Addressing by harshi446


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The ‘What’ and ‘Where’ of
   Addressing is necessary for any
    –   To talk: Appearance, name, …
    –   To call: Telephone numbers
    –   To mail: Postal address
    –   To visit: Postal address + directions
    –   To E-Mail: E-Mail addresses
    –   To instant message: ICQ#, AIM ID, etc.
   These ‘addresses’ allow us to uniquely
    identify the entity with which we wish to
Addressing a la Shoch
   Name/Identifier: What
    – Names normally identify the entity
    – If an entity moves, the name/identity will remain
      the same
   Address: Where
    – Addresses identify the location of the entity
    – If an entity moves, the address will change
   Route: How to get there
    – Routes identify the path to get to an entity
    – If an entity moves, the route will change

 Addressing deals with how to define an
  entity’s location (uniquely)
 Addressing is necessary for message
    – An address is the start and end point for
      the route
      • However, routing is another subject
    – Where do we want the message to go?
   We have already seen MAC addresses (for
    Ethernet and some other LANs):
    –   e.g. 02-60-8C-08-E1-0C
    –   6 octet address
    –   Globally unique
    –   Defined statically by the hardware manufacturer
   Most people are familiar with the IP
    addresses used by TCP/IP networks:
    –   e.g.
    –   4 octet address
    –   Not necessarily globally unique
    –   Defined dynamically by DHCP servers or
        negotiated by the operating system
IP Addressing

A Closer Look
IP Addresses

 TCP/IP networks use IP for the network layer
 IP defines 4 octet addresses
    – 4 billion possible addresses
   Usually written in the form A.B.C.D
    – A, B, C, and D are each 1 octet (0-255), normally
      written in decimal notation
    – Thus, IP addresses fall in the range: –
IP Addresses

   Originally intended for separate
    internets (interconnected LANs)
    – Thus, the 32 bit size was not a concern
    – 48 bits is generally considered a fairly safe
      size for globally unique addressing
    – Computers connected to ARPANET (and
      later incarnations) were just given
      consecutive addresses,,, …
IP Addresses

 Any computer connected to a TCP/IP
  network (e.g. the Internet) must have an
  IP address
 Further, any network interface card
  (NIC) using TCP/IP to access an
  network (e.g. the Internet) must have a
  different IP address
IP Addresses

 Even though there are 4 billion possible
  IP addresses, they are running out
 Here’s why:
    – Some of the bits are dedicated to header
      information (discussed later)
      • ½ the addresses for each lost bit
    – Addresses are categorized, and some of
      the categories are running out of
      addresses (while others are not)
Non-Classed Addresses

   Part of the address represented the network
    the computer resided on, and part
    represented the computer itself
    – Network: 7 bits (up to 128 networks)
    – Computer: 24 bits (up to 1.6 million computers on
      each network)
   Since there were very few networks on
    ARPANET originally, this wasn’t a problem
Address Classes

   When private organizations started
    joining the Internet, the needs became
    – Some (fewer) networks have multitudes of
      computers (thousands)
       • e.g. The @Home network
    – Some (many) networks have very few
      computers (a few hundred or less)
       • e.g. The Windsor Police Department
Address Classes

   Quickly, the addresses were separated
    into 3 classes (plus room for more
    classes if needed):
    – Class A: Fewer networks, many nodes
    – Class B: Medium networks, medium
    – Class C: Many networks, fewer nodes
   IP Address Classes
    Class A:
bit index: 0      1-7                 8-31
           0 network              host (machine)

    Class B:
bit index: 0 1           2-15                16-31
            1 0         network               host

    Class C:
bit index: 0 1 2         3-23                      24-31
            1 1 0       network                     host
IP Address Classes
   Class A:
    – Range: –
    – Networks: 128 max, Machines: 65537-1.6 million
    – e.g. huge networks, such as large
      military/government organizations (e.g. FBI), the
      @Home network, etc…
   Class B:
    – Range: –
    – Networks: 16384 max, Machines: 257-65536
    – e.g. Internet service providers (ISPs) (dial-up)
   Class C:
    – Range: –
    – Networks: 2 million max, Machines: 1-256
    – e.g. Small businesses
IP Address Classes

   The IP address classes are self-identifying
    – Which means that given the address, you can
      determine what class an address is
       • Actually, using only the first number
    – Examples:
       • (
           – 137 -> Class B
       • (@Home DHCP server)
           – 24 -> Class A
   Other IP Address Classes
    Class D:
bit index: 0 1 2 3                      4-31
          1 1 1 0             Multicast group address
     •These addresses are used to represent multicast groups
        •Discussed later

    Class E:
bit index: 0 1 2 3 4                   5-31
            1 1 1 1 0          Reserved for future use

     •These addresses were left open to be used and divided
     into classes as needed
Special IP Addresses
 Used to indicate that this machine is
    without an assigned IP
    – Used during bootstrapping (e.g. requesting an IP
      from a DHCP server)
 <all 0s (binary)><hostID>: Used to send
  messages to some machine on this network
 Used to send broadcast
  messages across this machine’s network
 <netID><all 1s (binary)>: Used to send
  broadcast messages to the specified network
 Used to send messages back to
  this machine (called loopback or localhost)
IP Addressing Comments

   In IP addressing:
    – 0’s usually represent ‘this’
    – 1’s usually represent ‘all’
   Broadcasting, although discussed here
    in terms of addressing, will be
    discussed further
   The address, does not normally
    exist on the network
    – Either as the source address or destination
      address of a packet
   The address is used internally by NICs
    – When a NIC receives a message addressed with to be transmitted, it passes the message
      directly to the receiver hardware
    – The receiver hardware returns the message to the
      operating system exactly as if the message were
      received from the network
       • However, the message never entered the network
Internal IP Addresses

   Depending on the address class needed by
    an organization, a range of internal
    addresses is available:
    – Class A: –
    – Class B: –
    – Class C: –
   IP routers outside a private (connection-
    shared) network, will not forward datagrams
    designated for addresses in these ranges
Multi-homed Machines

   There is no restriction preventing
    machines from participating in multiple
    – A machine could have multiple NICs
    – Each NIC would have its own MAC
    – On TCP/IP networks, each of these NICs
      would be given a different IP address
Multi-homed Machines
                                                          M    M                                     
                Class C private network
                M       M                M   M            M    M

                    M   M                M   M
                    Class B private network  
Multi-homed Machines                              
                Class C private network  

                    Class B private network
   Routers are multi-homed machines
    – They have a number of network ports, each of
      which represents a different path
   Routers use tables that relate destinations to
    network paths
    – Internet routers relate destination network
      addresses with one of their network ports
    – When a datagram arrives at a router:
       • Its destination address is used to determine the network
       • The network address is used to look up the destination
         port in the routing table
Network Addresses
 An IP address can be used to calculate the
  address of the network
 The machine address is passed through a
  filter (called a subnet filter):
    – This filter extracts the bits of the address that
      represent the network and sets the bits that
      represent the machine to zero
    – The filter determines which part of the address
      represent the network address, by using the
      subnet mask
Subnet Mask
 The subnet mask is a binary number, that has
  0s in the machine portion of the address, and
  1s in the network portion
 Most networks of each type use a constant
  subnet mask
    – Class A:
       (Binary: 11111111000000000000000000000000)
    – Class B:
       (Binary: 11111111111111110000000000000000)
    – Class C:
       (Binary: 11111111111111111111111100000000)
Using Subnet Masks

   Example:
    – Address:
    – Subnet Mask:
Address:     10001001110011110010000000000010
Mask:        11111111111111110000000000000000
Net Address: 10001001110011110000000000000000
   Network address:
Routing in Action
Network N1 (Class B)
              P2   *             P4        P3

Network N2 (Class C)                   Network N3 (Class A)
Address:                    Address:

Next Generation Addressing
in TCP/IP Networks

 Due to the limited nature of existing IP
  addressing (IPv4), a new version of IP
  addressing was developed
 This new scheme uses 16 octets for
  addresses, instead of 4 octets
 Written using hex notation:

IPv6 Features
 16 octet addresses (128 bits)
 Larger numbers of address classes
    – More accurate control of network/machine counts
   Variable-sized headers
    – Optional information can be placed into the header
      when needed
    – Reduces header size in most cases
   Extendible protocol
    – IPv6 allows for new header information to be
      added to support different protocols
IPv6 Features
   Automatically reconfigurable
    – Addresses can be automatically reassigned
    – e.g. when a certain number of nodes join the
      network, a different address class may be desired
   Autoconfigurable
    – The use of autoconfiguration (such as DHCP)
      allows dynamic private addressing and dynamic
      public addressing
IPv6 Datagram Format


 header   extension headers   data
IPv6 Header Format
0             4                   12                                   31

    version       traffic class              flow label

32                                     48             56           63

         payload length                 next header        hop limit

64                                     96                          128

         source address                     destination address
IPv6 Integration
   Will IPv6 replace IP addresses?
    – Who knows?
 Currently, temporary solutions have made
  IPv4 addresses capable of lasting longer than
  originally predicted
 If and when IPv6 is to be integrated, the
  process must be a transition
    – Closing the entire Internet down to convert
      hardware and software to IPv6 not going to
    – Some stations may take longer to transition than
      other stations
       • e.g. Bob’s Internet Shack vs. the Telus Network
IPv6 Integration
    NAT (network address translators) provide one
     example of such a temporary solution
    NATs provide three benefits:
    1.   NATs provide IP masquerading
         •   Messages using these addresses pass through a network
             address translator (NAT) to be transformed into external IPs
    2.   NATs provide IP sharing
         •   ISPs for example, have many customers, but significantly
             less at any given time are logged onto their system
             –   IP addresses can be assigned dynamically to these customers
                 when they log in
    3.   NATs provide schemes to allow networks to use either
         IPv4 or IPv6
         –   Addresses would be converted as they pass through a NAT
IPv6 Integration
   Another method that may be used for the
    transition between IPv4 and IPv6 is address
    –   IPv4 addresses could be embedded into IPv6
        •   Translation between the two types of addresses is
            possible without any other information
    –   Some problems exist with this approach, but in
        general it simplifies communication between
        IPv6 networks and IPv4
Special IPv6 Addresses
   0:0:0:0:0:0:0:0 Used to indicate that this
    machine is without an assigned IP
    – Used during bootstrapping (e.g. requesting an IP
      from a DHCP server)
   0:0:0:0:0:0:0:1 Used to send messages back
    to this machine (called loopback)
    – These two addresses are not valid on the actual
      network medium (same as with IPv4)
 00:… Reserved (including IPv4 and IPX
  address inclusion)
 FF:… Multicast addresses

Internetwork Packet
Exchange Addresses

 IPX was originally created to replace IP
 In reality, it is used primarily on LANs
 In conjunction with the SPX protocol,
  formed one of the two protocol suites
  used in Netware networks
    – SPX is to TCP, what IPX is to IP
   Still can be (although rarely is) used
    today in Windows networks
IPX Addresses

   IPX uses a 2 component address (like
    – The network portion (4 octets)
    – The machine portion (6 octets)
   Unlike IP, these sizes are constant
    – So there are no IPX address classes
   IPX uses sizes large enough to
    accommodate all categories of networks
IPX Addresses

   The network portion of an IPX address is 4
    octets (32 bits)
    – This allows for 4.29*109 networks (4 billion)
    – This is almost enough for everyone on earth to
      have their own network
   The machine portion of an IPX address is 6
    octets (48 bits)
    – This allows for 2.81*1014 machines on each
      network (281,475 trillion)
IPX Addresses

 The scalability of IPX addresses is not
  their only benefit
 The constant size of the network and
  machine address portions simplifies
  extracting each portion
    – As a result, machines that process IPX
      networks can process IPX datagrams more
      • Such as network nodes, routers, etc.
IPX Addresses

   Why 48 bits for the machine portion?
    – 48 bits allows for way too many machines, more
      than will be needed for many years
       • By the time machine IDs run out, network hardware and
         software will have been obsolete by many years!
    – Using 48 bits allows hardware to use the
      machine’s MAC address as the machine portion
       • This makes auto-configuration (dynamic IPX address
         assignment) easier/faster
IPX Addresses
   If IPX has been around since the NetWare
    days, why don’t we use it for the Internet,
    instead of IP?
    – Good question! Why don’t we?
    – Frankly, IPX has a bad reputation, because
      initially it was used with SPX and other bandwidth-
      munching protocols
    – IPX can be used in conjunction with TCP
      (TCP/IPX), and it would make an excellent
      replacement for TCP/IP
       • However, standards organizations (e.g. ISO) want to use
         the protocols they develop, and not ones developed by
         corporations, such as Novell
Fragmentation & Reassembly

 Packets can arrive out of order in
  connectionless networks
 Packets must be reordered during
 During fragmentation, the portion of data that
  each fragment represents must be identified
    – Since the length of a packet’s data can always be
      determined, all that is necessary is to use the
      offset of the start of the packet’s data in the larger
      data chunk
  Fragmentation & Reassembly

   0        1500        3000     4500        6000 6800

                   Logical Data Chunk

Length:1500 Length:1500 Length:1500 Length:1500 Length:800
Offset:0    Offset:1500 Offset:3000 Offset:4500 Offset:6000

 Packet 1    Packet 2     Packet 3      Packet 4   P5
Fragmentation & Reassembly

   Why do we use the data offset, and not just a
    sequence of numbers to determine packet
    – Sometimes, packets can be fragmented at one
      location, and must be re-fragmented at another
      location (such as while passing through a network
      incompatible with larger frame sizes)
    – These situations would require renumbering of all
      packets in the sequence, which is not always
  Fragmentation & Reassembly

Length:1500 Length:1500 Length:1500 Length:1500 Length:800
Offset:0    Offset:1500 Offset:3000 Offset:4500 Offset:6000

 Packet 1    Packet 2    Packet 3       Packet 4   P5

 Packet 1    Packet 2    P3a   P3b      Packet 4   P5

                Length:1000         Length:500
                Offset:3000         Offset:4000
Fragmentation & Reassembly

   Re-fragmentation (at gateways, routers, …) is
    – The re-fragmenting node must process each
      packet, fragmenting it into smaller packets
    – Another reassembling node must collect these
      packets and assemble them into larger packets
    – Each of these operations involved memory
      processing, which is expensive when applied to
      many packets per second
Fragmentation & Reassembly
   Re-fragmentation (particularly in routers)
    should be avoided at all costs
    – To virtually eliminate re-fragmentation in a
      network, the maximum transmission unit (MTU)
      should be determined and used as the packet size
    – A network’s MTU is the largest size that can be
      used for packets that will not result in any re-
      fragmentation by any routers, or other multi-
      homed nodes
    – Schemes for determining the MTU dynamically
      have been developed, but are beyond the scope
      of this course
IP Datagrams

 IP datagrams are packets sent over IP
  networks using connectionless messaging
 Datagrams can be used directly within
  network-capable programs by sending
  datagrams via UDP (user datagram protocol)
 Datagrams are used transparently by TCP to
  provide connection-based transport
       IP Datagrams
bits   Name       Description                                           Used For
4      Version    Version (equal to 4)                                  All
4      IHL        Header length                                         All
8      TOS        Type of Service (obsolete)                            -
16     Length     Total length of datagram (header included)            All
16     ID         Identifier: used in reassembly to identify packets    Reassembly
1      DF         Should the datagram be re-fragmented, if necessary?   Routers (re-frag)
1      MF         Are there more fragments in the sequence?             Reassembly
13     Offset     Offset of data that this datagram represents          Reassembly
8      TTL        Hop limit                                             Routers
8      Protocol   Transport protocol used for this packet (UDP, TCP)    Acknowledgement
16     Checksum   Checksum of the header                                All
32     SA         Source address                                        All
32     DA         Destination address                                   All
?      Options    Future features                                       -
?      Padding    Fills remaining space                                 -
    IP Datagram Routing

   When an IP-enabled router receives a datagram, it:
    – Receives a datagram through one of its ports
    – Deletes the datagram, if the hop count (TTL in IPv4, Hop limit
      in IPv6) has a non-positive value
    – If the hop count is positive, it is decremented and processing
    – Determine the destination address’ network address
    – Uses the destination network address to find an entry in the
      routing table
    – Uses the routing table entry to determine to which port the
      datagram should be sent
    – Sends the datagram through the correct port
       IPv6 Datagrams

bits   Name         Description                                            Used For
4      Version      Version (equal to 6)                                   All
8      TOS          Type of Service (status info)                          All
20     Flow label   Future features                                        -
16     Length       Length of data in the datagram (header not included)   All
8      Hop limit    Hop limit (decremented to zero)                        Routers
16     SA           Source address                                         All
16     DA           Destination address                                    All
?      H2H          Hop to hop header                                      Routing
?      SRH          Source routing header                                  Routing
?      FH           Fragment header                                        Reassembly
?      E2E          End to end options                                     Reassembly
    Header Checksums
   Networks sometimes result in corrupt data
   Information in the header is equally susceptible to this
   However, header information, when corrupt, can cause
    more serious difficulties
    – For example, the destination address may have a few bits
      changed, or the hop count, etc.
    – Corruption like this, is not always easy to detect and fix
    – Corrupt data (determined by another checksum) can be fixed
      by re-issuing the datagram
    – Header checksums are used to ease identification of header
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