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IP Addresses

IP Addresses. Universal address regardless of layer 2 architecture Each address is that of an interface , not necessarily a host A host may have more than one interface and therefore more than one IP address IP address affects the path selected to deliver data to a host. I1. I3. I4. R.

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IP Addresses

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  1. IP Addresses • Universal address regardless of layer 2 architecture • Each address is that of an interface, not necessarily a host • A host may have more than one interface and therefore more than one IP address • IP address affects the path selected to deliver data to a host

  2. I1 I3 I4 R A B I2 I5 Path Based on Address Host A sends data to host B at address I4 Host A sends data to host B at address I5

  3. I1 Host I2 I4 Router I3 I5 I7 I8 Bridge

  4. Datagram Delivery • In the Internet Protocol, the basic unit of data is called a datagram • Each datagram is individually addressed • Decide if destination is on a network to which device (host/router) is attached • If on the same network, deliver datagram directly (direct delivery) • If on other than a directly attached network, send datagram to a predefined router for delivery (indirect delivery)

  5. Datagram Delivery • Given a destination IP address, a datagram eventually needs direct delivery • But datagram must be encapsulated into the layer 2 architecture for delivery (frame) • How to find the layer 2 (MAC) address that is associated with the destination IP address? • Could build a table • Could develop a protocol

  6. Ethernet IB IC IA A B C PA PB PC Address Resolution Protocol(ARP) • At higher level protocol we wish to use addresses IA, IB, IC • At final delivery we need to use PA, PB, PC • We must therefore map IXPX

  7. O Ethernet IB IC IA A B C PA PB PC F • Station O needs to send a datagram to address IX • Station O sends a special frame to broadcast address • Frame contains address IX • Station IX is expected to respond with its Physical address • Station O retains this mapping in its memory (cache) • Station O also includes its own IP and Physical addresses in the frame

  8. Address Resolution Protocol • This is sort of a call for ‘Who on this network has addressIX’ • Response from target station contains its Physical address • Actual protocol is defined in RFC 826 • Frame format also defined • This is a general protocol, not unique to IP

  9. 11111..111 Source T Type = 0806 ARP Frame CRC

  10. 1111….111 Source T Type = 0806 ARP Frame CRC

  11. ARP Frame Hardware type: 1 for Ethernet Protocol type:0800 for Internet Protocol Hardware length:6 for Ethernet Protocol length:4 for IP Operation:1 for ARP Request, 2 for Reply Sender hardware address Sender protocol address Target hardware address:0s on a request Target protocol address

  12. ARP – The Protocol • A broadcast is received • Check the frame type – 0806 = ARP • Check Type and address of sender (Is) Update cache • Check Operation – Is it a Request • Check Target address (It) – Is it me? • Swaps Hardware and Protocol address • Inserts own Hardware address • Set Operation to Reply (2) • Sends response (unicast)

  13. Gratuitous ARP • Station send ARP request for its own IP address • Let other stations know its mapping and any changes in hardware address • Checks to see if any other station has this same IP address

  14. Proxy ARP • Sometimes it is desirable for one device (router) to respond on behalf of other hosts • Often used to respond for devices on the other end of a serial line connected to the router • If a station can respond for another, is this a security concern?

  15. Reverse ARP • Extension of ARP protocol to allow a station to obtain its IP address given its hardware addressed • Originally used with diskless stations • This function has been replaced with more modern protocols • BOOTP • DHCP

  16. Position of ARP and RARPin TCP/IP Stack

  17. Tools - Utilities Packet Capture – Analyzer http://www.ethereal.com Ethereal capture/analyzer Windows version Unix version Ethereal_XTRA on course Web site http://www.netgroup.polito.it/ WinPcap – required for ethereal Analyzer – another capture utility Windump

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