1. Chapter 6: Objectives
Explain how network layer protocols and services support communications across data networks.
Explain how routers enable end-to-end connectivity in a small to medium-sized business network.
Determine the appropriate device to route traffic in a small to medium-sized business network.
Configure a router with basic configurations.

2. The Network Layer

3. Encapsulation and Decapsulation
Data Link Header
IP Header
TCP Header
HTTP Header
Data Link Trailer
Data
Data Link Header
Data Link Header
Data Link Trailer
Data Link Trailer
IP Packet
IP Packet
Data Link Header
Data Link Header
Data Link Trailer
Data Link Trailer
IP Packet
IP Packet
Data Link Header
Data Link Header
Data Link Trailer
Data Link Trailer
IP Packet
IP Packet
Data Link Header
IP Header
TCP Header
HTTP Header
Data Link Trailer
Data

4. IPv / HLEN / Flag / S. IP / D. IP / …
Encapsulation
DATA
SEGMENT
S.P / D.P. / S.N. / Ack # / …
DATA
PACKET
IPv / HLEN / Flag / S. IP / D. IP / …
DATA (SEGMENT)
FRAME
Frame Header
DATA (PACKET)
Trailer

5. Functions of the Network LayerIP
Functions of the Network Layer IP
The network layer, or OSI Layer 3, provides services to allow end devices to exchange data across the network.
The network layer uses four basic processes:
Addressing end devices
Encapsulation
Routing
De-encapsulation

6. Network Layer Protocols
Common Network Layer Protocols
Internet Protocol version 4 (IPv4)
Internet Protocol version 6 (IPv6)
Legacy Network Layer Protocols
Novell Internetwork Packet Exchange (IPX)
AppleTalk
Connectionless Network Service (CLNS/DECNet)
Section

7. Characteristics of IPv4
Connectionless:
No connection is established before sending data packets.
Best effort delivery:
No additional overhead is used to guarantee packet delivery.
Makes it unreliable …?
Media independent:
Operates independently of the medium carrying the data.

8. Connectionless Service = Postal Service

9. Connectionless Service

10. Best Effort Delivery = Unreliable

11. Best Effort Delivery = Unreliable
IP is unreliable because it doesn’t have the capability to manage, and recover from, undelivered or corrupt packets.
TCP (if used) will manage the transmission reliability.
It also makes for a smaller IP header.
Less overhead = less delay in delivery = very fast.

12. IPv4 Media Independent
IP doesn’t care what type of media the packet is carried on.

13. MTU It is my job to reconstruct the packets.
The outgoing link has a large enough MTU but I don’t reconstruct packets.
MTU
The outgoing link has a smaller MTU so I have to fragment the packets.
IP Packet
IP Packet
IP Packet
IP Packet
IP Packet
Network link with larger MTU
Network link with smaller MTU
Network link with larger MTU
IP Packet
IP Packet
IP Packet
IP Packet
IP Packet
IP Packet
The Network layer does consider the maximum size of PDU that each medium can transport.
This is referred to as the Maximum Transmission Unit (MTU).
The Network layer determines how large to create the packets.
Routers may need to split up a packet when forwarding it from one media to a media with a smaller MTU.
This process is called fragmenting the packet or fragmentation.
This is similar to segmenting at the Transport layer but happens at the Network layer.

14. IPv4 Packet

15. IPv4 Packet
IP Header
Data (Payload)
IPv4 has been in use since 1983 when it was deployed on the Advanced Research Projects Agency Network (ARPANET).
An IPv4 packet has two parts:
IP Header - Identifies the packet characteristics.
Payload - Contains the Layer 4 segment information and the actual data.

16. IPv4 Header – Significant Fields
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Byte 1
Byte 2
Byte 3
Byte 4

17. IPv4 Header – Validation Fields
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Byte 1
Byte 2
Byte 3
Byte 4

18. Sample IPv4 Packet

19. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding

20. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Version (4 bits)
Indicates the version of IP currently used.
0100 = 4 and therefore IPv4
0110 = 6 and therefore IPv6

21. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
IP Header Length (4 bits)
Identifies the number of 32-bit words in the header.
The IHL value varies due to the Options and Padding fields.
The minimum value for this field is 5 (i.e., 5×32 = 160 bits = 20 bytes) and the maximum value is 15 (i.e., 15×32 = 480 bits = 60 bytes).

22. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Differentiated Services (8 bits)
Formerly called the Type of Service (ToS) field.
The field is used to determine the priority of each packet.
First 6 bits identify the Differentiated Services Code Point (DSCP) value for QoS.
Last 2 bits identify the explicit congestion notification (ECN) value used to prevent dropped packets during times of network congestion.

23. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Total Length (16 bits)
Sometimes referred to as the Packet Length.
Defines the entire packet (fragment) size, including header and data, in bytes.
The minimum length packet is 20 bytes (20-byte header + 0 bytes data) and the maximum is 65,535 bytes. .

24. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU.
When this happens, fragmentation occurs and the IPv4 packet uses the following 3 fields to keep track of the fragments

25. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Identification (16 bits)
Field uniquely identifies the fragment of an original IP packet.

26. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Flag (3 bits)
This 3-bit field identifies how the packet is fragmented.
It is used with the Fragment Offset and Identification fields to help reconstruct the fragment into the original packet.

27. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Fragment Offset (13 bits)
Field identifies the order in which to place the packet fragment in the reconstruction of the original unfragmented packet.

28. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Time-to-Live (TTL) (8 bits)
Used to limit the lifetime of a packet.
It is specified in seconds but is commonly referred to as hop count.
The packet sender sets the initial TTL value and is decreased by one each time the packet is processed by a router, or hop.
If the TTL field decrements to zero, the router discards the packet and sends an ICMP Time Exceeded message to the source IP address.
The traceroute command uses this field to identify the routers used between the source and destination.

29. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Protocol (8 bits)
Field indicates the data payload type that the packet is carrying, which enables the network layer to pass the data to the appropriate upper-layer protocol.
Common values include ICMP (1), TCP (6), and UDP (17).
Others: GRE (47), ESP (50), EIGRP (88), OSPF (89)

30. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Header Checksum (8 bits)
Field is used for error checking of the IP header.
The checksum of the header is recalculated and compared to the value in the checksum field.
If the values do not match, the packet is discarded.

31. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Source IP Address (32 bits)
Contains a 32-bit binary value that represents the source IP address of the packet.

32. Differentiated Services Destination IP Address
Version
IP Header Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Flag
Fragment Offset
Time-To-Live
Protocol
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Destination IP Address (32 bits)
Contains a 32-bit binary value that represents the destination IP address of the packet.

33. Sample IPv4 Headers
Section

34. Sample IPv4 Headers
Section

35. Sample IPv4 Headers
Section

36. IPv6 Packet
IPv4

37. Limitations of IPv4
Section
Since 1983, IPv4 has been updated to address new challenges.
However, even with changes, IPv4 still has three major issues:
IP address depletion
Internet routing table expansion
Lack of end-to-end connectivity

38. IP Address Depletion
IPv4 has a limited number of unique public IP addresses available.
Although there are approximately 4 billion IPv4 addresses, the increasing number of new IP-enabled devices, always-on connections, and the potential growth of less-developed regions have increased the need for more addresses.

39. Blocks Assigned in 1993

40. Blocks Assigned in 2000

41. Blocks Assigned in 2007

42. Blocks Assigned in 2010

43. IPv4 Address Depletion
In October 2010, less than 5% of the public IPv4 addresses remained unallocated.
Extensions to IPv4 including Network Address Translation (NAT), variable length subnet masks (VLSM) and classless inter-domain routing (CIDR) have helped to extend the life of IPv4. NAT in particular has been a major factor in slowing the depletion of addresses.
Despite these efforts, as of October 2010, less than 5% of the public IPv4 addresses remained unallocated.
Although estimates vary, this pool may be exhausted by early 2012.
As emerging nations and new ISPs come online, they will not be able to obtain blocks of IPv4 addresses. Although stop-gap measures such as NAT exist, the ultimate solution is IPv6.

44. November 30, 2010 Available Blocks: 71 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99
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45. IANA Runs out of IPv4
Monday, January 31, 2011 IANA allocated two blocks of IPv4 address space to APNIC, the RIR for the Asia Pacific region (39/8 and 106/8)
This triggered a global policy to allocate the remaining IANA pool of 5 /8’s equally between the five RIRs.
So, basically…

47. Internet Routing Table Expansion
A routing table is used by routers to make best path determinations.
As the number of servers (nodes) connected to the Internet increases, so too does the number of network routes.
These IPv4 routes consume a great deal of memory and processor resources on Internet routers.

48. Lack of End-to-End Connectivity
NAT
/24
RFC 1918 Private Address
Public IPv4 Address
Network Address Translation (NAT) is a technology commonly implemented within IPv4 networks.
NAT provides a way for multiple devices to share a single public IP address.
However, because the public IP address is shared, the IP address of an internal network host is hidden.
This can be problematic for technologies that require end-to-end connectivity.

49. IETF To The Rescue
To address these problems, the IETF it implemented solutions to solve these problems.
Short Term solutions included:
Subnetting
Variable-length subnet masking (VLSM)
Classless interdomain routing (CIDR)
Supernetting
Network Address Translation (NAT)
Private Addresses
However, its long term solution was IP version 6 (IPv6)

50. IPv6
IPv6 overcomes the limitations and provides the following improvements:
Increased address space
Improved packet handling
Eliminates the need for NAT
Integrated security

51. Increased Address Space
The 32-bit IPv4 address space provides approximately 4,294,967,296 unique addresses.
Of these, only 3.7 billion addresses are assignable, because the IPv4 addressing system separates the addresses into classes, and reserves addresses for multicasting, testing, and other specific uses.
IPv6 addresses are based on 128-bit hierarchical addressing as opposed to IPv4 with 32 bits.
340 undecillion addresses
This dramatically increases the number of available IP addresses.

52. Increased Address Space
Number name
Scientific Notation
Number of zeros
1 Thousand
103
1,000
1 Million
106
1,000,000
1 Billion
109
1,000,000,000
1 Trillion
1012
1,000,000,000,000
1 Quadrillion
1015
1,000,000,000,000,000
1 Quintillion
1018
1,000,000,000,000,000,000
1 Sextillion
1021
1,000,000,000,000,000,000,000
1 Septillion
1024
1,000,000,000,000,000,000,000,000
1 Octillion
1027
1,000,000,000,000,000,000,000,000,000
1 Nonillion
1030
1,000,000,000,000,000,000,000,000,000,000
1 Decillion
1033
1,000,000,000,000,000,000,000,000,000,000,000
1 Undecillion
1036
1,000,000,000,000,000,000,000,000,000,000,000,000
There are 4 billion IPv4 addresses
There are 340 undecillion IPv6 addresses
50 billion billion billion addresses for every person on earth

53. Do we need this many addresses?

54. Improved Packet Handling
The IPv6 header has been simplified with fewer fields.
This improves packet handling by intermediate routers and also provides support for extensions and options for increased scalability/longevity.

55. Destination IP Address
IPv6 Header
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Byte 1
Byte 2
Byte 3
Byte 4

56. Sample IPv4 Packet

57. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address

58. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Version (4 bits)
Indicates the version of IP currently used.
0100 = 4 and therefore IPv4
0110 = 6 and therefore IPv6

59. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Traffic Class (8 bits)
Field is equivalent to the IPv4 Differentiated Services (DS) field.
It also contains a 6-bit DSCP value used for QoS and a 2-bit ECN used for traffic congestion control.

60. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Flow Label (20 bits)
Field provides a special service for real-time applications.
It can be used to inform routers and switches to maintain the same path for the packet flow so that packets are not reordered.

61. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Payload Length (16 bits)
Field is equivalent to the Total Length field in the IPv4 header.
It defines the entire packet (fragment) size, including header and optional extensions

62. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Next Header (8 bits)
Field is equivalent to the IPv4 Protocol field.
It indicates the data payload type that the packet is carrying, enabling the network layer to pass the data to the appropriate upper-layer protocol.
This field is also used if there are optional extension headers added to the IPv6 packet.

63. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Hop Limit (8 bits)
Field replaces the IPv4 TTL field.
This value is decremented by one by each router that forwards the packet.
When the counter reaches 0 the packet is discarded and an ICMPv6 message is forwarded to the sending host, indicating that the packet did not reach its destination.

64. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Source Address (128 bits)
Field identifies the IPv6 address of the sending host.

65. Destination IP Address
Version
Traffic Class
Flow Label
Payload Length
Next Header
Hop Limit
Source IP Address
Destination IP Address
Destination Address (128 bits)
Field identifies the IPv6 address of the receiving host.

66. Sample IPv6 Headers
Section

67. Sample IPv6 Headers
Section

68. Sample IPv6 Headers
Section

69. Eliminates the Need for NAT
With such a large number of public IPv6 addresses, Network Address Translation (NAT) is not needed.
Customer sites, from the largest enterprises to single households, can get a public IPv6 network address.
This avoids some of the NAT-induced application problems experienced by applications requiring end-to-end connectivity.

70. Integrated Security
IPv6 natively supports authentication and privacy capabilities.
With IPv4, additional features had to be implemented to do this.