Network Guide to Networks, Fourth Edition

1
Network Guide to Networks, Fourth Edition

• Chapter 11
• In-Depth TCP/IP Networking

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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

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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

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Subnetting

• Separates network into multiple, logically
  defined segments (subnets)
• Each subnets traffic separated from every other
  subnets traffic
• Enhances security
• Subnetworks must be connected via routers or
  other Layer 3 devices
• Improves performance
• Data is selectively retransmitted
• Simplifies troubleshooting

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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

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Classful Addressing (continued)
Figure 11-1 Example IP addresses with classful
addressing
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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 hosts network ID given IP address
  and subnet mask, perform ANDing

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Subnet Masks (continued)
Table 11-1 Default subnet masks
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Subnet Masks (continued)
Table 11-2 ANDing
Figure 11-2 Example of calculating a hosts
network ID
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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)

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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

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Subnetting Techniques (continued)
Table 11-3 Class B subnet masks
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Subnetting Techniques (continued)
Table 11-4 Class C subnet masks
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Calculating Subnets

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

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Calculating Subnets (continued)
Figure 11-3 A router connecting several subnets
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Calculating Subnets (continued)
Figure 11-3 (continued) A router connecting
several subnets
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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

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CIDR (continued)
Figure 11-4 Subnet mask and supernet mask
Figure 11-5 Calculating a hosts network ID on a
supernetted network
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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

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Internet Gateways (continued)
Figure 11-6 The use of default gateways
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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 clients transmission reaches default
  gateway, it assigns clients transmission a valid
  IP address

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NAT (continued)
Figure 11-7 NAT through an Internet gateway
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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

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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

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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

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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 users SMTP server

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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

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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

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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

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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

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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

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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

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Nbtstat

• Given NetBIOS name, get IP address
• Common nbtstat switches
• -a displays a machines name table given its
  NetBIOS name
• -A displays a machines 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

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Nslookup

• Query DNS database from any network computer and
  find host name of a device by specifying its IP
  address, or vice versa
• Provides hosts IP address, primary DNS server
  name, and address holding record for this name
• Many options (switches)

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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

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Dig (continued)
Figure 11-11 Output of a simple dig command
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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

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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

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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

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Ipconfig

• TCP/IP administration utility for use with
  Windows NT, 2000, XP, and Server 2003 OSs
• Provides information about network adapters 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

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Winipcfg

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

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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

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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

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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

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VoIP (continued)
Figure 11-16 Accessing a VoIP network from
traditional telephones
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VoIP (continued)
Figure 11-17 Accessing a VoIP network from IP
phones
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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

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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

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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

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Summary (continued)

• The nslookup utility allows you to look up the
  DNS host name of a network node by specifying the
  nodes 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