of usable host addresses per subnet

Subnetting Techniques (continued)

Table 11
3: Class B subnet masks

Subnetting Techniques (continued)

Table 11
Class C subnet masks

Calculating Subnets

Formula for determining how to modify a default
subnet mask: 2n

n = number of bits in subnet mask that must be switched
from 0 to 1

Y = number of subnets that result

Extended network prefix: Additional bits used for
subnet information plus existing network ID

Class A, B, and C networks can all be subnetted

External routers pay attention to only the network
portion of devices’ IP addresses

Calculating Subnets (continued)

Figure 11
A router connecting several subnets

Calculating Subnets (continued)

Figure 11
3 (continued):
A router connecting several subnets

CIDR (Classless Interdomain Routing)

Classless routing or supernetting

Provides additional ways of arranging network and
host information in an IP address

Supernet: Subnet created by moving subnet
boundary to the left

Generates more usable IP addresses

CIDR notation (slash notation): network ID
followed by forward slash (/), followed by number
of bits used for extended network prefix

CIDR Block

CIDR (continued)

Figure 11
5: Calculating a host’s network ID on a supernetted

Figure 11
Subnet mask and supernet mask

Internet Gateways

Combination of software and hardware enabling two
different network segments to exchange data

Every device on a TCP/IP
based network has a
default gateway

First interprets outbound requests to other subnets

Interprets inbound requests from other subnets

Each node on network has one default gateway

May be network interface on a router

Must maintain routing tables as well

Core gateways make up the Internet backbone

Internet Gateways (continued)

Figure 11
The use of default gateways

NAT (Network Address Translation)

Default gateways can be used to “hide” IP numbers
assigned within an organization

Clients behind gateway may use any IP addressing

Even non
legitimate schemes

Must have legitimate IP address to exchange data with

NAT: when client’s transmission reaches default
gateway, it assigns client’s transmission a valid IP

NAT (continued)

Figure 11
NAT through an Internet gateway

ICS (Internet Connection Sharing)

Computer with Internet access (ICS host) configured
to translate requests to and from Internet on behalf
of other computers on network

Acts as DHCP server, DNS resolver, and NAT gateway
for clients on its LAN

Network adapter on ICS host assigned IP address of

Clients must be set up to obtain IP addresses

ICS host assigns clients IP addresses in range of through

Intranets and Extranets

Intranet: network or part of network that uses
based services to exchange information
within an enterprise

Used for supplying HTTP
accessible documents,

mail, file sharing, document management, and

Defined by its security policies

Extranet: network that uses Internet
like services
and protocols to exchange information within an
with certain, authorized users
outside of that organization

TCP/IP Mail Services

mail is most frequently used Internet service that
network administrators manage

Mail servers communicate with other mail servers to
deliver messages across Internet

Hundreds of software packages for mail servers exist

Sendmail, Microsoft Exchange Server, Lotus Notes,
Novell Groupwise

Mail clients send/retrieve messages to/from mail

Servers and clients communicate through TCP/IP
Application layer protocols

SMTP (Simple Mail Transfer Protocol)

Protocol responsible for moving messages between
mail servers over TCP/IP
based networks

Belongs to Application layer of TCP/IP Model

Relies on TCP at Transport layer

Operates from port 25

Relies on higher
level programs for instructions

Can only transport or hold mail

When configuring clients to use Internet e
must identify user’s SMTP server

MIME (Multipurpose Internet Mail Extensions)

Standard SMTP message format allows for lines that
contain 1000 ASCII characters max

Cannot handle pictures or formatted text

MIME: standard for encoding and interpreting
binary files, images, video, and non
ASCII character
sets within e
mail messages

Identifies each element of a message according to content

Works in conjunction with SMTP

POP (Post Office Protocol)

Application layer protocol used to retrieve messages
from mail servers

POP3 is most current and commonly used version

Mail delivered and stored on mail server until user
connects (via e
mail client) to retrieve messages

Mail deleted from server after retrieval

Minimizes use of server resources

Best suited to users who retrieve mail from same
workstation all the time

IMAP (Internet Message Access Protocol)

Developed as sophisticated alternative to POP3

IMAP4 is most current version

Users can store messages on mail server

IMAP4 provides the following features:

Retrieve all or only a portion of any mail message

Review messages and delete them while the messages
remain on the server

Create sophisticated methods of organizing messages on
the server

Share mailboxes in a central location

Additional TCP/IP Utilities

TCP/IP comes with complete set of utilities that can
help to track down most TCP/IP
related problems

e.g., Ping, Telnet, ARP

Nearly all TCP/IP utilities can be accessed from
command prompt on any type of server or client
running TCP/IP

Syntax may differ depending on OS

Options may differ according to OS


Displays TCP/IP statistics and details about TCP/IP
components and connections on a host

Port on which a particular TCP/IP service is running

Network connections currently established

Number of packets handled by network interface since

Number of data errors

Netstat (continued)

Common Netstat switches:

a lists all available TCP and UDP connections

e displays details about all packets that have been sent

n lists currently connected hosts according to their ports
and IP addresses (in numerical form)

p allows you to specify what type of protocol statistics to

r provides list of routing table information

s provides statistics about each packet transmitted by a
host, separated according to protocol type


Given NetBIOS name, get IP address

Common nbtstat switches:

a displays a machine’s name table given its NetBIOS

A displays a machine’s name table given its IP address

r lists statistics about names that have been resolved to IP
addresses by broadcast and by WINS

s displays a list of all the current NetBIOS sessions for a


Query DNS database from any network computer
and find host name of a device by specifying its IP
address, or vice versa

Provides host’s IP address, primary DNS server name,
and address holding record for this name

Many options (switches)


Domain information groper (dig): similar to

Provides more detailed information than nslookup

e.g., specifics about resource records associated with host name

Many switches

Must be explicitly installed on Windows systems

Dig (continued)

Figure 11
Output of a simple dig command


Query DNS registration database and obtain
information about a domain

Who is domain registered to?

Technical person responsible for domain?

Hosting entity?

DNS Server addresses?

Must install software to use on Windows systems

based alternatives exist


Traceroute (Tracert)

Uses ICMP to trace path from one node to another

Identifies all intermediate hops

Useful for determining router or subnet connectivity

Transmits series of UDP datagrams to specified

Increases TTL as path is discovered

Traceroute may stop before completing

Device problem on path

Device does not accept ICMP transmissions

Often indicates firewall

Traceroute (continued)

Common switches:

d instructs traceroute not to resolve IP addresses to host

h specifies maximum number of hops packets should
take when attempting to reach a host

Default is 30

w identifies timeout period for responses


TCP/IP administration utility for use with Windows
NT, 2000, XP, and Server 2003 OSs

Provides information about network adapter’s IP address,
subnet mask, and default gateway

Commonly used switches:

/? displays list of available switches

/all displays complete TCP/IP configuration information
for each network interface on device

/release releases DHCP
assigned addresses for all network

/renew renews DHCP
assigned addresses for all network


Same as ipconfig utility, but applies to Windows 9x
and Me OSs

Graphical interface


TCP/IP configuration and management utility used
type of systems

Similar to ipconfig on Windows systems

Commonly used switches:

a applies command to all interfaces on a device

down marks interface as unavailable to network

up reinitializes interface after it has been taken “down”

Complete list of switches found in man pages

VoIP (Voice over IP)

Use of packet
switched networks and TCP/IP to
transmit voice conversations

IP telephony

Objectives for implementing VoIP:

Lower costs for voice calls

Supply new or enhanced features and applications

Centralize voice and data network management

VoIP (continued)

VoIP callers can use:

Traditional telephone

IP telephones: telephones designed for TCP/IP

Softphones: computers equipped with microphone,
speaker, and VoIP client software

IP telephones must have unique IP addresses

More difficult to transmit voice signals over a
switched network than data signals

Internet telephony: VoIP carried via Internet

May also be carried over private lines

VoIP (continued)

Figure 11
Accessing a VoIP network from traditional

VoIP (continued)

Figure 11
Accessing a VoIP network from IP phones


Subnetting separates one network or segment into
multiple, logically defined segments, or subnets

Bits in a subnet mask that equal 1 indicate that
corresponding bits in an IP address contain network

Bits in a subnet mask that equal 0 indicate that
corresponding bits in an IP address contain host

CIDR allows the creation of supernets, or subnets
established by using bits that normally would be
reserved for network class information

Summary (continued)

Gateways facilitate communication between
different subnets

Every device on a TCP/IP
based network has a
default gateway

NAT allows a network administrator to “hide” IP
addresses assigned to nodes on a private network

ICS is a service that allows a network of computers
to share a single Internet connection through an ICS
host computer

Summary (continued)

SMTP is responsible for moving messages from one
mail server to another over TCP/IP

POP is a mail retrieval protocol

IMAP4 allows users to store messages on the mail
server, rather than always having to download them
to the local machine

The netstat utility displays TCP/IP statistics and the
state of current TCP/IP components and connections

Summary (continued)

The nslookup utility allows you to look up the DNS
host name of a network node by specifying the
node’s IP address, or vice versa

The traceroute utility useful for determining router
or subnet connectivity problems

VoIP is the use of packet
switched TCP/IP
networks to carry voice signals