1. 1 Network Address Translation

2. 2 Network Address Translation (NAT) Extension of original addressing scheme Motivated by exhaustion of IP address space Allows multiple computers to share single address Requires device to perform packet translation Implementations available –Stand-alone hardware device –IP router with NAT functionality embedded

3. 3 NAT Details Site –Obtains single, valid IP address –Assigns a private address to each computer –Uses NAT box to connect to Internet NAT –Translates addresses in IP datagrams

4. 4 Illustration Of NAT Single valid IP address needed Computers at site assigned private, nonroutable addresses

5. 5 NAT Example Site uses private network 10.0.0.0 / 8 internally –First computer assigned 10.0.0.1 –Second computer assigned 10.0.0.2 –And so on... Site obtains valid IP address (e.g., 128.210.24.6). Assume computer 10.0.0.1 sends to 128.211.134.4 –NAT translates IP source address of outgoing datagram –NAT translates destination address of incoming datagram

6. 6 Illustration Of NAT Translation Transparent to each end –Computer at site sends and receives datagrams normally –Computer in Internet receives datagrams from NAT box

7. 7 Implementation Of NAT NAT device stores state information in table Value entered in table when NAT box receives outgoing datagram for new destination

8. 8 Variants of NAT Basic NAT –Changes IP addresses Network Address and Port Translation (NAPT) –Changes IP addresses and protocol port numbers –Most popular form

9. 9 Network Address and Port Translation (NAPT) By far the most popular form of NAT Can change TCP or UDP protocol port numbers as well as IP addresses Allows –Multiple computers at site to communicate with single destination –Multiple users on given computer to communicate with the same destination

10. 10 TCP Splicing Popular use of NAPT Interconnects two independent TCP connections Performs segment rewriting Extremely efficient: avoids overhead of extracting data from one connection and sending to the other Uses extended translation table

11. 11 Example NAPT Translation Table Entry in table records protocol port number as well as IP address Port numbers reassigned to avoid conflicts