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Mapping Internet to Physical Addresses

Mapping Internet to Physical Addresses. 2 machines on a physical network can only communicate if they know each other’s physical address (PA) How does a router or host map an IP addr. to a PA? 2 machines with IP addresses I A and I B and physical addresses P A and P B

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Mapping Internet to Physical Addresses

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  1. Mapping Internet to Physical Addresses 2 machines on a physical network can only communicate if they know each other’s physical address (PA) How does a router or host map an IP addr. to a PA? 2 machines with IP addresses IA and IB and physical addresses PA and PB Devise a scheme so that high level programs can only work with IP address CS 428 Computer Networks

  2. Address Mapping • Must be done all along path from source to destination • 2 cases • Last step of delivering a packet • Delivery to host on physical network • All other steps • Delivery to router on physical network • Problem known as the ‘address resolution problem’ CS 428 Computer Networks

  3. Physical Addresses • 2 types • Ethernet type - large fixed address • ProNet type - token ring - small easily configurable • Resolution difficult for Ethernet, easy for ProNet • ProNet uses small intergers, allows user to choose the PA when installing the board CS 428 Computer Networks

  4. Physical Addresses • Key - choose IP and PA that have some part of them overlapping • User has choice when configuring card! • Example: 192.5.48.3 and PA 3 • Computing PA from IP becomes easy • PA = f (IA) CS 428 Computer Networks

  5. Dynamic Binding - ARP • No hope of encoding 48 bit addr into 32 bit IP addr • Use the Ethernet broadcast ability to solve the problem • No central DB and new hosts can be dynamically added • Host A wants to resolve IP addr IB CS 428 Computer Networks

  6. ARP • Host A broadcasts a special packet that asks the host with IP addr IB to respond with its PA PB • All hosts receive the packet. Only B recognizes its IP addr • Sends a reply with its PA • Host A uses the received PA to send the packet to host B CS 428 Computer Networks

  7. ARP • See figure 5.1 on page 80 • To reduce communication costs hosts keep ‘ARP caches’ to maintain recently acquired IP to PA binding information • Cache information can become ‘stale’ • Assume hosts A and B. Over time B crashes or leaves • No indication that host B not there • Use timeouts, typically 20 mins. CS 428 Computer Networks

  8. IP Addressing • All hosts on a given physical network share same prefix • network + host • Splitting IP addrs keep routing tables reasonable sizes • Class A - 8 network + 24 host • Class B - 16 network + 16 host • Class C - 24 network + 8 hosts CS 428 Computer Networks

  9. Addressing • Important - Individual sites have the right to modify addresses and routes within their intranet as long as it remains invisible to other sites CS 428 Computer Networks

  10. Addressing • Very important to have multiple physical networks use the same IP network address • To minimize the use of class B addresses we need to use as many class C addresses as possible CS 428 Computer Networks

  11. Transparent Routers • A network using a class A addr can be extended through a simple trick • Arrange a physical network to multiplex several host connections through a single host port • See figure 10.1 on page 149 • LAN does NOT have its own IP prefix CS 428 Computer Networks

  12. Transparent Routers • Hosts attached to it are assigned addresses as if they are directly connected to the WAN • Transparent router de-multiplexes datagrams that arrive from the WAN assigning them to appropriate hosts • Uses a table of addresses CS 428 Computer Networks

  13. Transparent Routers • Divide IP addresses into parts and encode information into unused parts • ARPANET is 10.0.0.0 ==> 10.p.u.i • Network (10) • Port on destination (p) • Destination (i) • (u) is UNUSED !!! CS 428 Computer Networks

  14. Transparent Routers • Transparent router can assign one host 10.1.1.5 and 10.1.2.5 • “Same” IP addr for 2 hosts on 2 separate networks • Advantage - Require fewer network addresses because LANs can share IP prefix • Disadvantage - Only works with class A on ARPANET CS 428 Computer Networks

  15. Proxy ARP • aka promiscuous ARP, the ARP hack • Maps single IP prefix into 2 physical networks • See figure 10.2 on page 150 • Applies only to networks that use ARP to bind internet addresses to PAs CS 428 Computer Networks

  16. Proxy ARP • With 2 networks A and B and 1 router R • R answers ARP requests on each network for hosts on the other network • It gives its PA as the addr matching PB then routes datagrams correctly • “In essence, R lies about IP to physical address binding” CS 428 Computer Networks

  17. Subnet Addresses • Subnetting most common of 3 techniques • Subnetting is a required part of IP addressing • Main router is interface to WAN • Routes datagrams to specific internal physical networks • See figure 10.3 on page 152 CS 428 Computer Networks

  18. Subnets • Example class B addr 128.10.0.0 • Break internal network into several ‘independent’ class C networks • 128.10.1.0 - 128.10.2.0 - 128.10.n.0 • Gateway to WAN upon receiving datagram discerns which local network gets packet CS 428 Computer Networks

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