Network+ Guide to Networks, Fourth Edition
Chapter 11
In-Depth TCP/IP Networking
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)
Table 11-2: ANDing
Figure 11-2: Example of calculating a host’s network ID
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 11-4: Subnet mask and supernet mask
Figure 11-5: Calculating a host’s network ID on a supernetted
network
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
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–
–
–
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
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–
–
–
–
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
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–
–
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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