Routing with Software and Hardware Addresses

1
Routing with Software and Hardware Addresses

• How routers use addresses at the Network and Data
  Link layers

2
Sender Puts Addresses in the Network Layer

• The sender's IP address is saved in the Network
  Layer header, along with the IP address for the
  recipient.
• This data stays in the Network Layer header until
  the intended recipient breaks down the header.

3
Sender Puts Addresses in the Data Link Layer

• The Data Link layer uses MAC addresses. If the
  recipient is outside the sender's network, the
  sender sets the Data Link Destination address to
  the MAC address of the router (default gateway)
  on his network, and sends the message as a frame
  to that router.
• If necessary, an ARP signal is sent to determine
  the MAC address of the default gateway router.

4
Router Changes Addresses in the Data Link Layer

• The router on the sender's network gets the
  frame, erases the sender and recipient addresses
  in the Data Link Layer, and picks a route to the
  recipient's network (still named on the header of
  the Network layer, remember?).
• The next router in a logical chain is selected.
• The next router's MAC address is written in the
  Data Link header as the Destination", and the
  current router's MAC address is written to the
  Data Link header as the Source". The frame is
  forwarded to the next router.

5
Routers Pass Frames, Changing Addresses in the
Data Link Layer

• The process in the step above is repeated until a
  router on the intended recipient's network gets
  the frame.

6
Final Router Delivers to the Recipients Data
Link Layer

• When a router on the intended recipient's network
  gets the frame, the final router's MAC address
  and the recipient's MAC address are written to
  the Data Link Layer header, and the frame is
  delivered.
• The frame is unpacked and handed to the IP
  protocol on the recipients Network layer.

7
What is the Source and Destination Address, at
Any Given Point?

• In these problems, the text asks you to determine
  the destination address of a frame, and an IP
  packet leaving a host.
• Remember the destination address of an IP packet
  is the final destination address.
• The destination address of a frame is always the
  MAC address of the next device that takes us
  closer to the final device.
• The text tries to confuse you by throwing
  switches into the mix. Switches are not relevant
  to this kind of problem.