1 00:00:00,730 --> 00:00:07,420 So in summary, when a device wants to communicate with another device in the same subnet, it will 2 00:00:07,420 --> 00:00:15,920 send a broadcast onto the local segment to find the MAC address of the device using the target IP address. 3 00:00:15,940 --> 00:00:19,930 So in this case, A sends the frame to the hub. 4 00:00:20,140 --> 00:00:27,040 The hub, because it's a multiport repeater, will send the frame out of all ports except the port on 5 00:00:27,040 --> 00:00:27,960 which it arrived. 6 00:00:27,970 --> 00:00:32,650 So both the router and host c receive the frame. 7 00:00:33,040 --> 00:00:39,790 Network interface cards will only accept unicast traffic destined to their MAC address, or they'll 8 00:00:39,790 --> 00:00:46,630 accept broadcast traffic as well as accepting multicast traffic for multicast addresses that they've 9 00:00:46,630 --> 00:00:47,830 subscribed to. 10 00:00:47,920 --> 00:00:53,890 The router has a mac address of G on interface fost ethernet zero zero, so the router will receive 11 00:00:53,890 --> 00:00:58,630 the broadcast and forward the request to higher layer protocols. 12 00:00:58,780 --> 00:01:06,250 The router will be able to see at layer three that this is an OP request for IP address ten 112. 13 00:01:06,790 --> 00:01:15,160 But the router in this example is configured with IP addresses ten one one 110 one two 100 So the router 14 00:01:15,160 --> 00:01:20,770 will drop the frame because the OP request is not for one of its IP addresses. 15 00:01:21,040 --> 00:01:27,820 Routers do not forward broadcasts, so this broadcast is not forwarded out of interface fast ethernet 16 00:01:27,890 --> 00:01:28,930 zero one. 17 00:01:28,930 --> 00:01:31,900 So the broadcast received by the router is dropped. 18 00:01:32,170 --> 00:01:38,920 The network interface card on PCC will receive the broadcast and see that this is an OP request for 19 00:01:38,920 --> 00:01:40,120 its IP address. 20 00:01:40,150 --> 00:01:43,170 So it will then reply with an OP reply. 21 00:01:43,180 --> 00:01:51,130 As we've seen in the Wireshark capture, PCC will update its OP cache to show that IP address ten 111 22 00:01:51,130 --> 00:01:57,070 is associated with MAC Address A and it will then send the frame to the hub. 23 00:01:57,310 --> 00:02:01,780 The hub will fold the frame out of all ports because it's a multiport repeater. 24 00:02:02,080 --> 00:02:07,750 The route in this example will receive a frame from the hub, but because the destination Mac address 25 00:02:07,750 --> 00:02:14,050 is a and it's not the Mac address of the router, which is G, the router will drop the frame. 26 00:02:14,320 --> 00:02:16,900 A will also receive a copy of the frame. 27 00:02:17,290 --> 00:02:23,050 When it receives the frame, it will accept it because the destination address is A and its Mac address 28 00:02:23,050 --> 00:02:23,740 is A. 29 00:02:23,770 --> 00:02:31,450 It will then update its OPP cache with an OP entry stating that IP address ten 112 is associated with 30 00:02:31,450 --> 00:02:32,530 MAC address C. 31 00:02:32,800 --> 00:02:38,050 So the OP reply allows A to update its OP cache. 32 00:02:38,470 --> 00:02:41,860 At this point, no user traffic has been transmitted. 33 00:02:41,890 --> 00:02:49,000 What's happened here is that the devices have simply worked out which MAC addresses are associated with 34 00:02:49,000 --> 00:02:50,380 which IP addresses. 35 00:02:50,440 --> 00:02:57,550 So A now knows that MAC address C is associated with IP address ten 112. 36 00:02:58,440 --> 00:03:06,120 The ping traffic can now be transmitted with a source MAC address of a which is the local machine destination 37 00:03:06,120 --> 00:03:07,330 MAC address of C. 38 00:03:07,350 --> 00:03:12,720 In other words, PCC, the destination MAC address was learned through OP. 39 00:03:13,020 --> 00:03:15,800 Source IP address is 10.1 1.1. 40 00:03:15,810 --> 00:03:18,900 Destination IP address is ten .1.122. 41 00:03:19,140 --> 00:03:25,650 When the hub receives the frame from PCA, it will repeat it out of all interfaces except the interface 42 00:03:25,650 --> 00:03:26,700 it arrived on. 43 00:03:26,700 --> 00:03:33,060 So the router will once again receive the frame but will drop it because its Mac address is G and the 44 00:03:33,060 --> 00:03:36,180 destination Mac address for this frame is C. 45 00:03:36,570 --> 00:03:43,410 PCC will also receive the traffic and will accept it because the destination Mac address is C and its 46 00:03:43,410 --> 00:03:44,910 local Mac address is C. 47 00:03:45,150 --> 00:03:50,610 The layer two headers will be stripped and the IP address information will be read by higher layer protocols. 48 00:03:50,820 --> 00:03:57,900 This is an ICMP echo packet, so the PC will reply with an echo reply message. 49 00:03:58,320 --> 00:04:05,400 C will send the frame to the hub with a source mac address of C Destination Mac address of A it knows 50 00:04:05,400 --> 00:04:08,790 the Mac address of A because of the previous OPP request message. 51 00:04:08,790 --> 00:04:15,540 So its OPP cache has a entry associating MAC address a with IP address ten 111. 52 00:04:15,870 --> 00:04:21,269 So source mac address in the frame is C destination mac address is a source IP address is ten one one 53 00:04:21,269 --> 00:04:24,390 to destination IP address is ten 111. 54 00:04:24,840 --> 00:04:30,240 When the frame is received by the hub, the hub will repeat it out of all ports except the one on which 55 00:04:30,270 --> 00:04:31,350 it was received. 56 00:04:31,620 --> 00:04:38,310 The router will receive the frame but will drop it because the destination mac address is a not to g 57 00:04:38,340 --> 00:04:39,990 the local Router's Mac address. 58 00:04:40,260 --> 00:04:46,530 When a receives the frame from the hub, it will accept it because the destination Mac address is a 59 00:04:46,530 --> 00:04:48,330 and the PC Mac address is a. 60 00:04:48,660 --> 00:04:54,030 It will then strip layer two headers and forward the information to higher layer protocols. 61 00:04:54,210 --> 00:04:59,280 In this case it was an echo reply, so the ping will show a success message. 62 00:04:59,310 --> 00:05:06,180 In other words, an echo request was sent to PCC and an echo reply message was successfully received 63 00:05:06,180 --> 00:05:06,840 back. 64 00:05:07,350 --> 00:05:13,470 So in my Wireshark capture, for example, I can filter on ICMP messages. 65 00:05:13,470 --> 00:05:22,080 I can see the initial echo request message sent from my PC to ten 00254, and I can then see the echo 66 00:05:22,080 --> 00:05:23,220 reply message. 67 00:05:23,610 --> 00:05:27,360 Notice, please, that these are unicast frames. 68 00:05:27,360 --> 00:05:35,370 Unicast firstly from my PC to the local router and then a unicast from the router to my local machine. 69 00:05:35,820 --> 00:05:41,310 The same information would be displayed if you were pinging another local device on the segment. 70 00:05:41,490 --> 00:05:49,740 As a user, you would see something similar to this ping ten 00254 and the ping was successful. 71 00:05:49,740 --> 00:05:57,030 So in this example, my PC successfully got a reply from the remote device that I was pinging.