Addressing, ARP

32-bit IP addresses are network-layer addresses for interfaces. They are used for layer 3 (network layer) forwarding.

• Can change (e.g. can be assigned a different address via DHCP from subnet machine is in)

MAC (or LAN or physical or Ethernet) addresses are the physical address of a link.

• Completely flat (does not change)
• Function is for “locally” getting a frame from one interface to another physically-connected interface (i.e. same network in the IP-addressing sense)
• 48 bit MAC addresses (for most LANs)
• Addresses are burned into NIC ROM
  • Sometimes software settable
• e.g. 1A-2F-BB-76-09-AD
  • Hex notation
• Each adapter on LAN has a unique LAN address

• MAC address allocation is administered by IEEE
  • Manufacturers buy a portion of the MAC address space to ensure uniqueness
• Compared to IP hiearchical addresses, MAC flat address is portable
  • Can move LAN card from one LAN to another
  • IP address depends on IP subnet to which a node is attached

Given the IP address of an interface, we must determine its MAC address.

• Done via ARP (address resolution) table
  • Each IP node (host, router) on LAN has table in memory
  • Entries have entries <IP address; MAC address; TTL>
    • TTL (time to live) is time after which address will be forgotten (typically 20 minutes)
• The given IP address is checked against network mask ⇒ if not under same subnet, need to route to first hop router
• e.g. suppose A wants to send a datagram to B (same LAN), and B’s MAC address is not in A’s ARP table
  • A broadcasts an ARP query packet (broadcast frame) containing B’s IP address
    • Destination MAC address = FF-FF-FF-FF-FF-FF
    • All nodes on LAN receive ARP query
  • B receives ARP packet and replies to A with its MAC address (i.e. MAC address for B)
    • Frame is directly sent to A’s MAC address (unicast)
    • i.e. link with IP address matching broadcasted packet responds
  • A will cache the IP-MAC address pair in its ARP table until information becomes stale (timeout)
    • Soft state ⇒ information times out unless refreshed
• e.g. suppose A wants to send a datagram to B (different LAN), B’s MAC address is not in A’s ARP table

  • A is configured with information of first-hop router from DHCP

  • A creates:

    • IP datagram with IP source A, destination B

    • Link-layer frame with R’s MAC address as destination (contains A-B IP datagram from above)

      

  • Frame is sent from A to R

  • Datagram is removed when frame is receieved, passed up to IP

  • R processes datagram for B’s network

    • R forwards datagram with IP source A, destination B:
    • R creates link-layer frame with B’s MAC address as destination (contains A-B IP datagram)