Any Questions?

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Any Questions?
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Chapter 4 IP Routing Static and Connected Routes

• IP Routing and Addressing
• Routes to Directly Connected Subnets
• Static Routes

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Do I know this?
Go through the Quiz- 5 minutes
4

• 1. A PC user turns on her computer, and as soon
  as the computer is up and working, she opens a
  web browser to browse http//www.ciscopress.com.
  Which protocol(s) would definitely not be used by
  the PC during this process?
• a. DHCP
• b. DNS
• c. ARP
• d. ICMP

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• 1. A PC user turns on her computer, and as soon
  as the computer is up and working, she opens a
  web browser to browse http//www.ciscopress.com.
  Which protocol(s) would definitely not be used by
  the PC during this process?
• a. DHCP
• b. DNS
• c. ARP
• d. ICMP
• Answer D

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• 2. A PC user turns on her computer, and as soon
  as the computer is up and working, she opens a
  command prompt. From there, she issues the ping
  2.2.2.2 command, and the ping shows 100 percent
  success. The PCs IP address is 1.1.1.1 with mask
  255.255.255.0. Which of the following settings
  would be required on the PC to support the
  successful ping?
• a. The IP address of a DNS server
• b. The IP address of a default gateway
• c. The IP address of an ARP server
• d. The IP address of a DHCP server

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• 2. A PC user turns on her computer, and as soon
  as the computer is up and working, she opens a
  command prompt. From there, she issues the ping
  2.2.2.2 command, and the ping shows 100 percent
  success. The PCs IP address is 1.1.1.1 with mask
  255.255.255.0. Which of the following settings
  would be required on the PC to support the
  successful ping?
• a. The IP address of a DNS server
• b. The IP address of a default gateway
• c. The IP address of an ARP server
• d. The IP address of a DHCP server
• Answer B

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• 3. Router 1 has a Fast Ethernet interface 0/0
  with IP address 10.1.1.1. The interface is
  connected to a switch. This connection is then
  migrated to use 802.1Q trunking. Which of the
  following commands could be part of a valid
  configuration for Router 1s Fa0/0 interface?
• a. interface fastethernet 0/0.4
• b. dot1q enable
• c. dot1q enable 4
• d. trunking enable
• e. trunking enable 4
• f. encapsulation dot1q

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• 3. Router 1 has a Fast Ethernet interface 0/0
  with IP address 10.1.1.1. The interface is
  connected to a switch. This connection is then
  migrated to use 802.1Q trunking. Which of the
  following commands could be part of a valid
  configuration for Router 1s Fa0/0 interface?
• a. interface fastethernet 0/0.4
• b. dot1q enable
• c. dot1q enable 4
• d. trunking enable
• e. trunking enable 4
• f. encapsulation dot1q
• Answer A and F

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• 4. A router is configured with the no ip
  subnet-zero global configuration command. Which
  of the following interface subcommands would not
  be accepted by this router?
• a. ip address 10.1.1.1 255.255.255.0
• b. ip address 10.0.0.129 255.255.255.128
• c. ip address 10.1.2.2 255.254.0.0
• d. ip address 10.0.0.5 255.255.255.252
• Answer C

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• 4. A router is configured with the no ip
  subnet-zero global configuration command. Which
  of the following interface subcommands would not
  be accepted by this router?
• a. ip address 10.1.1.1 255.255.255.0
• b. ip address 10.0.0.129 255.255.255.128
• c. ip address 10.1.2.2 255.254.0.0
• d. ip address 10.0.0.5 255.255.255.252
• Answer C

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• 5. Which of the following must be true before IOS
  lists a route as S in the output of a show ip
  route command?
• a. The IP address must be configured on an
  interface.
• b. The router must receive a routing update from
  a neighboring router.
• c. The ip route command must be added to the
  configuration.
• d. The ip address command must use the special
  keyword.
• e. The interface must be up and up.

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• 5. Which of the following must be true before IOS
  lists a route as S in the output of a show ip
  route command?
• a. The IP address must be configured on an
  interface.
• b. The router must receive a routing update from
  a neighboring router.
• c. The ip route command must be added to the
  configuration.
• d. The ip address command must use the special
  keyword.
• e. The interface must be up and up.
• Answer C

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• 6. Which of the following commands correctly
  configures a static route?
• a. ip route 10.1.3.0 255.255.255.0 10.1.130.253
• b. ip route 10.1.3.0 serial 0
• c. ip route 10.1.3.0 /24 10.1.130.253
• d. ip route 10.1.3.0 /24 serial 0

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• 6. Which of the following commands correctly
  configures a static route?
• a. ip route 10.1.3.0 255.255.255.0 10.1.130.253
• b. ip route 10.1.3.0 serial 0
• c. ip route 10.1.3.0 /24 10.1.130.253
• d. ip route 10.1.3.0 /24 serial 0
• Answer A

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• 7. Which of the following is affected by whether
  a router is performing classful or classless
  routing?
• a. When to use a default route
• b. When to use masks in routing updates
• c. When to convert a packets destination IP
  address to a network number
• d. When to perform queuing based on the
  classification of a packet into a particular
  queue

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• 7. Which of the following is affected by whether
  a router is performing classful or classless
  routing?
• a. When to use a default route
• b. When to use masks in routing updates
• c. When to convert a packets destination IP
  address to a network number
• d. When to perform queuing based on the
  classification of a packet into a particular
  queue
• Answer A

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• 8. A router has been configured with the ip
  classless global configuration command. The
  router receives a packet destined to IP address
  168.13.4.1. The following text lists the contents
  of the routers routing table. Which of the
  following is true about how this router forwards
  the packet?
• Gateway of last resort is 168.13.1.101 to network
  0.0.0.0
• 168.13.0.0/24 is subnetted, 2 subnets
• C 168.13.1.0 is directly connected,
  FastEthernet0/0
• R 168.13.3.0 120/1 via 168.13.100.3, 000005,
  Serial0.1
• a. It is forwarded to 168.13.100.3.
• b. It is forwarded to 168.13.1.101.
• c. It is forwarded out interface Fa0/0, directly
  to the destination host.
• d. The router discards the packet.

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• 8. A router has been configured with the ip
  classless global configuration command. The
  router receives a packet destined to IP address
  168.13.4.1. The following text lists the contents
  of the routers routing table. Which of the
  following is true about how this router forwards
  the packet?
• Gateway of last resort is 168.13.1.101 to network
  0.0.0.0
• 168.13.0.0/24 is subnetted, 2 subnets
• C 168.13.1.0 is directly connected,
  FastEthernet0/0
• R 168.13.3.0 120/1 via 168.13.100.3, 000005,
  Serial0.1
• a. It is forwarded to 168.13.100.3.
• b. It is forwarded to 168.13.1.101.
• c. It is forwarded out interface Fa0/0, directly
  to the destination host.
• d. The router discards the packet.
• Answer B

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Any Questions?
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IP Routing HOST Logic

• 1. When sending a packet, compare the destination
  IP address of the packet to the sending hosts
  perception of the range of addresses in the
  connected subnet, based on the hosts IP address
  and subnet mask.
• a. If the destination is in the same subnet as
  the host, send the packet directly to the
  destination host. Address Resolution Protocol
  (ARP) is needed to find the destination hosts
  MAC address.
• b. If the destination is not in the same subnet
  as the host, send the packet directly to the
  hosts default gateway (default router). ARP is
  needed to find the default gateways MAC address.

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IP Routing ROUTER logic

• 1. For each received frame, use the data-link
  trailer frame check sequence (FCS) field to
  ensure that the frame had no errors if errors
  occurred, discard the frame (and do not continue
  to the next step).
• 2. Check the frames destination data link layer
  address, and process only if addressed to this
  router or to a broadcast/multicast address.
• 3. Discard the incoming frames old data-link
  header and trailer, leaving the IP packet.
• 4. Compare the packets destination IP address to
  the routing table, and find the route that
  matches the destination address. This route
  identifies the outgoing interface of the router,
  and possibly the next-hop router.
• 5. Determine the destination data-link address
  used for forwarding packets to the next router or
  destination host (as directed in the routing
  table).
• 6. Encapsulate the IP packet inside a new
  data-link header and trailer, appropriate for the
  outgoing interface, and forward the frame out
  that interface.

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IP Routing Example
Pg 164
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WAN Routing Process
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IP Addressing and Subnetting

• For each subnet, recognize the following
• Subnetwork Address
• Range of Hosts
• Subnetwork Broadcast Address

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MAJOR IP addressing concepts

• Unicast IP addresses are IP addresses that can
  be assigned to an individual interface for
  sending and receiving packets.
• Each unicast IP address resides in a particular
  Class A, B, or C network, called a classful IP
  network.
• If subnetting is used, which is almost always
  true in real life, each unicast IP address also
  resides in a specific subset of the classful
  network called a subnet.
• The subnet mask, written in either dotted
  decimal form (for example, 255.255.255.0) or
  prefix notation form (for example, /24),
  identifies the structure of unicast IP addresses
  and allows devices and people to derive the
  subnet number, range of addresses, and broadcast
  address for a subnet.
• Devices in the same subnet should all use the
  same subnet mask otherwise, they have different
  opinions about the range of addresses in the
  subnet, which can break the IP routing process.

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MAJOR IP addressing concepts

• Devices in a single VLAN should be in the same
  single IP subnet.
• Devices in different VLANs should be in
  different IP subnets.
• To forward packets between subnets, a device
  that performs routing must be used. In this book,
  only routers are shown, but multilayer
  switchesswitches that also perform routing
  functionscan also be used.
• Point-to-point serial links use a different
  subnet than the LAN subnets, but these subnets
  only require two IP addresses, one for each
  router interface on either end of the link.
• Hosts separated by a router must be in separate
  subnets.

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Subnets in action
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Subnetting Review

• CD-only Appendix D, Subnetting Practice
• CD-only Appendix E, Subnetting Reference Pages
• CD-only Appendix H, ICND1 Chapter 12 IP
  Addressing and Subnetting
• Subnetting videos

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IP Forwarding and most specific route

• Often their might be overlapping routes in a
  routers table
• The use of autosummary
• Manual route summarization
• The use of static routes
• Incorrectly designed subnetting so that subnets
  overlap their address ranges

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Overlapping Routes

• When a particular destination IP address matches
  more than one route in a routers routing table,
  the router uses the most specific routein other
  words, the route with the longest prefix length.

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Multiple Routes in Action

• R1show ip route rip
• 172.16.0.0/16 is variably subnetted, 5 subnets, 4
  masks
• R 172.16.1.1/32 120/1 via 172.16.25.2,
  000004, Serial0/1/1
• R 172.16.1.0/24 120/2 via 172.16.25.129,
  000009, Serial0/1/0
• R 172.16.0.0/22 120/1 via 172.16.25.2,
  000004, Serial0/1/1
• R 172.16.0.0/16 120/2 via 172.16.25.129,
  000009, Serial0/1/0
• R 0.0.0.0/0 120/3 via 172.16.25.129, 000009,
  Serial0/1/0
• R1show ip route 172.16.4.3
• Routing entry for 172.16.0.0/16
• Known via rip, distance 120, metric 2
• Redistributing via rip
• Last update from 172.16.25.129 on Serial0/1/0,
  000019 ago
• Routing Descriptor Blocks
• 172.16.25.129, from 172.16.25.129, 000019
  ago, via Serial0/1/0
• Route metric is 2, traffic share count is 1
• What route will be used for
• 172.16.1.1, 172.16.1.2,172.16.2.3, and
  172.16.4.3.

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Multiple Routes in action

• 172.16.1.1 Matches all five routes longest
  prefix is /32, the route to 172.16.1.1/32.
• 172.16.1.2 Matches last four routes longest
  prefix is /24, the route to 172.16.1.0/24.
• 172.16.2.3 Matches last three routes longest
  prefix is /22, the route to 172.16.0.0/22.
• 172.16.4.3 Matches the last two routes longest
  prefix is /16, the route to 172.16.0.0/16.

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DNS, DHCP, ARP, and ICMP

• How are they used
• When do we need them

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DHCP

• Often gathered through DHCP
• The hosts IP address
• The associated subnet mask
• The IP address of the default gateway (router)
• The IP address(s) of the DNS server(s)

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DNS

• Used behind the scenes to convert Name to IP
  addres
• Web browser
• E-mail client
• Uses a cache on host to store information
  temporarily
• Only asks again for same address after timeout

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ICMP

• Often used for with PING
• Echo Request
• Echo Reply
• Also used to give information when trying to
  contact networks and hosts
• Destination unreeachable
• Host unreachable

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DHCP, DNS, ARP, ICMP

• Not used every packet
• 1. If not known yet, the host uses DHCP to learn
  its IP address, subnet mask, DNS IP addresses,
  and default gateway IP address. If already known,
  the host skips this step.
• 2. If the user references a host name not
  currently held in the hosts name cache, the host
  makes a DNS request to resolve the name into its
  corresponding IP address. Otherwise, the host
  skips this step.
• 3. If the user issued the ping command, the IP
  packet contains an ICMP Echo Request if the user
  instead used a typical TCP/IP application, it
  uses protocols appropriate to that application.
• 4. To build the Ethernet frame, the host uses the
  ARP caches entry for the next-hop deviceeither
  the default gateway (when sending to a host on
  another subnet) or the true destination host
  (when sending to a host on the same subnet). If
  the ARP cache does not hold that entry, the host
  uses ARP to learn the information.

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Any Questions?
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Fragmentation and MTU

• MTU-Maximum transmission unit
• The largest size IP packet that the network will
  carry
• Calculated for each interface
• Default for ethernet is 1500
• If a routers interface MTU is smaller than a
  packet that must be forwarded, the router
  fragments the packet into smaller packets.

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Fragmentation
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Fragmentation

• The IP header contains fields useful for
  reassembling the fragments into the original
  packet.
• The IP header includes an ID value that is the
  same in each fragmented packet, as well as an
  offset value that defines which part of the
  original packet is held in each fragment.
• Fragmented packets arriving out of order can be
  identified as a part of the same original packet
  and can be reassembled in the correct order using
  the offset field in each fragment.

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MTU

• mtu
• Used from the config-if prompt
• Sets the MTU for all Layer 3 protocols
• ip mtu
• Used from the config-if prompt
• Sets the value used for IP
• If both are configured on an interface, the IP
  MTU setting takes precedence on that interface.
  However, if the mtu command is configured after
  ip mtu is configured, the ip mtu value is reset
  to the same value as that of the mtu command.

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Any Questions?
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Directly Connected Subnets

• Router adds networks to the routing table if
  configured on an interface if
• The interface is in a working statein other
  words, the interface status in the show
  interfaces command lists a line status of up and
  a protocol status of up.
• The interface has an IP address assigned, either
  through the ip address interface subcommand or by
  using DHCP client services.

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Secondary IP Addressing

• Allows a single physical interface to accept
  traffic for more than one IP Address

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Configuring secondary address

• interface ethernet 0
• ip address 10.1.7.252 255.255.255.0 secondary
• ip address 10.1.2.252 255.255.255.0
• Yosemite show ip route connected
• 10.0.0.0/24 is subnetted, 4 subnets
• C 10.1.2.0 is directly connected, Ethernet0
• C 10.1.7.0 is directly connected, Ethernet0
• C 10.1.129.0 is directly connected, Serial1
• C 10.1.128.0 is directly connected, Serial0

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Subnet Zero

• The zero subnet (or subnet zero) is the one
  subnet in each classful network that has all
  binary 0s in the subnet part of the binary
  version of the subnet number. In decimal, the
  zero subnet happens to be the same number as the
  classful network number.
• For example in the 172.16.0.0 /18 the networks
  would be
• 172.16.0.0-172.16.63.255
• 172.16.0.64-172.16.127.255
• 172.16.128.0-172.16.191.255
• 172.16.192.0-172.16.255.255
• Which is the subnet 0?
• ip subnet-zero command allows use of the subnet 0
• Default since IOS 12.0

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Any Questions?
50
ISL and 802.1Q Configuration on Routers

• How to connect a router interface to a switch
  trunk port
• Router needs to be able to identify VLAN IDs
• Router needs an IP address for each VLAN
• Must be in the same subnet as the addreses of the
  PCs
• The routers address for that VLAN will be the
  default gateway for the PCs on that VLAN

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Router Forwarding Between VLANs

• Subinterface Configuration
• interface fastethernet 0/0.1
• ip address 10.1.1.1 255.255.255.0
• encapsulation isl 1
• interface fastethernet 0/0.2
• ip address 10.1.2.1 255.255.255.0
• encapsulation isl 2
• interface fastethernet 0/0.3
• ip address 10.1.3.1 255.255.255.0
• encapsulation isl 3
• VLAN IDs dont need to match those on switch, but
  it makes it easier

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802.1 q configuration

• Native VLAN can be configured two ways
• On the physical interace
• interface fastethernet 0/0
• ip address 10.1.1.1 255.255.255.0
• Or another subinterface
• Interface fastethernet 0/0.1
• ip address 10.1.1.1 255.255.255.0
• encapsulation dot1q 1 native
• interface fastethernet 0/0.2
• ip address 10.1.2.1 255.255.255.0
• encapsulation dot1q 2
• interface fastethernet 0/0.3
• ip address 10.1.3.1 255.255.255.0
• encapsulation dot1q 3

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Any Questions?
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Static Routing

• Least used method of updating routing table
• Use the ip route command
• ip route 10.1.2.0 255.255.255.0 10.1.128.252
• ip route 10.1.3.0 255.255.255.0 10.1.130.253

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Extended PING

• Extended Ping gives you more options, including
  choice
• Interface address to use as source IP
• Size of packets
• Number of packets

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PING

• The Cisco ping command uses, by default, the
  output interfaces IP address as the packets
  source address, unless otherwise specified in an
  extended ping. The first ping in Example 4-7 uses
  a source of 10.1.128.251, because the route used
  to send the packet to 10.1.2.252 sends packets
  out Albuquerques Serial0 interface, whose IP
  address is 10.1.128.251.
• Ping response packets (ICMP Echo Replies) reverse
  the IP addresses used in the received ping
  request to which they are responding. So, in this
  example, Yosemites Echo Reply, in response to
  the first ping in Example 4-7, uses 10.1.128.251
  as the destination address and 10.1.2.252 as the
  source IP address.

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58
Static Default Routes

• A default route is a special route that matches
  all packet destinations.
• When configured a router has a route of last
  resort

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Default Route configuration

• R1(config)ip route 0.0.0.0 0.0.0.0 168.13.1.101
• ip default-network
• To use the ip default-network command to
  configure a default route, the engineer relies on
  her knowledge that Dist1 is already advertising a
  route for classful network 10.0.0.0 to R1. R1s
  route to network 10.0.0.0 points to Dist1s
  168.13.1.101 address as the next-hop address.
  Knowing that, the engineer can configure the ip
  default-network 10.0.0.0 command on R1, which
  tells R1 to build its default route based on its
  learned route for network 10.0.0.0/8. Example
  4-10 shows several details about this scenario on
  R1.

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Default Route Summary

• Default static routes can be statically
  configured using the ip route 0.0.0.0 0.0.0.0
  nexthop- address or the ip default-network
  net-number command.
• When a router only matches a packet with the
  default route, that router uses the forwarding
  details listed in the gateway of last resort
  line.

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Classful and Classless Routing

• Classless routing causes a router to use its
  default routes for any packet that does not match
  some other route.
• Classful routing places one restriction on when a
  router can use its default route, resulting in
  cases in which a router has a default route but
  the router chooses to discard a packet rather
  than forwarding the packet based on the default
  route.

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Classful and Classless
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Classful and Classless

• Classless routing When a packets destination
  only matches a routers default route, and does
  not match any other routes, forward the packet
  using that default route.
• Classful routing When a packets destination
  only matches a routers default route, and does
  not match any other routes, only use the default
  route if this router does not know any routes in
  the classful network in which the destination IP
  address resides.

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Any Questions?