TCOM 509

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TCOM 509 Internet Protocols (TCP/IP)Lecture
06_bSubnetting,Supernetting, CIDRIPv6

• Instructor Dr. Li-Chuan ChenDate 10/06/2003

• Based in part upon slides of Prof. J. Kurose (U
  Mass)

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IP Addresses Review

• IP address is 32-bit in IPv4.
• IP address are designed with two levels of
  hierarchy, network portion and host portion.
• Classful addressing inefficient Class A and
  Class B waste many address spaces for each
  network.
• Solution use another level of hierarchy,
  subnetting. Further divide a network into smaller
  networks called subnets.

Number of Hosts per subnet 2 hostid2
Number of Subnets 2 subnetid
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Subnet Mask

• Class A

11111111 00000000 00000000 00000000
8 1s
Class B
11111111 11111111 00000000 00000000
16 1s
Class C
11111111 11111111 11111111 00000000
24 1s

• Class A subnet mask 255.0.0.0
• Class B subnet mask 255.255.0.0
• Class C subnet mask 255.255.255.0

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Subnet Example

• What is the subnetwork address if the destination
  address is 128.2.4.12 and the subnet mask is
  255.255.240.0?
• Apply the AND operation 10000000 00000010
  00000100 00001100 11111111 11111111
  11110000 00000000--------------------------------
  ------------------------------- 10000000
  00000010 00000000 00000000
• ? 128.2.0.0 subnetwork address

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Subnet Example

• A company has the Class B address 128.3.0.0. The
  company needs 1000 subnets. Design the subnets.
  (Explain in class.)

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Variable Length Subnet Mask

• Suppose a site with class C address and needs to
  have 5 subnets with hosts as follows 40, 40, 30,
  30.
• The site cannot use subnet mask of 26 bits. Why?
• Solution use variable length subnet mask (vlsm).
• First uses the 26-bit subnet mask
  (255.255.255.192) to divide the network into 4
  subnets.
• Then it applies with 27-bit subnet mask
  (255.255.255.224) to one of the subnets to divide
  it further into two smaller subnets.

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Supernetting

• Class A and B addresses are almost depleted.
• Class C is available, but most organization needs
  more than 256 hosts in the network.
• Solution use supernetting.
• Combine several class C networks to create a
  supernetwork (less number of 1s than default
  mask)
• A supernet mask is reverse of a subnet mask.

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Supernet Example

• A company needs to make a supernet out of its 8
  Class C address. What is the supernet mask?
• A supernet has a first address of 208.64.32.0 and
  a supernet mask of 255.255.248.0. How many blocks
  are in this supernet? What is the range of
  addresses? What is the total number of addresses?

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Classless Addressing

• What if a small home business only wants 8
  addresses?
• Solution
• Use classless addressing variable-length blocks
  that belongs to no class.
• The whole address space, 232, is divided into
  blocks of different sizes.
• Rules
• Number of blocks must be power of 2.
• The beginning address must be divisible by the
  number of addresses.

Boundary is flexible
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Classless Example

• Which of the following can be the beginning
  address of a block that contains 16 addresses?
• 208.64.32.32
• 208.16.44.44
• 18.20.40.60
• 128.29.3.72
• Rememer The beginning address must be divisible
  by the number of addresses.

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Classless InterDomain Routing (CIDR)

• CIDR notation
• w.x.y.z/n
• Where n denotes the number of bits that are the
  same in every address in the block.
• Examples
• A site is given a block with the beginning
  address and the prefix length 208.64.32.24/30.
  What is the range of the block?
• Beginning address 11010000 01000000 00100000
  00011000
• Ending address 11010000 01000000 00100000
  00011011
• ? Only 4 address in this block
• What is the network address of 208.64.32.82/27?Th
  e prefix length is 27 (must keep the first 27
  bits the same) and change the remaining bits to
  0s.? The network address is 208.64.32.64/27

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Routing in the Internet

• Autonomous Systems (AS) A collection of hosts
  and routers that are administered by a single
  authority.
• The Global Internet consists of AS interconnected
  with each other
• Stub AS small corporation one connection to
  other ASs
• Multihomed AS large corporation (no transit)
  multiple connections to other ASs
• Transit AS provider, hooking many ASs together
• Two-level routing
• Intra-AS administrator responsible for choice of
  routing algorithm within network
• Inter-AS unique standard for inter-AS routing
  BGP

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Subnetting

• Autonomous Systems (AS) A collection of hosts
  and routers that are administered by a single
  authority.
• The Global Internet consists of AS interconnected
  with each other
• Stub AS small corporation one connection to
  other ASs
• Multihomed AS large corporation (no transit)
  multiple connections to other ASs
• Transit AS provider, hooking many ASs together
• Two-level routing
• Intra-AS administrator responsible for choice of
  routing algorithm within network
• Inter-AS unique standard for inter-AS routing
  BGP

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Why different Intra- and Inter-AS routing ?

• Policy
• Inter-AS admin wants control over how its
  traffic routed, who routes through its net.
• Intra-AS single admin, so no policy decisions
  needed
• Scale
• hierarchical routing saves table size, reduced
  update traffic
• Performance
• Intra-AS can focus on performance
• Inter-AS policy may dominate over performance

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IPv6

• Initial motivation 32-bit address space
  completely allocated by 2008.
• Additional motivation
• Efficiency - header format helps speed
  processing/forwarding
• QoS - header changes
• new anycast address route to best of several
  replicated servers
• IPv6 datagram format
• fixed-length 40 byte header
• no fragmentation allowed

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IPv6 Header (Cont)
Priority identify priority among datagrams in
flow Flow Label identify datagrams in same
flow. (concept of flow
not well defined). Next header identify upper
layer protocol for data
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Other Changes from IPv4

• Checksum removed entirely to reduce processing
  time at each hop
• Options allowed, but outside of header,
  indicated by Next Header field
• ICMPv6 new version of ICMP
• additional message types, e.g. Packet Too Big
• multicast group management functions

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Transition From IPv4 To IPv6

• Not all routers can be upgraded simultaneous
• How will the network operate with mixed IPv4 and
  IPv6 routers?
• Two proposed approaches
• Dual Stack some routers with dual stack (v6, v4)
  can translate between formats
• Tunneling IPv6 carried as payload in IPv4
  datagram among IPv4 routers

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Dual Stack Approach
IPv6
IPv6
IPv6
IPv6
IPv4
IPv4
A-to-B IPv6
B-to-C IPv4
B-to-C IPv6
B-to-C IPv4
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Tunneling
tunnel
Logical view
IPv6
IPv6
IPv6
IPv6
Physical view
IPv6
IPv6
IPv6
IPv6
IPv4
IPv4
A-to-B IPv6
E-to-F IPv6
B-to-C IPv6 inside IPv4
B-to-C IPv6 inside IPv4