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Allocation Schemes

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Allocation Schemes CIDR representation and IPv6 allocations. IPv4 Subnet Masking Originally the network size was based on the first few bits (classful addressing ... – PowerPoint PPT presentation

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Title: Allocation Schemes


1
Allocation Schemes
  • CIDR representation and IPv6 allocations.

2
IPv4 Subnet Masking
  • Originally the network size was based on the
    first few bits (classful addressing)
  • Getting rid of address classes was painful!
  • routing protocols, stacks, applications
  • Modern IPv4 allows subnet boundaries anywhere
    within the address (classless addressing)
  • But decimal addresses still make figuring out
    subnets unnecessarily difficult. . .

3
CIDR
  • In IPv4 you would see representations like
  • 129.93.0.0/16
  • 129.93.0.0 255.255.240.0
  • 129.93.0.0/20
  • At the bit level this is
  • 10000001.01011101.1111 0000.00000000

Engineering Workshops
4
Reasons for CIDR
  • To try to preserve the address space.
  • To control the growth of the routing table.

5
IPv6 Notation
  • In IPv6 every address is notated
  • IPv6 address / Prefix Length
  • 20010468/35
  • 20010468/32
  • At the bit level
  • 0010 0000 0000 0001 0000 0100 0110 1000 000
    0/35
  • 0010 0000 0000 0001 0000 0100 0110 1000
    0/32

6
Allocation Strategies Example
  • We wish to allocate /48s out of the /35.
  • Which are available
  • 200104680000 through
  • 200104681fff
  • Recall the the bit structure is
  • 0010 0000 0000 0001 0000 0100 0110 1000 000
    0000000000000
  • 0010 0000 0000 0001 0000 0100 0110 1000 000
    1111111111111
  • So there are 8,192 /48s in a /35

7
Why Allocation ?
  • To try to control the growth of the routing table
    in the default free zone.
  • It is a necessary consequence of using a Provider
    based aggregatable address scheme.
  • It makes the address space more manageable.

8
How would Allocations work?
  • Suppose you wish to give out /40s in the /35.
  • 20010468000 0 0000 or 20010468/40
  • 20010468000 1 1111 or 200104681f00/40
  • Thus there are 32 /40s in the /35 each of which
    has 256 /48s.
  • 5 bits
  • 8 bits

9
How would Allocations work?
  • The same idea holds for /41s or /42s.
  • 20010468000 000000 or 20010468/41
  • 20010468000 111111 or 200104681f80/41
  • 20010468000 0000000 - 000 1111111
  • 20010468/42 200104681fd0/42

10
Mixed Allocations
  • The interesting case is how to handle mixed
    allocations.
  • Some sites need a /40 others a /42. How can you
    handle this case.
  • See
  • draft-ietf-ipv6-ipaddressassign-04
  • A flexible method for managing the assignment of
    bits of an IPv6 address block
  • A perl script is included.

11
Example
  • A provider has been assigned the 3ffe0b00/24
    prefix and wants to assign prefixes to its
    connected networks. Assume 8 bits for NLAs.
    NLA2, will use 10 bits for subNLAs.
  • TLA assigning to NLAs using lefmost bits
  • 10000000 assigned to NLA1
  • 01000000 assigned to NLA2
  • NLA2 assigning to its subNLAs using centermost
    bits
  • 0000010000 assigned to subNLA1
  • 0000100000 assigned to subNLA2
  • subNLAs use centermost bits and site nets
    assigned using rightmost bits.
  • Putting all bits together for subNLA3
  • TLA
    NLA2 subNLA3
  • 0011 1111 1111 1110 0000 1011 0100 0000 0000
    1100 00

  • lt-------gt lt------gt

12
Mixed Allocations
  • Here is the assignment
  • Take 3ffe3700/32. Out of that allocate
  • 34 2
  • 37 3
  • 38 5
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