1 / 18

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

luigi
Download Presentation

Mapping Internet to Physical Addresses

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related