1. NT1210 Introduction to Networking
Unit 8:
Chapter 8, The Internet Protocol (IP) 1

2. Objectives
Identify the major needs and stakeholders for computer networks and network applications.
Identify the classifications of networks and how they are applied to various types of enterprises.
Compare and contrast the OSI and TCP/IP models and their applications to actual networks.
Explain the functionality and use of typical network protocols. 2

3. Objectives
Differentiate among major types of LAN and WAN technologies and specifications and determine how each is used in a data network.
Explain basic security requirements for networks.
Use network tools to monitor protocols and traffic characteristics.
Plan and design an IP network by applying subnetting skills.
Explain the functionality of typical network protocols. 3

4. Objectives
Plan and design an IP network by applying subnetting skills.
Categorize TCP/IP protocols according to network model layers.
Describe how TCP/IP addressing moves data packets through networks. 4

5. Introducing the Internet Protocol (IP)
TCP/IP Model review: Layers 1 and 2 Protocols
Example LAN/WAN Standards and Types in the TCP/IP Model 5
Figure 8-1

6. Introducing the Internet Protocol (IP)
TCP/IP Model review: Upper layers define non-physical (logical) networking functions
Various Perspectives on the TCP/IP Model and Roles 6
Figure 8-2

7. Introducing the Internet Protocol (IP)
Network Layer protocols
IP: Most important protocol defined by Network layer
Almost every computing device on planet communicates, and most use IP to do so
Network layer also defines other protocols 7

8. Introducing the Internet Protocol (IP)
Network Layer protocols: Part 1
Name
Full Name
Comments
ICMP
Internetwork Control Message Protocol
Messages that hosts and routers use to manage and control packet forwarding process; used by ping command
ARP
Address Resolution Protocol
Used by LAN hosts to dynamically learn another LAN host’s MAC address
DHCP
Dynamic Host Configuration Protocol
Used by host to dynamically learn IP address (and other information) it can use
DNS
Domain Name System/Service
Allows hosts to use names instead of IP address; needs DNS server to translate name into corresponding IP address (required by IP routing process)
Other TCP/IP Network Layer Protocols 8
Table 8-1

9. Introducing the Internet Protocol (IP)
Network Layer protocols: Part 2
Name
Full Name
Comments
RIP
Routing Information Protocol
Application that runs on routers so that routers dynamically learn IP routing tables (used to route IP packets correctly); open standard protocol defined in RFC 2453
EIGRP
Enhanced Interior Gateway Routing Protocol
Proprietary routing protocol owned by Cisco Systems
OSPF
Open Shortest Path First
Open source routing protocol defined in RFC 2328
Other TCP/IP Network Layer Protocols 9
Table 8-1

10. Introducing the Internet Protocol (IP)
IPv6: Next generation of IP addressing.
Needed because IPv4 addresses exhausted.
128-bit long addresses: 2128 or 3.4x1038 or over 340 undecillion IPs, that’s 340 with 36 zero’s after it.
Customer usually gets /64 subnet, which yields 4 billion times IPs available in all of IPv4.
Comparison: Number of IPv4 addresses equal to weight of cat; number of IPv6 addresses equal to weight of Earth and provides enough IP addresses for every grain of sand on every beach on earth. 10

11. Introducing the Internet Protocol (IP)
Migration to IPv6 has taken over decade and still in process.
IPv6 originally defined back in mid-1990s.
June 6, 2012 – Was the scheduled IPv6 Day, official worldwide “switch over” day, moved up to February
IPv4 Vs. IPv6 Timeline 11
Figure 8-3

12. Introducing the Internet Protocol (IP)
IP defines many functions that work together with one ultimate goal: To send data from one host to another host through any TCP/IP network.
Most important functions include:
Creating end-to-end physical paths through TCP/IP network by interconnecting physical networks (LANs and WANs) using routers
Identifying individual hosts and groups of hosts using IP addressing
Routing (forwarding) IP packets to correct destination host
Example of a Post Office Sorting a Letter Sent to Hollywood, California 12
Figure 8-4

13. Introducing the Internet Protocol (IP)
IP is like Post Office
Example of a Post Office Sorting a Letter Sent to Hollywood, California 13
Figure 8-4

14. Introducing the Internet Protocol (IP)
Routers in IP network: Interconnect LANs and WANs via physical connectors called interfaces
Example: Cisco 1841 router with two built-in Gigabit Ethernet LAN interfaces that use RJ-45 connectors
Enterprise Class Router, LAN Interfaces, and WAN Interfaces 14
Figure 8-5

15. Introducing the Internet Protocol (IP)
IP interconnects LANs and WANs
Interconnected LANs and WANs: Redundancy, but No LAN/WAN Detail 15
Figure 8-7

16. Introducing the Internet Protocol (IP)
IPv4 Addresses
32 bits
Expressed in binary and dotted decimal forms
Source and destination IP addresses included in 20-byte IP header added to all IP packets
IPv4 Header Format and Fields 16
Figure 8-8

17. Introducing the Internet Protocol (IP)
Converting binary IP address to dotted decimal
Separate 32 bits into 4 groups of 8 bits each
Do binary-to-decimal conversion of each 8-bit number (each decimal value between 0 and 255)
Put period (dot) between each decimal number
Generic View of Converting from Binary IP Address to DDN Format 17
Figure 8-9

18. Introducing the Internet Protocol (IP)
Example: Converting binary IP address to dotted decimal
Converting Binary IP Address to DDN 18
Figure 8-10

19. Introducing the Internet Protocol (IP): Routing
Routing IP Packets from Source to Destination
IP addressing groups addresses into networks
All addresses with same value in first parts of addresses considered to be in one network
Example: All addresses that begin with 11, 12, 13, 14, or 15 in that particular network
Example IP Address Groupings: All with the Same First Octet in the Same Group 19
Figure 8-11

20. Introducing the Internet Protocol (IP): Routing
IP routing example with routing tables: PC11 in left LAN sends IP packet to address (LAN on upper right)
Following the steps in the Figure:
Host PC11, noticing that the destination is not on the same local LAN, sends the packet to the router on the same LAN as itself: R1.
R1 compares the IP packet’s destination IP address ( ) to R1’s IP routing table, matching the entry for 12.x.x.x (which means “all addresses that begin with 12). That routing table entry tells R1 to send the IP packet to R2 next.
R1 forwards the IP across the WAN from R1 to R2.
R2 compares the IP packet’s destination IP address ( ) to R2’s IP routing table, matching its entry for 12.x.x.x. That routing table entry tells R2 to send the packet directly to PC21 over R2’s LAN interface on the right.
R2 forwards the IP packet across the LAN from R2 to PC21.
Example IP Address Groupings: All with the Same First Octet in the Same Group 20
Figure 8-12

21. Introducing the Internet Protocol (IP): Routing
Routers build routing tables in two ways
Static configuration: Routes entered manually and do not change
Dynamic routing protocol: Application router uses to automatically learn new routes from other routers
Following the steps in the Figure:
R2 sends a routing protocol message listing an address grouping “12.x.x.x” (all addresses that begin with 12).
R1 adds a route to its routing table, listing that group of addresses, and listing R2 as the next-hop router. For any packets R1 wants to send to addresses in this group, R1 will send the packets to R2 next.
R1 using the routing protocol messages to advertise a route for 12.x.x.x to router R3.
R3 adds a route to its routing table, listing that group of addresses, and listing R1 as the next-hop router.
Routing Protocols Advertising All Addresses that Begin with 12 as One Route 21
Figure 8-13

22. Introducing the Internet Protocol (IP): Other Protocols
Domain Name System/Service (DNS): Mapping names to IP addresses
Users use names; IP routing uses numbers
DNS translates name into corresponding IP address
DNS client sends DNS Request message
DNS server returns DNS Reply
Following the steps in the Figure:
The user at PC11 wants to connect to “Server1”, but PC11 does not know Server1’s IP address. So, PC11 sends a DNS Request to the DNS Server.
The DNS Server finds that “Server1” is “ ” per its list, so it sends a DNS Reply back to PC11 with that information.
PC11 can now send a packet with destination IP address to Server1.
The figure shows how DNS works in one company, but it also works worldwide, as discussed in Chapter 9, “The Internet
DNS Name Resolution Request, Reply, and Packet to Server1 IP Address 22
Figure 8-14

23. Introducing the Internet Protocol (IP): Other Protocols
Layer 3 - Network
IP with its Support Protocols 23
Figure 8-15

24. IP Addressing on User LANs: Network Settings
Locations Need IP addresses
Each LAN and WAN interface on hosts and routers need IP address to communicate
IP Addresses Used on Every LAN/WAN Interface 24
Figure 8-17

25. IP Addressing on User LANs: Network Settings
IP Address grouping: Allows IP routing to work better
Routers list one number to represent each network (address group) in routing tables
In the figure on the right, one group of addresses bases the group on the first octet’s value, namely all addresses that begin with “11” in the first octet.
Two groups base the group on the value in the first two octets, as seen on the LANs on the right side of the figure.
The two IP address groups on the WAN links base the groups on the value in the first three octets.
IP Address Groupings: IP Networks 25
Figure 8-18

26. IP Addressing on User LANs: Network Settings
Original IPv4 RFC defined way to group IPv4 addresses using IP address classes (classful IP addressing)
Every possible IPv4 address falls into class
First Octet
Class
Purpose A
Reserved
Unicast addresses, in class A networks
127
Reserved for loopback testing B
Unicast addresses, in class B networks C
Unicast addresses, in class C networks D
Multicast addresses; not used as unicast IP addresses E
Experimental; not used as unicast IP addresses
Summary of IPv4 Address Classes Based on First Octet Values 26
Table 8-2

27. IP Addressing on User LANs: Network Settings
Class A includes lower half of IPv4 address space: All IPv4 address that begin with first octet between 0 and 127
Network ID
Class A IP Network Concept
Size (Number of Addresses)
All addresses with a first octet equal to 1
> 16,000,000
All addresses with a first octet equal to 2
All addresses with a first octet equal to 3
All addresses with a first octet equal to 4 …
Etc….
All addresses with a first octet equal to 126
Example Class A Networks 27
Table 8-3

28. IP Addressing on User LANs: Network Settings
Class B includes ¼ of IPv4 address space with first octet value from 128 – 191
Includes medium number (216) of medium sized IP networks for approximately 65,000 hosts per network
Network ID
Concept
Size (Number of Addresses)
All with a first two octets equal to 128.1
> 65,000
All with a first two octets equal to 128.2
All with a first two octets equal to 128.3
All with a first two octets equal to
All with a first two octets equal to
All with a first two octets equal to
Example Class B Networks 28
Table 8-4

29. IP Addressing on User LANs: Network Settings
Class C includes 1/8th of IPv4 address space with first octet between 192 and 223
Large number of small IP networks: over 2,000,000 IP networks, each with 256 IP addresses each
Network ID
Concept
Size (Number of Addresses)
All with a first three octets equal to
254
All with a first three octets equal to
All with a first three octets equal to
All with a first three octets equal to
All with a first three octets equal to
All with a first three octets equal to
Example Class C Networks 29
Table 8-5

30. IP Addressing on User LANs: Network Settings
LAN IP address classes summary
Summary of How Class Rules Break Down the IPv4 Address Space 30
Figure 8-20

31. IP Addressing on User LANs: Network Settings
Private addresses: Classful IP networks reserved for enterprises to use in their network designs
Can only be used on local LAN; can’t be routed through WAN (non-routable)
Not regulated by agencies such as ARIN or ICANN
Network ID
Concept
Size (Number of Addresses)
10.x.x.x
Class A Private IP addressing space
Over 16,000,000 networks of over 16,000,000 IPs each
x.x –
x.x
Class B Private IP addressing space
Over 65,000 networks of over >65,000 IPs each
x.x
Class C Private IP addressing space
Over 65,000 networks of 256 IPs each 31

32. IP Addressing on User LANs: Network Settings
Static IP address assignment: Manually configured
Static IP Address Assignment on Mac OS X 32
Figure 8-21

33. IP Addressing on User LANs: Network Settings
Most host OS’s allow static configuration of several network settings
Host IP Settings 33
Figure 8-22

34. IP Addressing on User LANs: Network Settings
Dynamic Host Configuration Protocol (DHCP) defines way hosts can lease IP address from DHCP network server so does not have to be configured statically
Operates on client-server concept
DHCP protocol defined by set of RFCs
Sample Network for DHCP Discussions 34
Figure 8-23

35. IP Addressing on User LANs: Network Settings
Example: IP address assignment design using both DHCP and statically assigned addresses
Location
Type
Range
Atlanta LAN
Static
DHCP
Boston LAN
San Fran LAN
Address Planning: Some Static, Some DHCP, for Every LAN 35
Table 8-6

36. IP Addressing on User LANs: Network Settings
Once DHCP server exists in network and has been configured with set of IP addresses to lease, DHCP clients can request IP addresses
DHCP Lease Process between a DHCP Client and Server 36
Figure 8-24

37. IP Addressing on User LANs: Network Settings
User can see results of DHCP process from computer
DHCP Client Configuration on Mac OS X 37
Figure 8-25

38. IP Addressing on User LANs: Network Settings
DHCP example: Crossing networks to access DHCP server
Following the steps in the Figure:
The server begins with a configuration for the Atlanta subnet, listing the IP addresses it can lease, and noting that all are open and available.
PC11 initiates the 4-message DHCP exchange to lease an IP address, with the server leasing address to the client.
PC11 now uses IP address
Remote DHCP Client in Boston 38
Figure 8-26

39. Short Break
Take 10 39

40. IP Routing with Focus on Layer 3
IP defines how to route packets across TCP/IP network
Some routing tasks must use logic from lower two layers because Network layer (3) cannot physically send bits
Network layer relies on Layers 1 and 2 logic for this
IP Routing Perspective, While Ignoring LAN/WAN Details 40
Figure 8-27

41. IP Routing with Focus on Layer 3
Router must have IP routing table with useful entries to route IP packets.
Routing table may list multiple routes.
Each IP route identifies network, as well as other information about how to send packets to that network.
Routers look at incoming packet’s destination IP address and compare it to list of network IDs in its routing table to determine where to send packet to destination. 41

42. IP Routing with Focus on Layer 3
Finding a classful network ID based on IP address
Five Classful Networks in a Small Corporate Network 42
Figure 8-28

43. IP Routing with Focus on Layer 3
Each route in routing table lists:
Information about how to match IP packets
Forwarding instructions that tell router where to forward packets to (e.g., next router)
Example: R1’s IP routing table shows five network IDs so it knows routes to all five networks
R1 Routing Table with Routes for Five Classful Networks 43
Figure 8-29

44. IP Routing with Focus on Layer 3
Router compares incoming IP packet’s destination address to information in its routing tables to find best route to destination
How Router R1 Uses its IP Routing Table: Match and Forward 44
Figure 8-30

45. IP Routing with Focus on Layer 3
Following the steps in the Figure:
PC11 decides to send a packet, destination , because of some user action (for example, the opening of a web page with PC21’s name as the URL).
PC11, based on its host IP routing logic, sends the IP packet over the LAN to router R1.
R1 performs IP routing, comparing the packet’s destination IP address ( ) to R1’s IP routing table. R1 matches its route for class B network , because destination address is in class B network
R1 forwards the packet, per its routing table, out interface S0/0, towards R2.
R2 uses the same routing logic as R1, but R2 matches its own route for class B network
R2’s route for network has different forwarding instructions. R2 forwards the packet out R2’s local interface F0/0 (FastEthernet 0/0).
Routing from End-to-End: Multiple Cooperative Routing Decisions 45
Figure 8-31

46. IP Routing with Focus on Layer 3: Subnetting
Classful IP networks and wasted IP addresses
Subnetting: Process of subdividing network to create smaller groups of consecutive IP addresses
Subnets (subdivided networks): Smaller groups of addresses
Numbers of Classful Networks, and Their Sizes 46
Figure 8-32

47. IP Routing with Focus on Layer 3: Subnetting
Example: Several subnets created by subnetting network
Each subnet has subnet/network ID
Subdividing (Subnetting) Class A Network 47
Figure 8-33

48. IP Routing with Focus on Layer 3: Subnetting
Example continued: IP addresses and networks replaced with five subnets of network
Sample Corporate Network Using Subnets of Network 48
Figure 8-34

49. IP Routing with Focus on Layer 3: Subnetting
Subnet mask: Shows how much of IP address for each device is in common to all IPs in subnet
Example (/24): First three octets (first 24 bits) must be equal for all subnets in network
PC11 sends packet to PC21 (destination IP address )
R1 will have route for PC21’s subnet (network ID )
Routing Logic with Subnets and Masks 49
Figure 8-35

50. IP Routing with Focus on Layer 3: Subnetting
Classful networks have default subnet mask based on each class
Class A: (8 bits)
Class B: (16 bits)
Class C: (24 bits)
If subnet mask anything other than default, then subnetting being used
Routing Logic with Subnets and Masks 50
Figure 8-35

51. IP Routing with Focus on Layer 3: Subnetting
How to calculate subnets
Determine network class (A, B, or C)
Determine EITHER number of hosts needed for each subnet OR how many subnets needed
Determine how many bits needed to provide correct number of hosts/subnets; last subnet is NOT usable
Calculate IPs for each subnet; first IP identifies subnet (Network ID) and last IP identifies broadcast address
Determine subnet mask (total number of bits for network/subnet ID) 51

52. IP Routing with Focus on Layer 3: Subnetting
Example: Calculating subnets for network
Class: C
Number of subnets needed: 14
Number of bits needed to supply 14 subnets: 3
Number of bits left to determine number of IPs per subnet: 5 (results in 32 IPs per subnet)
Subnet mask: (default 16 bits + 3 more bits for subnetting = 19 bits) 52

53. IP Routing with Focus on Layer 3: Subnetting
Subnet No.
Network ID
Host Range IPs
Broadcast IP – 1 – 2 – 3 – 4 – 5 – 6 – 7 – 53

54. IP Routing with Focus on Layer 3: Subnetting
What happens when PC11 sends IP packet to PC12: Same subnet
PC11 determines its own IP address and subnet mask ( and )
PC11 decides determines destination is in same subnet
PC11 sends packet directly to PC12 without going through router R1
IP Host Routing Logic: Local Destination 54
Figure 8-36

55. IP Routing with Focus on Layer 3: Subnetting
What happens when PC11 sends IP packet to PC12: Different subnets
Host’s default gateway (default router) setting tells it where to send packets when they have destination address in different subnet
Default gateway tells host IP address of router attached to its LAN
Router then consults its routing table and determines how to deliver packet
IP Host Routing Logic: Remote Destination 55
Figure 8-37

56. IP Routing with Layer 1, 2, and 3 Interactions
Encapsulation: Action taken by lower layer when it takes data from higher layer and adds header (and sometimes trailer) to higher layer’s data
Example: PC11 opened web browser and tried to connect to URL at web server: PC11 creating bits it uses to send to server S1 (web server)
Following the steps in the Figure; top to bottom:
The web browser creates the HTTP message, an HTTP GET request, with which the web client asks the web server to get a web page and return the web page to the client.
The TCP function on PC11 encapsulates the HTTP message into a TCP segment by adding the TCP header to the HTTP message.
The IP function on PC11 encapsulates the TCP segment into an IP packet by adding the IP header.
Encapsulation Review: Application, Transport, and Network Layers 56
Figure 8-38

57. IP Routing with Layer 1, 2, and 3 Interactions
PC encapsulating IP packet into Ethernet frame (step 4)
Sending bits over LAN cable into network (step 5)
Encapsulation Review: Data Link Layer 57
Figure 8-39

58. IP Routing with Layer 1, 2, and 3 Interactions
De-encapsulation: On the destination side
Following the de-encapsulation steps in the Figure:
Server S1 physically receives the bits in this frame (layer 1).
Server S1 processes the Ethernet header and trailer, and eventually discards them (layer 2).
Server S1 processes the IP header, and eventually discards it (layer 3).
Server S1 processes the TCP header, and eventually discards it (layer 4).
Server S1 processes the HTTP message (layer 7).
De-encapsulation on a Receiving Host (S1) 58
Figure 8-40

59. IP Routing with Layer 1, 2, and 3 Interactions
Addressing frames and packets when crossing SAME subnet: Both MAC and IP addresses in Ethernet frame and encapsulated IP packet
IP and Ethernet Addresses, PC11 to server S1, Same Subnet 59
Figure 8-42

60. IP Routing with Layer 1, 2, and 3 Interactions
To learn destination MAC address, sending device uses Address Resolution Protocol (ARP) and info in ARP table
Address
Short Answer
Long Answer
Source MAC
On NIC
Given to Ethernet NIC by manufacturer; sending host can find MAC on NIC hardware.
Source IP
Configuration
Either through static configuration or DHCP
Destination MAC
ARP
From its ARP table, or if not found, by using ARP protocol and sending ARP Request and waiting for ARP Reply from destination
Destination IP
User
Either typed or clicked by user
How a Sending IP Host Knows What Addresses to Use 60
Table 8-9

61. IP Routing with Layer 1, 2, and 3 Interactions
Discovering MAC addresses using ARP: ARP Request and ARP Reply
ARP Request (ARP Broadcast): Sending device queries for MAC address of destination device; sends Request as broadcast to all other devices on subnet
Example: PC11 wants to send packet to server S1 (in same subnet) but does not know S1’s MAC address; PC11 sends ARP Request to all devices on subnet
Following the steps in the Figure:
PC11 builds a sends an ARP Request, a LAN broadcast, listing the IP address for the host that PC11 wants to reply with its MAC address:
The LAN switch floods the LAN broadcast frame out all ports, because that is how LAN switches process all LAN broadcasts.
All three of the other IP hosts think about the request. The two hosts that do not use IP address (PC12 and S3) silently ignore the ARP Request. S1 sees its own IP address ( ), and decides to reply.
ARP Request (Broadcast) 61
Figure 8-43

62. IP Routing with Layer 1, 2, and 3 Interactions
ARP Reply: Lists IP address ARP Request asked about with corresponding MAC address of that host
Example: ARP Reply that server S1 makes in response to PC11’s ARP Request
ARP Reply is unicast since ARP Request generated from one particular device
ARP Reply (Unicast) 62
Figure 8-44

63. IP Routing with Layer 1, 2, and 3 Interactions
Routing data between different subnets
IP packets in network act like person traveling to destination, using all forms of transportation; packet goes from end-to-end
Data Link frames act like individual vehicles used for only part of trip (e.g., just train); frames never leave their own LAN/WAN
Example, IP Packet End-to-End, Data Link Heads Stay on LAN or WAN 63
Figure 8-45

64. IP Routing with Layer 1, 2, and 3 Interactions
Addressing frames and packets when crossing subnets example: PC11 ( ) sends IP packet to PC21 ( )
Hosts sit on different LANs (may also be in different subnets)
Following the steps in the Figure:
PC11 puts its own IP address as the source IP ( ), and PC21’s IP address as the destination ( ).
R1 does not change the IP addresses as part of the routing process.
R2 does not change the IP addresses as part of the routing process.
IP Addresses Stay the Same Through End-to-End Path 64
Figure 8-46

65. IP Routing with Layer 1, 2, and 3 Interactions
Example: PC11 sends IP packet to PC21
PC11’s logic tells it to send packet to default router because destination is in different network or subnet
PC11 encapsulates packet inside Ethernet frame with destination MAC address R1
Ethernet Frames Use MAC on that LAN (Only) 65
Figure 8-47

66. IP Routing with Layer 1, 2, and 3 Interactions
Removing/adding Data Link headers: Router removes IP packet from incoming Data Link frame (de- encapsulation) and then adds new Data Link header and trailer before sending packet (encapsulation)
Steps router goes through:
De-encapsulates IP packet from inside Data Link frame
Makes routing decision using packet’s destination IP address and its own IP routing table, identifying correct outgoing interface
Encapsulates packet into new Data Link frame that works on outgoing interface
Sends packet through outgoing interface to destination
Routers Discard Old and Add New Data Link Framing 66
Figure 8-48

67. IP Routing with Layer 1, 2, and 3 Interactions
Example: When R1 receives packet destined to subnet on R2
R1 extracts IP packet
Consults its routing table and determines it needs to send packet to network connected to its S0 interface (outgoing interface)
Adds PPP header/trailer to frame and sends packet out S0
R2 receives PPP frame on its S1 interface (incoming interface)
Removes IP packet and consults its routing table and determines that it needs to use its F1 interface (outgoing interface)
Encapsulates IP packet inside new Ethernet frame and sends frame out F1 to destination
Routers Discard Old and Add New Data Link Framing 67
Figure 8-48

68. IP Routing with Layer 1, 2, and 3 Interactions
Using ARP with routers: R2 needs to deliver IP packet to host PC21
R2 builds Ethernet header with PC21’s MAC address as destination
If R2 does not know PC21’s MAC address (i.e., it is not in its ARP table), R2 uses ARP to learn MAC address
When R2 receives ARP Reply with PC21’s MAC address, sends frame
Following the steps in the Figure:
R2 wants to forward an IP packet to PC21, but R2 does not see PC21’s IP address ( ) in R2’s ARP table.
R2 sends an ARP Request looking for PC21’s MAC address.
PC21 sends an ARP Reply, supplying its MAC address ( ).
R2 now forwards the IP packet, with destination MAC address
Example of Router R2 Using ARP to Learn a Local Host’s MAC Address 68
Figure 8-49

69. Summary, This Chapter…
Described the main functions of the TCP/IP network layer in regards to its focus on either physical or logical functions, and the focus on the network or endpoint hosts.
Listed three major functions defined by IP.
Listed common TCP/IP network layer functions in addition to IP.
Examined a figure of an Enterprise TCP/IP network and determine where IP address groups (IP networks or subnets) would be needed. 69

70. Summary, This Chapter…
Looked at any IP version 4 address and determined its class, and if a unicast IP address, determined the class A, B, or C network ID of the network in which it resides.
Listed the four IP settings typically set on IP hosts during static configuration.
Described the layer 3 logic used by routers when routing IP packets.
Described an IP host’s layer 3 logic when routing IP packets. 70

71. Summary, This Chapter…
Explained the basic ideas of how the IP subnetting process subdivides a classful network into smaller groups.
Predicted the MAC and IP addresses used by two hosts on the same LAN subnet when they send IP packets to each other.
Predicted the MAC and IP addresses used throughout an IP packet’s journey from a host in one subnet to a host in another subnet. 71

72. Questions? Comments? 72