Classful Internet Addresses

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Classful Internet Addresses

• Chapter 4

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Universal Identifiers

• For our TCP/IP internet, we need an addressing
  scheme
• To hide the physical network details
• To give the appearance of one, big network
• Universal Communication Service
• Allows any host to communicate with any other
• Need a globally accepted method of identifying
  attached computers

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• Host identifiers classified as
• Name identify what an object is
• Address identify where it is
• Route Tell how to get there
• Refer to successively lower levels of host
  identifiers
• In general,
• People prefer pronounceable names
• Machines do better with compact representation
• Standardized to compact, binary address

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Original Classful Scheme

• Each host is assigned a 32-bit integer
• Cleverly chosen to make routing efficient
• Encode info about the network of the host
• Prefix identifies a network
• Each address is a (netid, hostid) pair
• Each address is self-identifying
• Quickly extract netid or host id

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What does an address specify?

• Said Address identifies a host
• Like the Hertz commercial (not exactly!)
• Consider a router connecting 2 networks
• IP address has a netid and hostid
• Cant have single IP address (must have 2)
• Multi-homed hosts
• Computer with 2 or more connections
• IP ADDRESSES SPECIFY A
• CONNECTION TO A NETWORK
• (not an individual computer)

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Network Broadcast Addressing

• Encoded addresses make routing efficient
• Also allows address to refer to networks as well
  as hosts
• Hostid 0 refers to the network itself
• Directed broadcast
• Hostid of all 1s (to all hosts of final router)
• Due to error in Unix release, hostid of all 0s
  is used, too
• Has some valid netid
• Powerful
• Single packet from source (traveling the
  internet)
• Delivered to all hosts on the destination network

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• Limited broadcast
• Broadcast aimed at the local network
• Independent of the assigned IP address
• Consists of 32 1s
• Host can use this as part of the startup
• Before it learns its IP address
• Before it learns the IP address for the local
  network
• Once local network IP address is learned,
  directed broadcast should be used

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All-0s Address

• Address of thirty-two 0 bits is reserved
• Used when host needs to communicate
• But does not yet know its IP address
• Temporarily at startup
• Host will have to obtain its IP address
• Send a datagram via limited broadcast
• Use address 0 to identify itself
• Receiver understands host does not yet know its
  IP address
• Uses a special method to send a reply

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Extension to Classful Addressing

• Need unique network prefix for each network
• In 1980s, LANs caught on
• Address space would soon be exhausted
• Developed subnet addressing
• Multiple physical networks share a prefix
• In 1990s, came up with supernetting
• Classless addressing scheme
• Allows prefix/suffix boundary to be arbitrary
• Wait until chapter 9..

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Multicast Addressing

• Unicast packet to single computer
• Broadcast packet to all computers on the network
• Multicast
• Packet delivered to a subset of hosts
• Class D of original scheme is reserved for this
• Wait until chapter 16.

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Addressing Weaknesses

• Addresses refer to network connection
• If host computer moves, need new IP address
• Bad for laptops!
• Chapter 18 covers mobility problems
• Growing pains
• If a Class C network gets more than 255 hosts,
  must change to Class B address
• No way to do gracefully

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• Causes problems in routing
• A host may have two connections
• Path taken by packets to same host depends on the
  address used
• Sometimes confusing to humans
• Packets to same host will behave differently
• Knowing only one IP address for the destination
  may not be enough

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Dotted Quad

• 32-bit IP address
• 10000000 00001010 00000010 00011110
• is written as
• 128.10.2.30
• With classful addressing
• Class Lowest Address Highest Address
• A 1.0.0.0 126.0.0.0
• B 128.1.0.0 191.255.0.0
• C 192.0.1.0 223.255.255.0
• D 224.0.0.0 239.255.255.255
• E 240.0.0.0 255.255.255.254

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• Not all possible addresses are used
• In particular, 127.0.0.0
• In the Class A range
• Reserved for loopback
• TCP/IP testing
• Inter-process communication on the local computer
• When loopback address is the destination
• Protocol software in the computer processes it
• Does not send across the network
• Hosts and routers should not forward

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Summary of Special Addresses
1
2
2
2
3
1 Invalid dest 2 Invalid source
3 Never appear on network
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Internet Addressing Authority

• Network address prefixes must be unique
• Private internet
• Whatever address prefixes you want
• Connecting to global Internet
• Cant duplicate other organizations
• Network portion assigned by central authority

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• Originally done by IANA
• Internet Assigned Number Authority
• Until fall 1998, Jon Postel ran the IANA
• Late 1998 he died
• Set up ICANN to take over the job
• Internet Corporation for Assigned Names Numbers
• Addresses names other constants used in
  protocols
• Usually not interact with central authority
• Contact local ISP for prefix
• Assigns network portion of addresses
• Organizations assign suffixes to hosts as desired

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Reserved Address Prefixes

• IBM has been assigned 9.0.0.0
• ATT has been assigned 12.0.0.0
• Private internets can use these on their own
  networks
• Usually not a good idea to use globally assigned
  prefixes
• Most sites eventually connect to Internet
• Prefixes have been reserved for private use
  (Chap 9)

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What classes are the networks?
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B
A
B
What classes are the networks?
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Endianness (byte ordering)

• Suppose 32-bit hexidecimal value 12345678 is
  stored in byte-addressable memory
• Two ways to store it
• MSB lowest LSB
  lowest
• (big-endian)
  (little-endian)

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• Direct copying of bytes from one machine to
  another may change the data
• Need a standard for byte-ordering
• Must understand addresses, packet lengths, etc.
• TCP/IP defines network byte order
• Host or router converts binary items to the
  standard before sending a packet
• Re-converts at the other end
• User data is exempt applications do as want
• Send MSB first (big-endian)

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Summary

• TCP/IP uses 32-bit IP addresses
• Two parts network prefix host suffix
• Original scheme used classes
• A _____ networks with ______ to _______ hosts
• B _____ networks with ______ to _______hosts
• C______networks with up to ___________hosts
• (not counting reserved prefixes for private nets)

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• IP addresses refer to network connections
• Hosts with multiple connections have multiple IP
  addresses
• Advantage scheme allows addressing
• A specific host
• A network
• All hosts on a network
• Disadvantage
• May need to know multiple addresses for a
  multi-connected machine
• TCP/IP protocols include byte-ordering standard

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