Giáo trình CCNA - Chương 4

CCNA – Semester4
Module 1
Scaling IP Address
Objectives
• Scaling networks with NAT and PAT
• Dynamic Host Configuration Protocol
Scaling networks with
NAT and PAT
Introducing NAT
• NAT is designed to conserve IP addresses and enable
networks to use private IP addresses on internal
networks.
• These private, internal addresses are translated to
routable, public addresses.
NAT terms
• Inside local address – The IP address assigned to a host on the inside network (not a
legitimate IP address-RFC 1918 private address).
• Inside global address – A legitimate IP address represents one or more inside local IP
addresses to the outside world.
• Outside local address – The IP address of an outside host as it is known to the hosts
on the inside network.
• Outside global address – The IP address assigned to a host on the outside network.
The owner of the host assigns this address.
Major NAT and PAT features
• NAT translations can be used for a variety of
purposes and can be either dynamically or
statically assigned.
• Dynamic NAT is designed to map a private IP
address to a public address.
• Overloading, or Port Address Translation (PAT),
maps multiple private IP addresses to a single

public IP address.
• Realistically, the number of ports that can be
assigned a single IP address is around 4000.
Major NAT and PAT features
NAT advantages
• Eliminates reassigning each host a new IP
address when changing to a new ISP.
• Conserves addresses through application port-
level multiplexing.
• Protects network security.
• Reduce address overlap occurrence.
NAT disadvantages
• Translation introduces switching path delays
and processing overhead.
• Loss of IP end-to-end traceability.
• Certain application will not function with NAT
enabled.
Cisco IOS NAT
Configure static NAT
Static translation
Configure dynamic NAT
Configure dynamic NAT
Configure dynamic NAT
Dynamic NAT example
Configure PAT
Clear NAT table
Show NAT translation
Troubleshooting NAT and PAT configuration
• Use the following steps to determine whether NAT is
operating as expected:

– Based on the configuration, clearly define what NAT is supposed to
achieve.
– Verify that correct translations exist in the translation table.
– Verify the translation is occurring by using show and debug
commands.
– Review in detail what is happening to the packet and verify that
routers have the correct routing information to move the packet along.
Debug IP NAT
Dynamic Host Configuration
Protocol
Introducing DHCP
• Dynamic Host Configuration Protocol (DHCP)
works in a client/server mode.
• DHCP enables DHCP clients on an IP network to
obtain their configurations from a DHCP server.
• Less work is involved in managing an IP
network when DHCP is used.
• The DHCP protocol is described in RFC 2131.
Easy IP
• Cisco routers can use a Cisco IOS feature set,
Easy IP, to offer an optional, full-featured DHCP
server.
• Easy IP leases configurations for 24 hours by
default.
• The DHCP service is enabled by default on
versions of Cisco IOS that support it.
• To disable DHCP service, use the no service dhcp
command.
BOOTP and DHCP
• Both protocols are client/server based and use UDP

ports 67 and 68. Those ports are still known as BOOTP
ports.
• The four basic IP parameters:
– IP address
– Gateway address
– Subnet mask
– DNS server address
DHCP Operation
DHCP Messages
• If the client detects that
the address is already
in use on the local
segment it will send a
DHCPDECLINE
message and the
process starts again.
• If the client received a DHCPNACK from the server after
sending the DHCPREQUEST, then it will restart the process
again.
• If the client no longer needs the IP address, the client sends a
DHCPRELEASE message to the server.
CISCO IOS DHCP
• Cisco IOS DHCP server always checks to make
sure that an address is not in use before the
server offers it to a client.
• By default, router will issue 2 ICMP echo
requests, or will ping, to a pool address before
sending the DHCPOFFER to a client.
Configuring DHCP
Excluding IP

Key DHCP Server Commands
Verifying DHCP operation
• To verify the operation of DHCP, the command show
ip dhcp binding can be used.
• To verify that messages are being received or sent
by the router, use the command show ip dhcp server
statistics.
Troubleshooting DHCP
Network services
• Remote clients will broadcast to locate servers
while routers, by default, will not forward client
broadcasts beyond their subnet.
IP helper-address
• Remote clients will broadcast to locate servers while
routers, by default, will not forward client broadcasts
beyond their subnet.
• When possible, administrators should use the ip helper-
address command to relay broadcast requests for these
key UDP services.
• By default, the ip helper-address command forwards the
following eight UDP services:
–Time
–TACACS
– DNS
– BOOTP/DHCP Server
– BOOTP/DHCP Client
– TFTP
– NetBIOS Name Service
– NetBIOS datagram Service
DHCP Relay

Summary
• NAT and PAT concepts and operation
• NAT and PAT configuration and
troubleshooting
• Comparison between DHCP and BOOTP
• Cisco Router DHCP configuration
• DHCP relay with ip helper-address
Module 2
WAN Technologies
Objectives
• Identify the devices used in a WAN and list
standards
• Differentiate between packet-switched and circuit-
switched WAN technologies
• Describe equipment involved in the implementation
of various WAN services
• Compare and contrast WAN design models
WAN Technology Overview
Parts of a WAN service
Modem
• In order for the local loop to carry data, a device
such as a modem is needed to prepare the data for
transmission.
DTE and DCE
• Devices that put data
on the local loop are
called data circuit-
terminating equipment,
or data communications
equipment (DCE).

• The customer devices that pass the data to the DCE are
called data terminal equipment (DTE).
WAN Line Types and Bandwidth
WAN Devices
WAN Devices: Modem
• A modem is needed if the local loop is analog rather than
digital.
• Modems transmit data over voice-grade telephone lines by
modulating and demodulating the signal.
WAN Devices: CSU/DSU
• The communications link
needs signals in an
appropriate format.
• For digital lines, a channel
service unit (CSU) and a
data service unit (DSU) are
required.
• The CSU/DSU may also be
built into the interface card
in the router.
WAN standard organizations
WAN standard
• WANs use the OSI reference model, but focus
mainly on Layer 1 and Layer 2.
• WAN standards typically describe both physical
layer delivery methods and data link layer
requirements, including physical addressing,
flow control, and encapsulation.