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

The Original Classful Addressing Scheme

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

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

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

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)

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

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

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)

Effect of Supernetting on Routing (cont)

A

DC

B

Z

Y

X

W

V

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

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

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

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)

CIDR Masks

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

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:

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

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

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

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:

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

Binary Trie Structure (cont)

• A search for the address: 10010010 11110000 00000000 00000001

• A search for the address: 10110111 11110000 00000000 00000001

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