Network Address Translation (NAT)

Transcription

1 Network Aress Translation (NAT)

2 NAT Motivation IPv4 was running out of aresses ISPs only want to limit a customer to one IP aress at any time, but customers want multiple evices to be online Engineers invente Network Aress Translation (NAT) asa way to solve both problems Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

3 NAT Operation Conceptually, NAT evice is locate between computers at a site an the rest of the Internet Site Only nees one globally-vali IP aress Can have multiple local hosts using the Internet Local host has full Internet access Service is transparent No change in protocols on local hosts No change in protocols on Internet servers Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

4 Conceptual Organization Of NAT Internet NAT Device Internet site with multiple computers from the Internet, site appears to be a single host NAT is sai to be in-line From the Internet, site appears to be a single computer From within the site, each computer appears to have an inepenent connection to the Internet Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

5 Aresses Use by NAT NAT evice runs a DHCP server to han out IP aresses to computers at the site Aresses assigne are IPv6 link-local or IPv4 private Block Description /8 Class A private aress block /16 Class B private aress block /12 16 contiguous Class B blocks / contiguous Class C blocks NAT translates source an/or estination aresses in atagrams that pass between the site an the Internet Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

6 NAT Variants Basic NAT Only translates IP aresses Selom use in practice NAPT Translates IP aress an transport-layer port numbers Most wiely-use type of NAT Twice NAT Works with DNS server Provies NAPT plus ability to accept incoming communication Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

7 Example Of Basic NAT Suppose NAT box has globally-vali IP aress of Computer at a site has private aress Computer contacts Internet site Resulting translation is: to the Internet SRC = DST = NAT SRC = DST = SRC = SRC = DST = vali aress DST = host at site with private aress Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

8 Implementation Of NAT NAT evice keeps an internal translation table Table stores translations for both outgoing an incoming atagrams Values fille in automatically when computer at site first sens atagram to the Internet Translation table for previous example Direction Fiel Ol Value New Value IP Source out IP Destination no change -- IP Source no change -- in IP Destination Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

9 Transport-Layer NAT (NAPT) Hanles TCP, UDP, an ICMP Translates TCP/ UDP protocol port numbers as well as IP aresses Permits multiple computers at a site to contact the same Internet service simultaneously without interference Examples: Two computers at a site ownloa songs from itunes Three computers at a site contact Google simultaneously Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

10 Example Of NAPT Translation Suppose Computers at site have private aresses assigne from private aress block / 16 Two computers at the site each contact TCP port on computer NAPT chooses a new port number for each an translates Dir. Fiels Ol Value New Value out IP SRC:TCP SRC : :40001 out IP SRC:TCP SRC : :40002 in IP DEST:TCP DEST : :30000 in IP DEST:TCP DEST : :30000 Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.

11 NAT In Practice Many consumer proucts have NAT built in Examples: Cable an DSL moems Wireless routers Note that most wireless routers provie both wire an wireless network connections; they provie NAT on all connections Internet connection wireless router DSL or cable moem Computer Networks an Internets -- Moule Spring, 2014 Copyright All rights reserve.