Module 10 IP Addressing and Formatting

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Module 10IP Addressing and Formatting
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• Textbook sections
• BF Chapter 4 IP addressing
• BF Chapter 5 Subnetting and Supernetting
• BF Chapter 7 Internet Protocol (IP) Sections 7.1,
  7.2, 7.3, and 7.4
• Topics
• IP Addressing
• Classes
• Special Addresses
• Subnetting
• Supernetting
• Classless interdomain routing (CIDR)
• IP Format
• Fields
• Fragmentation
• Option Field

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TCP/IP Reference Model

• Andrew S. Tanenbaum
• Thus the ability to connect multiple networks
  together in a seamless way was one of the major
  design goals from the very beginning.
• Given the DODs worry that some of its precious
  hosts, routers, and internetwork gateways might
  get blown to pieces at a moments notice, another
  major goal was that the network be able to
  survive loss of subnet hardware, with existing
  conversions not being broken off. In other
  words, DOD wanted connections to remain intact as
  long as the source and destination machines were
  functioning, even if some of the machine or
  transmission lines in between were suddenly put
  out of operation.

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1. IP Addressing - Classes
BF Figure 4-3 Internet address classes
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1. IP Addressing - Classes
BF Figure 4-4 Classes using dotted-decimal
notation
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Table 4.2 Special addresses
1. IP Addressing - Special Addresses
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BF Figure 4-6 Example of network addresses Note
The network itself is considered an entity with
an IP address in which the hostid part is set to
zero.
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BF Figure 4-7 Example of direct broadcast
address Note Used by a router to send a packet
to all hosts in a specific network
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BF Figure 4-8 Example of limited broadcast
addresses Note Used to broadcast on the local
network. However, a router will block a packet
having this type of address to confine the
broadcasting to the local network.
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BF Figure 4-9 Example of this host on this
network address
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1. IP Addressing - Special Addresses

• This host on this network
• Used by a host at bootstrap time when it does not
  know its IP address
• The host sends an IP packet to a bootstrap server
  using this address as the source address and a
  limited broadcast address as the destination
  address (Blocked by a router)
• Is a class A address regardless of the network.
  It reduces the number of networks in class A by
  one (stating bit is 0)

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BF Figure 4-10 Example of specific host on
this network Note These addresses allow hosts
to refer to their own network without knowing its
Netid. (but they have to know its class to know
how many 0s to include)
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BF Figure 4-11 Example of loopback address Note
Used for debugging network software
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1. IP Addressing - Special Addresses

• Loopback address
• An application such as ping can send a packet
  with a loopback address as the destination
  address to see if the IP software is capable of
  receiving and processing a packet.
• The loopback address can be used by a client
  process (a running application program) to send a
  message to a server process on the same machine.

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1. IP Addressing - Subnetting

• Subnet
• Two-level hierarchical addressing
• The hosts cannot be organized into groups, and
  all of the hosts are at the same level. The
  organization has one network with many hosts
• Subnet-addressing (Three-level hierarchical
  addressing)
• it is oblivious to the network outside the
  organization
• External view
• a host outside this organization would still see
  the original address structure with two levels
• Internal view
• Inside the organization the local network
  administrator is free to choose any combination
  of lengths for the subnet and host ID field
• Masking
• A process that extracts the address of the
  physical network from an IP address. Masking can
  be done whether we have subnetting or not.

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1. IP Addressing - Subnetting
Figure 8.6 Introducing another hierarchical level
through subnet addressing
Original
Net ID
Host ID
1 0
address
Subnetted
Net ID
Host ID
1 0
Subnet ID
address
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BF Figure 5-6 Applying bit-wise-and operator to
achieve masking
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1. IP Addressing - Supernetting

• Supernetting
• Although class A and B address are almost
  depleted, class C addresses are still available.
  However, the space of a class C address, with a
  maximum number of 254 host addresses, may not
  satisfy the needs of an organization. Even a
  midsize organization may need more addresses.
• One solution is supernetting. An organization
  can apply a block of class C addresses. The
  organization can then use these addresses in one
  supernetwork.
• Supernet mask

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BF Figure 5-19 Supernet mask
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• Classless interdomain routing (CIDR)
• CIDR reduces the number of entries in a routing
  table by using a supernet mask and the lowest
  network address of a supernet to represent the
  member networks
• Bit-count mask
• Writing and describing network addresses as four
  dotted-decimal octets followed by a
  four-dotted-decimal octet network mask has always
  been somewhat cumbersome.
• A more precise and compact way of describing the
  address space was desired when assigning CIDR
  block of addresses. A forward slash, /, followed
  by the number of bits set to one in the network
  mask, is used instead of the four-octet
  dotted-decimal mask. For example, a network mask
  of 255.255.0.0 has 16 bits of ones, so it can be
  written as /16. A network mask of 255.255.252.0
  has 22 bits of ones, so it can be written as /22.
  This type of mask is known as a bit-count mask.
• Combined with an IP network address, the network
  shorthand of 131.108.0.0/16 can be used to
  represent 131.108.0.0 mask 255.255.0.0.
  Likewise, 206.220.224.0/22 can be used to
  represent 206.220.224.0 mask 255.255.252.0 (which
  itself is a CIDR block representation the Class C
  addresses 206.220.224.0 through 206.220.227.0
  each with mask 255.255.255.0)

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BF Figure 5-22 CIDR
Lowest network address of a supernet
Supernet mask
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2. IP Format - Fields

• IP packet

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BF Figure 7-1 IP datagram
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BF Figure 7-4 Multiplexing
Table 7.3 Protocols
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2. IP Format - Fragmentation

• The need to perform fragmentation
• One of the problems of providing a host-to-host
  service over a heterogeneous collection of
  networks is that different type of networks tends
  to have different maximum frame sizes. For
  example, an Ethernet can accept packets up to
  1500 bytes long, while token-ring packets may be
  4,464 bytes long
• Two solutions
• Make sure that all IP datagrams are small enough
  to fit inside a frame of any network
• Provide a means by which packets can be
  fragmented and reassembled when they are too big
  to go over a given network.
• The central idea is that every network type has a
  maximum transmission unit (MTU), which is the
  largest packet size that it can carry in a frame

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2. IP Format - Fragmentation

• The need to perform fragmentation
• When a host sends an IP datagram, it can choose
  any size that it wants. A reasonable choice is
  the MTU of the network to which the host is
  directly attached. Then fragmentation will only
  be necessary if the path to the destination
  includes a network with a smaller MTU.
• If the transport protocol that sits on top of IP
  give IP a packet larger than the local MTU, than
  the source host must fragment it.
• Fragmentation typically occurs in a router when
  it receives a datagram that it wants to forward
  over a network that has an MTU that is smaller
  than the received datagram

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BF Figure 7-5 MTU
BF Table 7.4 MTUs for different networks (in
bytes)
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2. IP Format - Fragmentation

• Fragmentation
• When a datagram is fragmented, each fragment has
  its own header with most of the field repeated.
• A datagram can be fragmented several times before
  it reaches the final destination.
• A datagram can be fragmented by the source host
  or any router in the path.
• The reassembly of the datagram is done only by
  the destination host, because each fragment
  becomes an independent datagram.

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2. IP Format - Fragmentation

• Identification field
• This 16-bit field identifies a datagram
  originating from the source host. The
  combination of the identification and source IP
  address must uniquely define a datagram as it
  leaves the source host
• Flag field
• The first bit is reserved.
• The second bit is called the do not fragment bit.
  When set, it means the datagram should not be
  fragmented.
• The third bit is called the more fragment bit.
  When set, it means the datagram is not the last
  fragment.
• Fragmentation offset
• This 13-bit field shows the relative position of
  the fragment with respect to the whole datagram.
  Its is the offset of the data in the original
  datagram measured in units of eight bytes.

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2. IP Format - Fragmentation
BF Figure 7-7 Fragmentation example
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BF Figure 7-8 Detailed fragmentation example
Total length Data 4,000 bytes Header 20 bytes
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2. IP Format Option Field
BF Figure 7-9 Option format
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2. IP Format Option Field
BF Figure 7-10 Categories of options
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2. IP Format Option Field

• No operation option
• Used to align the next option on a 16-bit or
  32-bit boundary
• One-byte option
• No operation option is used as a filler between
  options.
• End of option option
• Used to align the payload data on a 16-bit or
  32-bit boundary
• One-byte option
• End of option is used for padding at the end of
  the option field.
• Restriction
• After this option, the receiver looks for the
  payload data.
• If more than one byte is needed for alignment,
  some no operation option must be used before the
  end of option option.

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BF Figure 7-11 No operation option
BF Figure 7-12 End of option option
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2. IP Format Option Field
BF Figure 7-13 Record route option
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2. IP Format Option Field
BF Figure 7-14 Record route concept
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2. IP Format Option Field
Source routing Routing decision performed at the
source before the packet is sent. The route
consists of the list of nodes that the packet
should traverse on the way to the destination.
BF Figure 7-15 Strict source route option
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2. IP Format Option Field
BF Figure 7-16 Strict source route concept
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2. IP Format Option Field
BF Figure 7-18 Timestamp option
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2. IP Format Option Field
BF Figure 7-19 Use of flag in timestamp
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2. IP Format Option Field
BF Figure 7-20 Timestamp concept Note Time is
expressed in milliseconds from midnight