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 –
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: ( , 3) specifies three network addresses:

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: – (216 class C addresses)
B: – (216 class C addresses)
C: – (216 class C addresses)
D: – (216 class C addresses)

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

11. Effect of Supernetting on Routing (cont)
Assume: customer X leases the addresses ( , 256) from ISP A
ISP A’s routing table:
A route to each of A’s subscribers:
( , 256) goes to X
A route to each other ISP:
( , 216) goes to B
( , 216) goes to C
( , 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:
Mask:
Dotted decimal =
Prefix: (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: /21
Specifies the starting address ( )
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: /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:
Is that /16
Is that /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:
A search for the address:

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