NATs Network Address Translators

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NATs (Network Address Translators)

• Henning Schulzrinne
• Dept. of Computer Science
• Columbia University
• Fall 2003

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NATs

• Network address translation local, LAN-specific
  address space translated to small number of
  globally routable IP addresses
• Motivation
• scarce address space
• cost about 9k/year for up to 262,000 addresses
• prevent home broadband users from running servers
  at home
• security prevent unsolicited inbound requests
• avoid renumbering if provider changes
• most small/mid-sized LANs inherit address space
  from ISP

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Prevalence of NATs

• Claim 50 of broadband users are behind NATs
• All Linksys/D-Link/Netgear home routers are NATs
• Measurement for Quake III users, about 17-25
  using NAT (May/June 2001)

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NAT details

• RFC 1631 (first description)
• RFC 1918 (private-use addresses)
• RFC 2663
• RFC 2776
• RFC 3022
• RFC 3027
• RFC 3235
• RFC 3424
• RFC 3489 (STUN)

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NAT types

• All use net-10/8 (10...) or 192.168/16
  (172.16/12 also available)
• Address translation
• Address-and-port translation (NAPT)
• most common form today, still called NAT
• one external (global) IP address