1. NATs (Network Address Translators)
Henning Schulzrinne
Dept. of Computer Science
Columbia University
Fall 2003

2. 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

3. 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)

4. 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)

5. NAT types
All use net-10/8 (10.*.*.*) or /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