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Supernetting

Supernetting. Recall: subnetting allows an organization to share a single IP network address among multiple physical networks Supernetting (a.k.a. classless addressing ) allows the addresses assigned to an organization to span multiple IP network addresses.

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Supernetting

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  1. Supernetting • Recall: subnetting allows an organization to share a single IP network address among multiple physical networks • Supernetting (a.k.a. classless addressing) allows the addresses assigned to an organization to span multiple IP network addresses

  2. The Original Classful Addressing Scheme

  3. Classful Addresses • The different classes were different sizes: • Less than 17,000 class B network addresses • More than 2,000,000 class C network addresses • The classes differed in popularity: • Class B addresses were very popular and almost exhausted • Class C addresses were hardly used at all

  4. Supernetting • Assign an organization a block of plentiful addresses (class C) rather than a single scarce (class B) address • Example: • An organization wants to connect to the Internet • The organization would prefer a class B address • Plans to subnet its various physical networks using the third octet of the IP address to represent the subnet • This would allow the organization to have 254 physical networks with up to 254 hosts per network

  5. Supernetting (cont) • Example (cont): • Instead of getting a class B address, the organization is given 256 contiguous class C addresses • E.g. 192.17.0.0 – 192.17.255.0 • Then: • The organization can have up to 256 physical networks (each with its own class C network address) • Each physical network can have up to 254 hosts • Result: a block of plentiful addresses (class C) substituted for a single scarce (class B) address

  6. Extending Supernetting • A few large commercial Internet Service Providers (ISPs) provide Internet connectivity • ISPs are assigned a large chunk of contiguous network addresses • Organizations contract with an ISP and are assigned one or more network address(es)

  7. Effect of Supernetting on Routing • Problem: • Recall: Routers (potentially) have an entry in their routing table for each unique network • Assigning an organization 256 class C addresses might require 256 routing table entries • Assigning an organization 1 class B address would require 1 routing table entry • The information that Internet routers must store and exchange increases dramatically

  8. Effect of Supernetting on Routing (cont) • Solution: Classless Inter-Domain Routing (CIDR) • Collapse a block of contiguous network addresses into a single pair • Example: (192.5.48.0, 3) specifies three network addresses: • 192.5.48.0 • 192.5.49.0 • 192.5.50.0

  9. Effect of Supernetting on Routing (cont) • Assume: a small number of ISPs each with a large block of addresses • Example: four large ISPs: • A: 195.0.0.0 – 195.63.255.0 (216 class C addresses) • B: 195.64.0.0 – 195.127.255.0 (216 class C addresses) • C: 195.128.0.0 – 195.191.255.0 (216 class C addresses) • D: 195.192.0.0 – 195.255.255.0 (216 class C addresses)

  10. A D C B Z Y X W V Effect of Supernetting on Routing (cont)

  11. Effect of Supernetting on Routing (cont) • Assume: customer X leases the addresses (195.17.0.0, 256) from ISP A • ISP A’s routing table: • A route to each of A’s subscribers: • (195.17.0.0, 256) goes to X • A route to each other ISP: • (195.64.0.0, 216) goes to B • (195.128.0.0, 216) goes to C • (195.192.0.0, 216) goes to D • Result: CIDR shortens routing tables

  12. CIDR Address Blocks and Bit Masks • No need to restrict network numbers to class C addresses • No need to use an integer to specify the block size • Instead: two items specify a block of addresses: • The lowest address in the block (32-bit IP address) • A 32-bit mask that divides addresses into a prefix and a suffix • Prefix – common to all addresses in the block • Suffix – differentiates unique address in the block

  13. CIDR Address Blocks and Bit Masks (cont) • Example: a CIDR block of 2048 addresses: • Starting address: 128.211.168.0 • Mask: 11111111 11111111 11111000 00000000 • Dotted decimal = 255.255.248.0 • Prefix: 10000000 11010011 10101 (the first 21 bits) • Suffix: the last 11 bits

  14. CIDR Notation • CIDR Notation (or slash notation) is a shorthand for representing both the starting address and mask • Example: 128.211.168.0/21 • Specifies the starting address (128.211.168.0) • Specifies the number of bits in the prefix (21) • Specifies the suffix (32-21 = last 11 bits)

  15. CIDR Masks • Note: /8, /16, and /24 prefixes correspond to the traditional class A, B, and C divisions

  16. Advantage of Classless Addressing • Flexibility in allocating blocks of various sizes • Assume: an ISP has the following block of addresses: 128.211.0.0/16 • Can assign one customer 2048 addresses in the /21 range: • Can assign another customer 4 addresses in the /29 range:

  17. Classless Addressing • Treats IP addresses as arbitrary integers rather than as part of a predefined class structure • Allows a network administrator to assign addresses in contiguous blocks • Number of addresses in a block must be a power of two • Allows for: • Flexibility in assigning blocks of addresses • Ease of management of addresses

  18. Private Addresses • Some prefixes have been reserved for private networks (i.e. networks not part of the global Internet) • These addresses are called private addresses (or nonroutable addresses) because they should not be used on the Internet

  19. Additional Routing Concerns • The original classful addressing scheme was self-identifying: • A router could determine the network address simply by looking at the address • Classless addresses are not self-identifying: • A router cannot determine the division between the prefix and the suffix from the address • Example: 128.211.176.213 • Is that 128.211/16 • Is that 128.211.176/8 • Is that something else

  20. Additional Routing Concerns (cont) • Classless routing tables a usually stored in a hierarchical data structure called a binary trie • A tree with paths determined by the data stored • A unique prefix identifies each data item • Example:

  21. Binary Trie Structure • Interior nodes (circles) correspond to two or more prefixes • Leaf nodes (squares) correspond to a unique prefix and contain an address and mask

  22. Binary Trie Structure (cont) • A search for the address: 10010010 11110000 00000000 00000001 • A search for the address: 10110111 11110000 00000000 00000001

  23. Summary • Problem: IP v4 addresses (especially class B) would be exhausted • Solutions: • Supernet addressing - a block of plentiful addresses (class C) substituted for a single scarce (class B) address • Classless Inter-Domain Routing - collapse a block of contiguous network addresses into a single pair to keep routing tables short

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