Lecture15 - Tue sday, May 20 Midte #2 rm S core areposte s...

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1 Tuesday, May 20 Midterm #2 Scores are posted Come to my office if you want to go over the exam Program #1 Grading is in progress Lab #5 is posted Due Friday, May 30 Lab schedule is changed!! Homework #4 is posted Due Thursday, May 22 (BOC) Program #2 is posted Due Thursday, June 5
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2 Today's Topics Network Address Translation (NAT) Internet Control Message Protocol (ICMP) Route computation
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3 Sharing an IP address Home networks, other small LANs Expensive to have unique IP address for each host Want to share internet access through just one IP address Want to maintain security/privacy Install router … but how does it work?
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4 Network Address Translation NAT is an extension of the original IP addressing scheme Motivated by exhaustion of IP address space Allows multiple computers at one site to share a single global IP address Requires a device to perform packet translation In-line configuration All traffic entering or leaving the network must go through the NAT device Should be transparent to all users Virtual private connection
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5 NAT: Network Address Translation local network uses just one IP address as far as outside world is concerned (external address) range of addresses not needed from ISP: just one IP address for all devices can change addresses of devices in local network without notifying outside world can change ISP / external address without changing addresses of devices in local network devices inside local net not explicitly addressable by outside world (a security plus).
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6 NAT: Network Address Translation 10.0.0.1 10.0.0.2 10.0.0.3 10.0.0.4 138.76.29.7 local network (e.g., home network) 10.0.0/24 rest of Internet Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers
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7 Implementation To send datagram out to the internet from a computer in the private network: Computer constructs datagram with source address and destination address, sends to NAT box NAT box translates the source address in the datagram to the site 's IP address NAT keeps source and destination addresses in its translation table Note: checksum must be recalculated and datagram must be reconstructed
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8 Implementation To forward an incoming datagram from the internet to a computer in the private network: Datagrams arrive addressed to the site 's IP address NAT finds source and destination addresses in its translation table NAT changes the destination address in the datagram to the internal address for the target computer NAT reconstructs the datagram (with new checksum, etc.) and forwards it to the computer in the private network
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9 Implementation Software solutions Standard PC with
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Lecture15 - Tue sday, May 20 Midte #2 rm S core areposte s...

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