1.) (6 points total) Consider an IP network composed of two networks (13.0.0.0/8
ID: 3640650 • Letter: 1
Question
1.) (6 points total) Consider an IP network composed of two networks (13.0.0.0/8 and 18.0.0.0/8)connected by a router, R, having interfaces 1 and 2. The IP addresses of hosts A, B, C, and D, as well as
of interfaces R1 and R2, are given in the figure below. Let the Ethernet address of host A be called
MAC_A, of R1 be called MAC_R1, and so forth.
A B 13.0.0.1 18.0.0.1 C D
| | +-----+ | |
--------------------------|1 R 2|--------------------------------------
13.0.0.3 13.0.0.4 +-----+ 18.0.0.7 18.0.0.8
explain the process by which host A sends some application layer data
to host D. Again, your explanation can stop at the point when A's data reaches D's Ethernet interface.
Assume for this question that Host A has never sent data before to the router, or to Host D.
Explanation / Answer
I can't really figure out your diagram, but I'm guessing that there are two hosts with two routers between them. What happens is A tries to send a packet to 18.0.0.8, but since it doesn't know where it is, it will want to send it to it's default gateway. In order to do this, it will send out an ARP request, asking for a MAC address. This takes the form of a broadcast, basically yelling "HEY!! WHO HAS MY GATEWAY ADDRESS 13.0.0.4?!" To which R1 responds "That's me, no need to shout, just send your traffic to MAC_R1". Host A then makes a mental note (in an arp address table) that IP address 13.0.0.1 is at MAC_R1. He then sends the packet intended to HostD to R1. It's worth noting at this point that HostA has no need to know the entire path to HostD. It lets R1 worry about that, he's a router, it's his job. R1 will then consult his routing tables, which may have been statically created, or broadcast by his neighbor via routing protocols such as OSPF, IGRP, RIP, or any number of protocols. He sees the entry "Send anything for the 18.0.0.0/24 network out Interface towards R2" So he does so. R2 receives the packet, and sees the destination packet "18.0.0.8" and says "Hey! I have a network connected that uses that address range! It must be over there! Assuming that HostD is a quiet introvert and has never communicated with the Router before, the router will need to yell an ARP request broadcast, as did HostA earlier, "HEY! IS THERE AN 18.0.0.8 IN THE HOUSE? I HAVE SOME MAIL FOR YOU!!" To which HostB responds "Hey, my mac address is MAC_D. You can send it over here, if you want, I guess." Router 2 sends it to the address MAC_D, and everyone is happy.
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