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Q and A, Ch. 21

Q and A, Ch. 21. IS333, Spring 2014 Victor Norman. Universal Addressing Scheme. Q: Apart from the fact that arbitrary pairs of application programs can communicate without knowing the type of network hardware, what are the other needs for a universal addressing scheme?

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Q and A, Ch. 21

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  1. Q and A, Ch. 21 IS333, Spring 2014 Victor Norman

  2. Universal Addressing Scheme Q: Apart from the fact that arbitrary pairs of application programs can communicate without knowing the type of network hardware, what are the other needs for a universal addressing scheme? A: That’s about the only thing I can think of…

  3. Dotted-decimal notation Q: I still don't understand dotted decimal. Are we supposed to know how to convert from 32 bit to dotted decimal? If so how do I even begin to do that? A: An IP address is 4 bytes = 32 bits. One byte is a decimal number from 0 to 255. So, you convert each byte of the address to decimal and put a . between each part.

  4. Non-computers have IP addresses? Q: The chapter talks about computers having IP addresses but do other devices like printers have them as well? A: Yes! Anything that needs to talk on the network needs to have an IP address.

  5. No hierarchical structure? Q: Could the Internet have been designed with no hierarchical structure of IP addresses so none would be wasted? A: It could but it wouldn’t scale. Packets are routed based on their network part only. This allows one routing entry for hundreds or thousands of hosts. Without a hierarchy, you’d have to have an entry for every host.

  6. Suffix not needed? Q: If you apply a mask to an IP address, you only see the prefix right? Why? Don't you need to know the suffix as well? A: The mask is applied to the address in a host or router when deciding how to route the packet. It is not applied anywhere else. And, routing is based on the network an address belongs to, so to make this decision, the host/router doesn’t need to know the suffix.

  7. Need for classless addressing Q: Can you explain the need for classless addressing? A: IANA began to see that lots of addresses were being wasted. If your company needed 300 addresses, you couldn’t get a class C address (254 addresses max), so you got a class B, with 65534 addresses, most of which went unused. And, the number of class B networks was going down fast…

  8. Lots of IP addresses available? Q: Aren't there so many IP addresses available, that it would never matter if we were wasteful? A: No! The last set of addresses was given out by the IANA to a regional address registrar last year. We are now out of IP addresses.

  9. More memory for CIDR? Q: Did it use a lot more memory to start storing a 32 bit mask along with every 32 IP address? A: The question is, where is this extra memory required? The IP packet header didn’t change at all. Routing tables didn’t change either – because subnetting was available before CIDR. The only thing that changed is routing protocols, and they take more memory now, and the fact that ISPs can hand out non-/8, /16, and /24 blocks of addresses.

  10. IP Address Reuse Q: Are IP addresses reused, and does a server store what IP addresses are being used and then know when one is no longer being used? A: IP addresses are re-used in a network that assigns them via DHCP. And, they could be reused in the Internet when one company’s address block is not used anymore.

  11. Review of the test

  12. Switching algorithm • in memory have a table that maps between port and mac address. • receive frame on port n, with source mac smac and dest mac dmac. • look up smac in table. Add entry if not there. Update entry if there, and port is not n. • if dmac is bcast (all 1s), send to all ports, except n. • look up dmac in table: if not there, send frame to all ports except n. If there, send frame to only port associated with dmac. • go through all entries in table and remove old ones.

  13. Practice questions • Need a network with 2000 hosts. What class network would you get in the old days when we did classful networking? With classless addressing, your ISP would give you a network with what mask?

  14. Practice questions (2) • For network 200.201.220.16/28: • how many hosts can you have? • what is the broadcast address? • what is highest IP address?

  15. Practice (3) • You get 153.106.96.0/20 from your ISP. • How many bits for host part? • What is the limited broadcast address? • You want 16 subnets, so what is your internal subnet mask? • What is the lowest host IP address for subnet 3 (starting with 0)?

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