Module 5.1: Internet Protocol

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Module 5.1 Internet Protocol

• TCP/IP Suite
• IP Addressing
• ARP
• RARP
• DHCP

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Some Protocols in TCP/IP Suite
3
Address Types
4
Address Relations and Encapsulation
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IP Addressing

• IP address is 32 bits
• Originally had 2 level of hierarchy network ID
  and host ID
• The network ID is assigned by the Internet
  Network Information Center (InterNIC).
• Routers use only network ID
• Five classes of IP ? This is called Classful
  Addressing
• Identify class by looking at the first byte
• 0-127 gives A, 128-191 gives B, and 192 to 223
  gives C
• A gives 126 networks with 16 million hosts
• B gives 16,382 networks with 64k hosts
• C gives 2 million networks with 254 hosts

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IP Addressing (Cont.)

• IP addresses are written in dotted-decimal
  notations
• Subnetting
• Calss B address can support 64000 hosts! Hard to
  manage on a single network.
• Adds another hierarchical level called the
  subnet.
• Every host has its own IP address and its subnet
  mask. Hence, this host can determine, if a
  destination IP address is
• On its own subnet
• On different subnet, but same own network
• On different network

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IP Addressing (Cont.)
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Classful vs. Classless Addressing
Only applicable on directly connected links
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Classless Inter-Domain Routing (CIDR)

• Classless Inter-Domain Routing is based on
  variable-length subnet masking (VLSM) to allow
  allocation on arbitrary-length prefixes.
  Variable-length subnet masks are mentioned in RFC
  950.
• CIDR is principally a bitwise, prefix-based
  standard for the interpretation of IP addresses.
• These groups, commonly called CIDR blocks, share
  an initial sequence of bits in the binary
  representation of their IP addresses.
• IPv4 CIDR blocks are identified using a syntax
  similar to that of IPv4 addresses a four-part
  dotted-decimal address, followed by a slash, then
  a number from 0 to 32 A.B.C.D/N.

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CIDR Blocks

• All of these CIDR prefixes would be used, at
  different locations in the network.
• Outside of MCI's network, the 208.128.0.0/11
  prefix would be used to direct to MCI traffic
  bound not only for 208.130.29.33, but also for
  any of the roughly two million IP addresses with
  the same initial 11 bits.
• Within MCI's network, 208.130.28.0/22 would
  become visible, directing traffic to the leased
  line serving ARS.
• Only within the ARS corporate network would the
  208.130.29.0/24 prefix have been used.

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Special Addresses

• There are several IP addresses that are reserved
  for special purposes and are not available for
  assignment for hosts.
• Any address with a first octet value of 127 is a
  loopback address.
• A loopback address is used by a host to
  communicate with itself through TCP/IP. It is
  also used for testing and diagnostics.

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Special Addresses (cont.)

• 255 in either the host id or the network id
  designates a broadcast.
• A message sent to 255.255.255.255 is broadcast to
  every host on the local network. A message sent
  to 196.1.64.255 is broadcast for every host on
  network 196.1.64.
• 0 in both host id and network id can only appear
  as a source address in BOOTP protocol in
  bootstrapping when host is determining its own
  address.
• Address Allocation for Private Intranets
• 10 A single Class A network.
• 172.16 through 172.31 16 contiguous class B
  network
• 192.168.0 through 192.168.255 256 contiguous
  Class C networks.
• Any organization can use any address in these
  ranges without reference to any other
  organization.
• Routers in networks do not use private addresses,
  e.g. ISP. Routers quietly discard all routing
  information regarding these addresses.
• Hosts having only a private IP address do not
  have IP layer connectivity to the Internet. They
  have to go through NAT gateway.

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ARP

• How does a machine map an IP address to its Data
  Link layer (hardware or MAC) address?
• This is needed by the source host (A) who needs
  to send an IP packet to another host (B)
• if B is on same IP network as A, address frame
  to B
• if B on a separate network, address frame to a
  router
• TCP/IP solution Address Resolution Protocol
  (ARP). See next figure.
• ARP cache or table is used by hosts to avoid
  sending ARP request every time. ARP table
  entries age after a certain time (30 seconds).

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RARP

• How does a diskless machine (X terminal)
  determine its IP address?
• When bootstrap code starts execution on a
  diskless machine, it must use the network to
  contact a server to obtain the machines IP
  address
• Usually, a machines IP address is kept on disk
  where OS finds it at startup
• RARP is the protocol used to solve the reverse
  problem solved by ARP
• Given a physical address, get the corresponding
  IP address
• The RARP server must be located on the same
  physical network as the host.

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Encapsulation of ARP and RARP messages in
Ethernet frames
Note The types above are unique. The length
field is constant which is 2 bytes. The value of
both types are more than 1500 bytes.
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BOOTP and DHCP

• A host requires three elements to connect to the
  Internet
• IP address
• subnet mask
• nearby router
• BOOTP (Bootstrap Protocol) provided such
  information to hosts.
• DHCP (Dynamic Host Configuration Protocol) builds
  on the capability of BOOTP.
• DHCP is widely used because it provides a
  mechanism for assigning temporary IP addresses to
  hosts.
• ISP uses DHCP to maximize the usage of their
  limited IP address space.
• Typical messages used by DHCP
• DHCP Discover broadcast initiated by host
• DHCP Offer reply by DHCP server(s) offering
  service to host
• DHCP request used by host to request info from
  a particular server.
• DHCP ACK reply by the selected server to host.

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Passing of IP packet.