Part 4 : Network Layer

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Part 4 Network Layer
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Role and Position of Network Layer

• Network layer in the Internet model is
  responsible for carrying a packet from one
  computer to another
• It is responsible for host-to-host delivery.
• Position of network layer

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Duties of Network Layer
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Chapter 19Host-to-host Delivery Interworking,
Addressing, and Routing
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19.1 Internetworks

• The physical and data link layers of a network
  operate locally

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Links in an Internetwork
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Network Layer in an Internetwork
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Network Layer at the Source
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Network Layer at a Router
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Network Layer at the Destination
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Switching

• Virtual circuit approach relationship between
  all packets belonging to a message is preserved
  a single route is chosen, and all packets take
  that route
• Datagram approach each packet is treated
  independently of all others thus, packets in
  the same message can take different routes, and
  possibly arrive out of order

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Datagram Approach
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Internet as a Connectionless Network

• In a connection-oriented service, the source
  first makes connection with the destination
  before sending a packet.
• They are sent on the same path in sequential
  order.
• In a connectionless service, the network layer
  protocol treats each packet independently, with
  each packet having no relationship to any other
  packet.

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19.2 Addressing

• For a host to communicate with any other host
• Need a universal identification system
• Need to name each host
• Internet address or IP address is a 32-bit
  address that uniquely defines a host or a router
  on the internet
• The IP addresses are unique in the sense that two
  devices can never have the same address. However,
  a device can have more one address.

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Notation

• Binary notation
• 01110101 10010101 00011101 11101010
• 32 bit address, or a 4 octet address or a
  4-byte address
• Decimal point notation

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Notation (contd)

• Hexadecimal Notation
• - 8 hexadecimal digits
• - Used in network programming

0111 0101 1001 0101 0001 1101 1110 1010
75 95 1D
EA
0x75951DEA
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Classful Addressing

• Occupation of address space
• In classful addressing, the address space is
  divided into five classes A, B, C, D, and E.
• Finding the class in binary notation

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Classful Addressing (contd)

• Finding the address class

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Classful Addressing (contd)

• Finding the class in decimal notation

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Example 4

• Find the class of each address
• a. 227.12.14.87
• b. 252.5.15.111
• 134.11.78.56
• Solution
• a. The first byte is 227 (between 224 and 239)
  the class is D.
• b. The first byte is 252 (between 240 and 255)
  the class is E.
• c. The first byte is 134 (between 128 and 191)
  the class is B.

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Netid and Hostid

• Each IP address is made of two parts netid and
  hostid.
• Netid defines a network hostid identifies a host
  on that network.

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Netid and Hostid (contd)

• IP addresses are divided into five different
  classes A, B, C, D, and E

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Classes and Blocks

• Blocks in class A
• Class A is divided into 128 blocks with each
  block having a different netid.
• Millions of class A addresses are wasted.

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Classes and Blocks (contd)

• Class B is divided into 16,384 blocks with each
  block having a different netid

Many class B addresses are wasted.
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Classes and Blocks (contd)

• Class C is divided into 2,097,152 blocks with
  each block having a different netid.

The number of addresses in a class C block is
smaller than the needs of most organizations
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Classes and Blocks (contd)

• Class D addresses are used for multicasting
  there is only one block in this class.
• Class E addresses are reserved for special
  purposes most of the block is wasted.

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Network Address

• The network address is the first address.
• The network address defines the network to the
  rest of the Internet.
• Given the network address, we can find the class
  of the address, the block, and the range of the
  addresses in the block
• In classful addressing, the network address
  (the first address in the block) is the one that
  is assigned to the organization.

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Network Address (contd)

• Network address an address with the hostid all
  set to 0s

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A Sample Internet with Classful Address

• Token Ring LAN (Class C), Ethernet LAN (Class B),
  Ethernet LAN (Class A) , Point-to-point WAN, A
  Switched WAN

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Subnetting and Supernetting

• Subnetting
• A network is divided into several smaller
  networks with each subnetwork (or subnet) having
  its subnetwork address
• Supernetting
• Combining several class C addresses to create a
  larger range of addresses
• IP Addresses are designed with two levels of
  hierarchy

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Subnetting

• Classes A, B, C in IP addressing are designed
  with two levels of hierarchy (not subnetted)
• Netid and Hostid

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Subnetting (contd)

• Further division of a network into smaller
  networks called subnetworks
• R1 differentiating subnets

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Subnetting (contd)

• Three levels of hierarchy netid, subnetid, and
  hostid

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Subnetting (contd)

• Three steps of the routing for an IP datagram
• Delivery to the site, delivery to the subnetwork,
  and delivery to the host
• Hierarchy concept in a telephone number

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Default Masks

Class In Binary In Dotted-Decimal Using Slash
A 11111111 00000000 00000000 00000000 255.0.0.0 /8
B 11111111 11111111 00000000 00000000 255.255.0.0 /16
C 11111111 111111111 11111111 00000000 255.255.255.0 /24

• When a router receives a packet, it needs to
  route it
• Uses mask to determine the subnetwork address
• Routers outside the organization use default
  mask
• Routers inside use a subnet mask

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Comparison of a default mask and a subnet mask

• Number of subnets is determined by number of
  extra 1s in the subnet mask.
• 2n 23 8 subnets

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Supernetting

• A block of class x addresses
• For example,
• An organization that needs 1,000 addresses can be
  granted four class C addresses

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Supernetting (contd)

• 4 class C addresses combine to make one
  supernetwork

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19.3 Routing

• Next-hop routing

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Routing (contd)

• Network-specific routing

• Dont have an entry for every host connected to
  the same
• physical network
• Instead, only have one entry to define the
  destination network

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Routing (contd)

• Host-specific routing

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Routing (contd)

• Default routing

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Static and Dynamic Routing Tables

• Static routing table containing information
  entered manually
• Dynamic routing table
• updating periodically using one of the dynamic
  routing protocols such as RIP, OSPF, or BGP
• Whenever there is a change in the Internet, the
  dynamic routing protocols update all the tables
  in the routers.

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20.2 IP datagram
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IP Datagram (contd)

• Version for IP version4, it is 4
• Header Length Defining the length of the
  datagram header in 4 byte words

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IP Datagram (contd)

• Differentiated Services
• The first 6 bits codepoint subfield (DSCP
  differentiated services code point)
• Values for codepoints

Category Codepoint Assigning Authority
1 XXXXX0 Internet
2 XXXX11 Local
3 XXXX01 Temporary or experiment
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IP Datagram (contd)

• Total Length head data
• Defining the total length of the datagram
  including the header
• Length of data total length header length
• Limited to 65,535 (216 1) bytes
• Encapsulation of a small datagram in an Ethernet
  Frame

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IP Datagram (contd)

• Fields related to fragmentation
• Identification 16 bit-field
• Datagram id that is originated by the source host
• Therefore, Source IP address datagram id
  (identification)
• All fragments having same identification number
• Identification No. to be used for the destination
  in reassembling the datagram
• Flags 3 bit-field
• D Do not fragment (1)
• If it can not pass the datagram through any
  available physical network, it discards the
  datagram and send ICMP error message to the
  source host
• M More fragment (0)
• 0 last fragment or only fragment

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IP Datagram (contd)

• Fragmentation offset 13-bit field
• Showing relative position of this fragment with
  respect to the whole datagram
• Measured in units of 8 bytes forcing hosts or
  routers that fragment datagrams to choose the
  size of each fragment so that the first byte
  number is divisible by eight

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IP Datagram (contd)

• Time to live
• Used to control the maximum number of hops
  (routers) visited by the datagram
• If the value is Zero, the routers discarded
• If the source wants to confine the packet to the
  local network, it can store 1 in this field

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IP Datagram (contd)

• Fragmentation
• The format and size of the received frame depend
  on the protocol used by the physical network

MTU (Maximum Transfer Unit) When a datagram
is encapsulated in a frame, the total size of the
datagram must be less than this maximum size
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IP Datagram (contd)

• MTUs for different networks

• Hyperchannel Network Systems Corporation, 1988
  (RFC 1044)

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IP Datagram (contd)

• Protocol
• Defining the higher level protocol that uses the
  services of the IP layer
• TCP, UDP, ICMP, and IGMP
• Multiplexing data from different higher level
  protocols

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IP Datagram (contd)

• Example of Checksum Calculation

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20.4 IPv6 Address

• IPv6 address consists of 16 octets it is 128
  bits long