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Classful Internet Addresses

Classful Internet Addresses. Chapter 4. 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

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Classful Internet Addresses

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  1. Classful Internet Addresses Chapter 4

  2. 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

  3. 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

  4. 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

  5. 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 • Can’t 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)

  6. 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 1’s (to all hosts of final router) • Due to error in Unix release, hostid of all 0’s is used, too • Has some valid netid • Powerful • Single packet from source (traveling the internet) • Delivered to all hosts on the destination network

  7. Limited broadcast • Broadcast aimed at the local network • Independent of the assigned IP address • Consists of 32 1’s • 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

  8. 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

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

  10. 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….

  11. Addressing Weaknesses • Addresses refer to network connection • If host computer moves, need new IP address • Bad for laptops! • Chapter 18 covers mobility problems • Chapter 22 covers DHCP • Growing pains • If a Class C network gets more than 255 hosts, must change to Class B address • No way to do gracefully

  12. 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

  13. 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 1.0.0.0 127.0.0.0 B 128.0.0.0 191.255.0.0 C 192.0.0.0 223.255.255.0 D 224.0.0.0 239.255.255.255 E 240.0.0.0 255.255.255.254

  14. 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

  15. Summary of Special Addresses 1 2 2 2 3 1 – Invalid dest 2 – Invalid source 3 – Never appear on network

  16. Internet Addressing Authority • Network address prefixes must be unique • Private internet: • Whatever address prefixes you want • Connecting to global Internet: • Can’t duplicate other organizations • Network portion assigned by central authority

  17. 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

  18. Reserved Address Prefixes • IBM has been assigned 9.0.0.0 • AT&T 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)

  19. What classes are the networks?

  20. B A B What classes are the networks?

  21. Error in text: should be: 128.10.2.6 ISP9.0.0.0 Error in text: should be: 128.210.0.50

  22. 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)

  23. 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)

  24. Summary • TCP/IP uses 32-bit IP addresses • Two parts: network prefix; host suffix • Original scheme used classes • A: _____ networks with up to _______ hosts/net • B: _____ networks with up to _______hosts/net • C:______networks with up to _______hosts/net (not counting reserved prefixes for private nets)

  25. 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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