Chabot College

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Chabot College

• ELEC 99.08
• Network Address Translation

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Shortcomings of Subnetting

• Waste (up to 50)
• Lots of addresses are unusable
• first last subnet
• first last host in each subnet
• Rigidity
• Subnet structure is inflexible
• same size for every subnet
• subnet with largest no. of hosts determines size
  for all

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Case Study 4CNet IP Address Allocation to
Chabot-Las Positas

• Algorithm hosts/254 class Cs
• Fails to consider network structure.
• Subnets needed for
• Broacast control
• WAN links
• Access policy enforcement

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Case Study 4CNet IP Address Allocation to
Chabot-Las Positas

• Can we afford the waste rigidity of subnetting?
• YES, if we run NAT a large private address
  space.

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Network Address Translation (NAT)

• Private address space inside network
• 4CNet-assigned addresses outside network
• Translation performed by Cisco PIX
  (Private-Internet Exchange)
• (Can also be performed by a router.)

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How NAT works...
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Default Translation

• The PIX assigns a single address to all traffic
  sent to the internet.
• The PIX uses TCP sequence numbers to map the
  returning traffic from established sessions to
  internal host addresses.
• Supports gt 16,000 simultaneous sessions.

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Address Mapping
Internal hosts can be mapped to both internal
(private) and external IP addresses
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What problems does NAT solve?

• Allows a huge address space for your net.
• Allows creation of many subnets with many hosts.
  (e.g. Class B network subnetted 255.255.255.0).
• Address waste doesnt matter.
• Rigidity doesnt matter (subnets can be oversized
  to allow for growth)
• Allows flexible access policies and firewalling.

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What problems does NAT solve?

• and youll never have to renumber your network
  again.

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What tradeoffs result?

• Capital cost of the box.
• Single point of failure in connection to
  internet.
• Dual DNS required But this is also an advantage
  lets you decide what the world gets to learn
  about your internal network.

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Dual DNS