hmw1-solutions.pdf - ECE470 Homework Assignment#1 Solutions...

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ECE470 Homework Assignment #1 Solutions Problem 1. Forwarding operations in a switched network. Consider the network shown in Figure 1. There are two hosts in the network, labelled H1 and H2. Figure 1: Forwarding operations in a switched network. 1. Supply all needed IP addresses. Explain your methodology for choosing the addresses (e.g., random, divine intervention, VLSM, my mom told me so, etc.). 2. The system boots up. Host H1 sends a 100-byte file to host H2. Explain clearly all the forwarding operations in this system. 3. The system boots up. Host H1 sends a 100-byte file to host H333333. Explain clearly all the forwarding operations in this system. Solution 1. Anand, the TA, told me that his mom told him to “use 2 addresses per black line” in the figure. There are five vertical lines, one horizontal and two diagonal ones, so we need 8 * 2 = 16 IP addresses. Let’s put all of them into a single subnet, hanging out of the router’s FA0/0 interface. Choose a /24 mask and the 10.12.100.0 subnet ID. Table 1 provides a possible address assignment. In this assignment, 10.12.100.1 is the address assigned to the interface fa0/2 of the router. This will act as the gateway (next routing hop) for all the hosts connected in this layer 2 network. Let’s assign 10.12.100.11 and 10.12.100.12 to H1 and H2 respectively. Let’s assume that there is some loop avoiding protocol running which has pruned the CoreB - SW1 and CoreA - SW2 links (i.e., the two cross links). (You can give your own answer for a topology with loops.) Can you find how many gaps in the address space are there? Has this addressing plan left room for growth, in terms of both hosts and redundant links? 2. The answer is based on the IP address allocation shown in Table 1. As the question suggests, this is a scenario after boot, so we can safely assume that the ARP caches in all switches are empty. (What other assumptions could you have made?) Once the host boots up and is ready to send a packet, the first thing it needs is to know DA-MAC, the “MAC address of H2”. It has the IP address of H2 but it needs DA-MAC to populate the DA field in
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