Reason for IP addresses
  Q: I'm still not super clear on the need for IP 
  addresses. I think it's for: 1. provide unique address 
  for each machine on the internet; 
  A: Yes
  2.to send packet regardless the physics of the 
  hardware(?); 
  A: Yes
  3.identify the type of network a host or the 
  destination machine is on.
  A: No.
    Role of IP address in routing
  Q: When a packet is sent across networks, is the IP 
  address used to carry the packet all the way to the 
  destination? or does it carry the packet to the correct 
  LAN and the MAC address takes care of the rest?
  A: All the way.  The destination IP address does not 
  change. When it gets to the last router, the router 
  sends the packet to the destination with the 
  destinations MAC address in Ethernet header.
         IP Address Parts
  • Every IP address has two (or 3) parts:
   –Network part: uniquely identifies the network 
    universally.
   –Host part: uniquely identifies the host on the network.
  • Why is this done?
   –Routing is done on the network part of an IP address.
   –For scalability.
  • Part of the host part can be used as a subnet part, 
   within an organization.
      Classful IP Addressing
  • (the old way)
  • Figure 21.1
  • 4 kinds of addresses
  • Didn’t have to specify the mask, because you 
   could figure it out from the address.
  • Exercise: what class does 200.201.202.203 
   belong to?
            Address masks
   Q: Can you explain address masks a bit more?
   A: An address mask indicates with 1 bits the part of an 
   address that is the network part. The 1 bits are all 
   consecutive and at the “left-most” part of the address.  
   • Can be shown as /n  first n bits are 1s  which means 
    the first n bits are the network part. 
   • E.g., Class C address  24-bit network part  
    255.255.255.0  /24
   NOTE: masks only needed in classful addressing to 
   indicate the subnet part.