Transcript Chapter 1 - Introduction

Computer Networks and Internets, 5e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 23
Support Protocols
and
Technologies
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Topics Covered
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23.1 Introduction
23.2 Address Resolution
23.3 The Address Resolution Protocol (ARP)
23.4 ARP Message Format
23.5 ARP Encapsulation
23.6 ARP Caching and Message Processing
23.7 The Conceptual Address Boundary
23.8 Internet Control Message Protocol (ICMP)
23.9 ICMP Message Format and Encapsulation
23.10 Protocol Software, Parameters, and Configuration
23.11 Dynamic Host Configuration Protocol (DHCP)
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Topics Covered
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23.12
23.13
23.14
23.15
23.16
23.17
23.18
23.19
DHCP Protocol Operation and Optimizations
DHCP Message Format
Indirect DHCP Server Access Through a Relay
Network Address Translation (NAT)
NAT Operation and Private Addresses
Transport-Layer NAT (NAPT)
NAT and Servers
NAT Software and Systems for Use at Home
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23.1 Introduction
• This chapter
– continues the discussion of internetworking by introducing four key
support technologies:
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•
•
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address binding
error reporting
bootstrapping
address translation
– Each technology handles a small problem
– When combined with other protocols
• each makes a significant contribution to the overall functionality
• Future chapters
– extend the discussion of internetworking
• by focusing on transport layer protocols and Internet routing protocols
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23.2 Address Resolution
• A crucial step of the forwarding process requires a translation:
– forwarding uses IP addresses
– a frame transmitted must contain the MAC address of the next hop
– IP must translate the next-hop IP address to a MAC address
• The principle is:
– IP addresses are abstractions
• provided by protocol software
– Network does not know how to locate a computer from its IP address
• the next-hop address must be translated to an equivalent MAC address
• Translation from a computer's IP address to an equivalent
hardware address is known as address resolution
– And an IP address is said to be resolved to the correct MAC address
• Address resolution is local to a network
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23.2 Address Resolution
• One computer can resolve the address of another computer only if both
computers attach to the same physical network
– A computer never resolves the address of a computer on a remote network
– Address resolution is always restricted to a single network.
• For example, consider the simple internet in Figure 23.1
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23.3 The Address Resolution Protocol (ARP)
• What algorithm does software use to translate?
– The answer depends on the protocol and hardware addressing
• here we are only concerned with the resolution of IP
• Most hardware has adopted the 48-bit Ethernet
• In Ethernet: Address Resolution Protocol (ARP)
• Consider Figure 23.2
– Suppose B needs to resolve the IP address of C
– B broadcasts a request that says:
“I'm looking for the MAC address of a computer that has IP address C”
– The broadcast only travels across one network
– An ARP request message reaches all computers on a network
– When C receives a copy of the request along other hosts
• Only C sends a directed reply back to B that says:
“I'm the computer with IP address C, and my MAC address is M”
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23.3 The Address Resolution Protocol (ARP)
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23.4 ARP Message Format
•
Rather than restricting ARP to IP and Ethernet
– The standard describes a general form for ARP messages
– It specifies how the format is adapted for each type of protocol
• Choosing a fixed size for a hardware address is not suitable
– New network technologies might be invented that have addresses
larger than the size chosen
– The designers included a fixed-size field at the beginning of an ARP
message to specify the size of the hardware addresses being used
• For example, when ARP is used with an Ethernet
– the hardware address length is set to 6 octets
• because an Ethernet address is 48 bits long
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23.4 ARP Message Format
• To increase the generality of ARP
– the designers also included an address length field
• ARP protocol can be used to bind an arbitrary high-level
address to an arbitrary hardware address
• In practice, the generality of ARP is seldom used
– most implementations of ARP are used to bind IP addresses to
Ethernet addresses
• Figure 23.3 illustrates the format of an ARP message
– when the protocol is used with an IP version 4 address (4 octets) and
Ethernet hardware address (6 octets)
– each line of the figure corresponds to 32 bits of an ARP message
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23.4 ARP Message Format
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23.4 ARP Message Format
• HARDWARE ADDRESS TYPE
– 16-bit field that specifies the type of hardware address being used
– the value is 1 for Ethernet
• PROTOCOL ADDRESS TYPE
– 16-bit field that specifies the type of protocol address being used
– the value is 0x0800 for IPv4
• HADDR LEN
– 8-bit integer that specifies the size of a hardware address in bytes
•
PADDR LEN
– 8-bit integer that specifies the size of a protocol address in bytes
•
OPERATION
– 16-bit field that specifies whether the message
• request (the field contains 1) or
• response (the field contains 2)
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23.4 ARP Message Format
• SENDER HADDR
– HADDR LEN bytes for the sender's hardware address
• SENDER PADDR
– PADDR LEN bytes for the sender's protocol address
• TARGET HADDR
– HADDR LEN bytes for the target's hardware address
• TARGET PADDR
– PADDR LEN bytes for the target's protocol address
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23.4 ARP Message Format
• As Figure 23.3 shows
• An ARP message contains fields for two address bindings
– one binding to the sender
– other to the intended recipient, ARP calls it target
• When a request is sent
– the sender does not know the target's hardware address
(that is the information being requested)
• therefore, field TARGET HADDR in an ARP request can be filled with zeroes
(0s) because the contents are not used
• In a response
– the target binding refers to the initial computer that sent the request
– Thus, the target address pair in a response serves no purpose
• the inclusion of the target fields has survived from an early version of the
protocol
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23.5 ARP Encapsulation
• When it travels across a physical network
– an ARP message is encapsulated in a hardware frame
• An ARP message is treated as data being transported
– the network does not parse the ARP message or interpret fields
• Figure 23.4 illustrates ARP encapsulation in an Ethernet
frame
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23.5 ARP Encapsulation
• The type field in the frame header specifies that the frame
contains an ARP message
• A sender must assign the appropriate value to the type field
– before transmitting the frame
• And a receiver must examine the type field
– in each incoming frame
• Ethernet uses type field 0x806 to denote an ARP message
• The same value is used for both ARP requests/ responses
– Frame type does not distinguish between types of ARP messages
– A receiver must examine the OPERATION field in the message
• to determine whether an incoming message is a request or a response
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23.6 ARP Caching and Message Processing
• Sending an ARP request for each datagram is inefficient
– Three (3) frames traverse the network for each datagram
(an ARP request, ARP response, and the data datagram itself)
• Most communications involve a sequence of packets
– a sender is likely to repeat the exchange many times
• To reduce network traffic
– ARP software extracts and saves the information from a response
• so it can be used for subsequent packets
– The software does not keep the information indefinitely
• Instead, ARP maintains a small table of bindings in memory
• ARP manages the table as a cache
– an entry is replaced when a response arrives
– the oldest entry is removed whenever the table runs out of space or
after an entry has not been updated for a long period of time
– ARP starts by searching the cache when it needs to bind an address
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23.6 ARP Caching and Message Processing
• If the binding is present in the cache
– ARP uses the binding without transmitting a request
• If the binding is not present in the cache
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ARP broadcasts a request
waits for a response
updates the cache
and then proceeds to use the binding
• The cache is only updated when an ARP message arrives
(either a request or a response)
• Algorithm 23.1 outlines the procedure for handling an
incoming ARP message
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23.6 ARP Caching and Message Processing
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23.6 ARP Caching and Message Processing
• Check the text book for details of the algorithm
• For optimization, it is necessary to know two facts:
– Most computer communication involves two-way traffic
• if a message from A to B, probability is high that a reply will be from B back to A
– Each address binding requires memory
• a computer cannot store an arbitrary number of address bindings
• The first fact explains why extracting the sender's address
binding optimizes ARP performance
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23.7 The Conceptual Address Boundary
• ARP provides an important conceptual boundary between
MAC addresses and IP addresses:
– ARP hides the details of hardware addressing
– It allows higher layers of software to use IP addresses
• There is an important conceptual boundary imposed
between the network interface layer and all higher layers
• Figure 23.5 illustrates the addressing boundary
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23.7 The Conceptual Address Boundary
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23.8 Internet Control Message Protocol (ICMP)
• IP defines a best-effort communication service
– datagrams can be lost, duplicated, delayed, or delivered out of order
• It seems best-effort service does not need error detection!
– But IP attempts to avoid errors and to report problems when they
occur
• We have already seen one example of error detection in IP:
– Header checksum is used to detect transmission errors
– When a host creates an IP datagram
• the host includes a checksum that covers the entire header
– Whenever a datagram is received
• the checksum is verified to ensure that the header arrived intact
• The IP header contains a TIME TO LIVE field used to
prevent a datagram from circulating forever
– if the forwarding tables in routers incorrectly introduce a circular path
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23.8 Internet Control Message Protocol (ICMP)
• Response to a checksum error is following:
– Datagram must be discarded immediately without more processing
– The receiver cannot trust any fields in the datagram header
• because the receiver cannot know which bits were altered
– The receiver cannot send an error message back to the sender
• because the receiver cannot trust the source address in the header
– Thus, the receiver has no option but to discard the damaged datagram
• IP includes a companion protocol, ICMP
– It is used to report errors back to the original source
• IP and ICMP are co-dependent
– IP depends on ICMP to report errors
– and ICMP uses IP to carry error messages
• Many ICMP messages have been defined
• Figure 23.6 lists key ICMP messages and their purpose
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23.8 Internet Control Message Protocol (ICMP)
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23.8 Internet Control Message Protocol (ICMP)
•
As in Figure 23.6, ICMP contains two message types:
– messages used to report errors
– messages used to obtain information
• For example
– Time Exceeded and Destination Unreachable are for reporting errors
• when a datagram cannot be delivered successfully
• A destination is unreachable if no route exists to the address
• A datagram times out if
– either the TTL count in the header expires or
– fragments of the datagram do not arrive before the timer expires
•
Echo Request and Echo Reply do not correspond to an error
– Instead, they are used by the ping application to test connectivity
– When a host receives an echo request message
• ICMP software on a host or router sends an echo reply that carries the same
data as the request
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23.9 ICMP Message Format and
Encapsulation
• ICMP uses IP to transport each error message:
– when a router has an ICMP message to send
• it creates an IP datagram and encapsulates the ICMP message in it
– the ICMP message is placed in the payload area of the IP datagram
– the datagram is then forwarded as usual
• with the complete datagram being encapsulated in a frame for transmission
• Figure 23.7 (below) illustrates the two levels of encapsulation
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23.9 ICMP Message Format and
Encapsulation
• ICMP messages do not have special priority
– They are forwarded like any other datagram, with one minor exception
• If an ICMP error message causes an error
– no error message is sent
• The reason should be clear:
– the designers wanted to avoid the Internet becoming congested
carrying error messages about error messages
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23.10 Protocol Software, Parameters,
and Configuration
• Once a host or router has been powered on, OS is started and the
protocol software is initialized
• How does the protocol software in a host or router begin operation?
• For a router, the configuration manager must specify initial values
for items such as
–
–
–
–
the IP address for each network connection
the protocol software to run
and initial values for a forwarding table
the configuration is saved, and a router loads the values during startup
• Host configuration usually uses a two-step process, known as
bootstrapping
– A protocol was invented to allow a host to obtain multiple parameters with a
single request, known as the Bootstrap Protocol (BOOTP)
– Currently, DHCP is used to take care of most configuration needed
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23.11 Dynamic Host Configuration
Protocol (DHCP)
• Various mechanisms have been created to allow a host
computer to obtain parameters
• An early mechanism known as the Reverse Address
Resolution Protocol (RARP) allowed a computer to obtain
an IP address from a server
• ICMP has Address Mask Request and Router Discovery
messages
– can obtain the address mask used and the address of a router
• Each of the early mechanisms was used independently
– requests were broadcast and a host typically configured layers from
lowest to highest
• DHCP allows a computer to join a new network and obtain
an IP address automatically
– The concept has been termed plug-and-play networking
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23.11 Dynamic Host Configuration
Protocol (DHCP)
• When a computer boots
– the client computer broadcasts a DHCP Request
– the server sends a DHCP Reply
• DHCP uses the term offer to denote the message a server sends
• and we say that the server is offering an address to the client
• We can configure a DHCP server to supply two types of addresses:
– permanently assigned addresses as provided by BOOTP or
– a pool of dynamic addresses to be allocated on demand
• Typically, a permanent address is assigned to a server, and a dynamic
address is assigned to an arbitrary host
• In fact, addresses assigned on demand are not given out for an arbitrary
length of time
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23.11 Dynamic Host Configuration
Protocol (DHCP)
• DHCP issues a lease on the address for a finite period
– The use of leases allows a DHCP server to reclaim addresses
• When the lease expires
– the server places the address to the pool of available addresses
– this allows the address to be assigned to another computer
• When a lease expires, a host can choose to relinquish the
address or renegotiate with DHCP to extend the lease
– Negotiation occurs concurrent with other activity
• Normally, DHCP approves each lease extension
– A computer continues to operate without any interruption
– However, a server may be configured to deny lease extension for
administrative or technical reasons
– DHCP grants absolute control of leasing to a server
– If a server denies an extension request
• the host must stop using the address
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23.12 DHCP Protocol Operation and
Optimizations
• DHCP includes several important details that optimize
performance, such as
• Recovery from loss or duplication
– DHCP is designed to insure that missing or duplicate packets do not
result in misconfiguration
– If no response is received
• a host retransmits its request
– If a duplicate response arrives
• a host ignores the extra copy
• Caching of a server address
– once a host finds a DHCP server
• the host caches the server's address
• Avoidance of synchronized flooding
– DCHP takes steps to prevent synchronized requests
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23.13 DHCP Message Format
• DHCP adopted a slightly modified version of the BOOTP
message format
• Figure 23.8 illustrates the DHCP message format
– OP specifies whether the message is a Request or a Response
– HTYPE and HLEN fields specify the network hardware type and the length
of a hardware address
– FLAGS specifies whether it can receive broadcast or directed replies
– HOPS specifies how many servers forwarded the request
– TRANSACTION IDENTIFIER provides a value that a client can use to
determine if an incoming response matches its request
– SECONDS ELAPSED specifies how many seconds have elapsed since the
host began to boot
• Except for OPTIONS (OP), each field in a DHCP
message has a fixed size
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23.13 DHCP Message Format
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23.13 DHCP Message Format
• Later fields in the message are used in a response to carry
information back to the host that sent a request
– if a host does not know its IP address, the server uses field YOUR IP
ADDRESS to supply the value
– server uses fields SERVER IP ADDRESS and SERVER HOST
NAME to give the host information about the location of a server
– ROUTER IP ADDRESS contains the IP address of a default router
• DHCP allows a computer to negotiate to find a boot image
– To do so, the host fills in field BOOT FILE NAME with a request
– The DHCP server does not send an image
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23.14 Indirect DHCP Server Access
Through a Relay
• DHCP broadcasts on the local network to find a server
• DHCP does not require each individual network to have a
server
– Instead, a DHCP relay agent forwards requests and responses
between a client and the server
• At least one relay agent must be present on each network
– and the relay agent must be configured with the address of the
appropriate DHCP server
• When the server responds
– the relay agent forwards the response to the client
• It may seem that using multiple relay agents is no better
than using multiple DHCP servers
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23.14 Indirect DHCP Server Access
Through a Relay
• Network managers prefer to manage multiple relay agents
for two reasons
• First
– In a network with one DHCP server and multiple relay agents,
administration of addresses is centralized into a single device
• Thus, a network manager does not need to interact with multiple devices to
change the lease policy or determine the current status
• Second
– Many commercial routers contain a mechanism that provides DHCP
relay service on all the networks to which the router attaches
• Relay agent facilities in a router are usually easy to
configure
– and the configuration is unlikely to change
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23.15 Network Address Translation (NAT)
• The Internet has expanded and addresses became scarce
– subnet and classless addressing (CIDR) were introduced to help
conserve addresses
• Another mechanism was invented that allows multiple
computers at a site to share a single, globally valid IP
address, known as Network Address Translation (NAT)
• NAT provides transparent communication
– a host in the Internet always appears to receive communication from
a single computer rather than from one of many computers at the site
• NAT runs as an in-line service
– It must be placed on the connection between the Internet and a site
• Most implementations embed NAT in another device
– such as a Wi-Fi wireless access point or an Internet router
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23.15 Network Address Translation (NAT)
• Figure 23.9 (below) illustrates a typical arrangement of a
site that uses NAT
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23.16 NAT Operation and Private
Addresses
• The goal of NAT is to provide an illusion
• When viewed from the Internet
– the site appears to consist of a single host computer that has been
assigned a valid IP address
– all datagrams sent from the site appear to originate from one host
– and all datagrams sent to the site appear to be sent to one host
• When viewed from a host in the site
– the Internet appears to accept and route private addresses
• A single IP address cannot be assigned to multiple
computers
– if two or more computers use the same address
• conflicts arise because multiple computers will respond to an ARP request
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23.16 NAT Operation and Private
Addresses
• NAT solves the problem by using two (2) types of addresses
– The NAT device itself is assigned a single globally-valid IP address
• as if the NAT device were a host on the Internet
– Each computer at the site is assigned a unique private address
• also known as a nonroutable address
• Figure 23.10 (below) lists address blocks that the IETF has
designated as private
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23.16 NAT Operation and Private
Addresses
• Private addressing is only used inside a site
• Before a datagram from the site can be allowed onto the
Internet
– NAT must translate the private IP into a globally valid IP address
• NAT must translate the globally valid IP address in an
incoming packet to a private address
– before transferring a datagram to a host at the site
• Basic function of NAT is a two-way translation, such as
– the source address translation
• as a datagram passes from the site to the Internet and
– the destination address translation
• as a datagram passes from the Internet to the site
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23.16 NAT Operation and Private
Addresses
• Figure 23.11 (below) illustrates the translations that occur in
each direction
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23.16 NAT Operation and Private
Addresses
• Most implementations of NAT use a translation table
– to store the information needed to rewrite addresses
• Figure 23.12 (below) shows a translation table that
corresponds to the address mapping in Figure 23.11
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23.17 Transport-Layer NAT (NAPT)
• NAT handles situations in which each host at a site
communicates with a unique server in the Internet
• If two hosts at the site attempt to communicate with remote
server X,
– the translation table will contain multiple entries for X
– and NAT will not be able to route incoming datagrams
• Basic NAT also fails in some situations, such as
– Consider problems when two or more applications running on a
given host at a site attempt simultaneous communication with
different destinations on the Internet
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23.17 Transport-Layer NAT (NAPT)
• A variation of NAT, called Network Address and Port
Translation (NAPT) avoids such problems:
– It allows a site to have arbitrary numbers of applications running on
arbitrary hosts
• all communicating simultaneously with arbitrary destinations throughout the
Internet
– most networking professionals assume the term NAT means NAPT
• Applications use protocol port numbers to distinguish among
services
• In addition to a table of source and destination addresses
– NAPT uses port numbers to associate each datagram with a TCP or
UDP flow
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23.17 Transport-Layer NAT (NAPT)
• Instead of stopping at the IP-layer
– NAPT operates on transport-layer headers
• NAPT entries contain a 4-tuple of source and destination IP
addresses and protocol port numbers
• To avoid a conflict
– NAPT must choose an alternative TCP source port for the connections
• Figure 23.13 (below) shows one possibility (web-server)
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23.18 NAT and Servers
• NAT system builds a translation table automatically
– by watching outgoing traffic and establishing a new mapping
whenever an application at the site initiates communication
• Automatic table construction does not work well for
communication initiated from the Internet to the site
– For example, if multiple computers at a site each run a web server
• the NAT device cannot know which computer should receive an incoming
web connection
• A variant of NAT called Twice NAT has been created to
allow a site to run multiple servers:
– Twice NAT arranges for the NAT system to interact with the site's
DNS server
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23.19 NAT Software and Systems for
Use at Home
• NAT is especially useful at a residence or small business
that has a broadband connection
– it allows a set of computers to share the connection
• without requiring the customer to purchase additional IP addresses
• A NAT software can make a PC act as a NAT device
• Also, dedicated NAT hardware systems are available at low
cost
– Such systems are usually called wireless routers
– The terminology is slightly misleading because such routers also
provide wired connections for host computers
• Figure 23.14 illustrates how such a router is connected
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23.19 NAT Software and Systems for
Use at Home
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