; CIDR Mask
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  • pg 1
									A supernetwork
Comparison of subnet, default,
    and supernet masks
 We need to make a supernetwork out of 16 class C blocks. What is
 the supernet mask?


We need 16 blocks. For 16 blocks we need to change four 1s to 0s in the default
mask. So the mask is
                  11111111 11111111 11110000 00000000
A supernet has a first address of and a supernet mask of A router receives three packets with the following destination
Which packet belongs to the supernet?

We apply the supernet mask to see if we can find the beginning address. AND               AND AND

Only the first address belongs to this supernet.
 A supernet has a first address of and a supernet mask
 of How many blocks are in this supernet and what
 is the range of addresses?


The supernet has 21 1s. The default mask has 24 1s. Since the
difference is 3, there are 23 or 8 blocks in this supernet. The blocks
are to The first address is The
last address is
                 Classless Addressing

Number of Addresses in a Block
There is only one condition on the number of addresses in a block; it
must be a power of 2 (2, 4, 8, . . .). A household may be given a block
of 2 addresses. A small business may be given 16 addresses. A large
organization may be given 1024 addresses.

Beginning Address
The beginning address must be evenly divisible by the number of addresses.
For example, if a block contains 4 addresses, the beginning address must be
divisible by 4. If the block has less than 256 addresses, we need to check only
the rightmost byte. If it has less than 65,536 addresses, we need to check only
the two rightmost bytes, and so on.
 Which of the following can be the beginning address of a block that
 contains 16 addresses?

The address is eligible because 32 is divisible by 16.
The address is eligible because 80 is divisible by 16.
Which of the following can be the beginning address of a block that
contains 1024 addresses?

To be divisible by 1024, the rightmost byte of an address should be 0
and the second rightmost byte must be divisible by 4. Only the
address meets this condition.
         Mask and slash Notation
• In subnetting, an organization is given a first address + a
  subnet mask
• In supernetting, an organization is given a first address + a
  supernet mask
• Similarly, in classless addressing it is given the first
  address + mask
• X.Y.Z.W/16 means the mask is (class B)
• X.Y.Z.W/17 means the mask is (own class)
• CIDR (Classless Interdomain Routing)
   – Address is classless
   – Routing is interdomain
A small organization is given a block with the beginning address
and the prefix length (in slash notation). What is
the range of the block?


The beginning address is To find the last address we
keep the first 29 bits and change the last 3 bits to 1s.
     Beginning:11001111 00010000 00100101 00011000 (24)
    Ending : 11001111 00010000 00100101 00011111 (33)
              There are only 8 addresses in this block.
        Finding the network address
What is the network address if one of the addresses is

The prefix length is 27, which means that we must keep the first 27
bits as is and change the remaining bits (5) to 0s. The 5 bits affect
only the last byte. The last byte is 01010010. Changing the last 5 bits
to 0s, we get 01000000 or 64. The network address is
An organization is granted the block The
organization needs to have four subnets. What are the subnet
addresses and the range of addresses for each subnet?

The suffix length is 6. This means the total number of addresses in
the block is 64 (26). If we create four subnets, each subnet will have
16 addresses.
Let us first find the subnet prefix (subnet mask). We need four subnets,
   which means we need to add two more 1s to the site prefix. The subnet
   prefix is then /28.
Subnet 1: to
Subnet 2 : to
Subnet 3: to

Subnet 4: to
An ISP is granted a block of addresses starting with The ISP
needs to distribute these addresses to three groups of customers as follows:

1. The first group has 64 customers; each needs 256 addresses.
2. The second group has 128 customers; each needs 128 addresses.
3. The third group has 128 customers; each needs 64 addresses.

Design the subblocks and give the slash notation for each subblock. Find out
how many addresses are still available after these allocations.
Configuration for routing example
Mask                      Dest.                      Next Hop                   I.                             --                     m0                     -                     m2                     -                     m1
-------------------------------------------------------------------------------------                      m0
-------------------------------------------------------------------------------------                         m0                        m0
-------------------------------------------------------------------------------------                                  m0
     Data structures and algorithms For
              Classless lookup
• One major consideration is speed
   – Speed of finding next hop
   – Speed of making changes to values in the table
• CIDR address is not self-identifying
       • Router cannot determine the subdivision between prefix and suffix by
         looking at the address
• In Classfull addressing Hashing is used to optimize speed
  lookup, ( router extracts network portion and uses it as a
  hash key), if same hash function
       • Host key precedes
       • Subnet key precedes
       • Network key
• Hashing does not work well in CIDR
                 Classless lookup
• Searching by mask length
   – Iterate over all possible divisions between prefix and suffix
   – Given a destination address D:
       • Algorithm tries using 32 bits of D, then 31 and so on …
       • For each possible size M, the router extracts M bits from D, assumes
         it contains a network prefix and looks up the prefix in the table

• Worst case is 32 searches when no routes exists
• To find a traditional class B address it needs 16 searches
• And the default route for 31 searches
        Classless lookup (cont.)
• Searching using a binary trie structure
   – To avoid inefficient searches, classless routing tables are usually
     stored in a hierarchical data structure

   – A binary trie is a tree with paths determined by the data stored

           32 bit address                         Unique Prefix
           00110101 00000000 00000000 00000000                001
           01000110 00000000 00000000 00000000               0100
           01010110 00000000 00000000 00000000               0101
           01100001 00000000 00000000 00000000                011
           10101010 11110000 00000000 00000000               1010
           10110000 00000010 00000000 00000000              10110
           10111011 00001010 00000000 00000000              10111
                Classless lookup (cont.)
• Searching using a binary trie structure
       – A search for address 10010010 11110000 00000000 00000001 fails because no
         entry in the trie with 100
       – Each exterior node in must contain a 32 bit address A and a mask M.
       – If search reaches an exterior node then conventional look up algorithm is used

                     0           1

   0                                 0
                                                             What if there is more than
                                                             one prefix ???

                 0                                           In practice, routing table could
                                                             Have a network-specific route
                         1                                   And a particular subnet of the
                                                             Same network ????
                                                 0       1

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