Network Address Translation 1 Network Address Translation NAT

Network Address Translation 1

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 2

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 3

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

NAT Example • Site uses private network 10. 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 5

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

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

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

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 9

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 10

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