Chapter 11

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Chapter 11

In
-
Depth TCP/IP Networking

Network+ Guide to Networks, Fourth Edition

Objectives


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


Explain the differences between public and private
networks


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

Subnetting


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
devices


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
-
1:
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
address


“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
-
2:
Example of calculating a host’s network ID

Table 11
-
2:
ANDing

Reserved Addresses


Certain types of IP addresses reserved for special
functions


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
-
4:
Class C subnet masks

Calculating Subnets


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


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
-
3:
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
network

Figure 11
-
4:
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
-
6:
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
scheme


Even non
-
legitimate schemes


Must have legitimate IP address to exchange data with
Internet


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

NAT (continued)

Figure 11
-
7:
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
192.168.0.1


Clients must be set up to obtain IP addresses
automatically


ICS host assigns clients IP addresses in range of
192.168.0.2 through 192.168.0.255

Intranets and Extranets


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


Used for supplying HTTP
-
accessible documents,

e
-
mail, file sharing, document management, and
collaboration


Defined by its security policies


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

TCP/IP Mail Services


E
-
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


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
-
mail,
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
type


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

Netstat


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
activation


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
list


-
r provides list of routing table information


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

Nbtstat


Given NetBIOS name, get IP address


Common nbtstat switches:


-
a displays a machine’s name table given its NetBIOS
name


-
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
machine

Nslookup


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)

Dig


Domain information groper (dig): similar to
nslookup


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
-
11:
Output of a simple dig command

Whois


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


Web
-
based alternatives exist


e.g., www.arin.net

Traceroute (Tracert)


Uses ICMP to trace path from one node to another


Identifies all intermediate hops


Useful for determining router or subnet connectivity
problems


Transmits series of UDP datagrams to specified
destination


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
names


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


Default is 30


-
w identifies timeout period for responses

Ipconfig


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
interfaces


/renew renews DHCP
-
assigned addresses for all network
interfaces

Winipcfg


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


Graphical interface

Ifconfig


TCP/IP configuration and management utility used
on UNIX
-
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
transmission


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
packet
-
switched network than data signals


Internet telephony: VoIP carried via Internet


May also be carried over private lines

VoIP (continued)

Figure 11
-
16:
Accessing a VoIP network from traditional
telephones

VoIP (continued)

Figure 11
-
17:
Accessing a VoIP network from IP phones

Summary


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
information


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


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
e
-
mail server to another over TCP/IP
-
based
networks


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
-
based
networks to carry voice signals