1 00:00:19,720 --> 00:00:27,450 We can also answer this question, How many broadcast domains are there in network 1? 2 00:00:27,570 --> 00:00:35,700 So if I go back all the way to the original ARP message and then click capture forward the ARP message 3 00:00:35,690 --> 00:00:39,060 is sent to the Hub. 4 00:00:39,090 --> 00:00:42,580 Notice it's a broadcast at layer 2. 5 00:00:42,600 --> 00:00:53,730 So what happens to broadcast traffic it gets flooded so we have one broadcast domain because a broadcast 6 00:00:53,790 --> 00:00:58,120 sent to a hub is flooded. 7 00:00:58,910 --> 00:01:03,390 So single broadcast domain in network 1 8 00:01:06,380 --> 00:01:15,140 we can also prove that by rerunning a simulation so let's rerun the simulation. 9 00:01:15,260 --> 00:01:24,620 I'm only going to look at ARP and ICMP traffic but on PC 1 what I'm gonna do now is send a broadcast 10 00:01:24,680 --> 00:01:35,900 to 10 11 255 so this is a broadcast, I'll only send two packets notice the broadcast traffic is sent 11 00:01:35,930 --> 00:01:48,800 to the hub when we look at the packet, source address is PC 1 we can see that again by looking at the 12 00:01:48,860 --> 00:01:50,480 MAC address. 13 00:01:50,480 --> 00:01:56,610 So notice the MAC address is PC 1, destination is a broadcast. 14 00:01:56,960 --> 00:02:05,780 So the destination MAC address is set to that destination IP address is set to a broadcast 255 255 255 15 00:02:06,370 --> 00:02:07,500 in Packet Tracer. 16 00:02:07,700 --> 00:02:14,850 Source IP address is PC 1. Notice the broadcast goes to everyone. 17 00:02:14,850 --> 00:02:23,320 So it's a single broadcast domain these devices will reply back but the traffic is flooded out of all 18 00:02:23,320 --> 00:02:24,370 ports. 19 00:02:24,370 --> 00:02:33,920 Notice we're getting a collision here so I'll reset the simulation and let's look at another problem 20 00:02:35,170 --> 00:02:51,380 if PC1 sends a ping to PC 4 and PC 2 sends a ping to PC 4 what's going to happen? So they're 21 00:02:51,380 --> 00:02:54,530 both sending packets into the network. 22 00:02:57,990 --> 00:02:58,760 In this example 23 00:02:58,760 --> 00:03:12,790 PC 2 send an ARP, because it doesn't know the MAC address of PC 4 so here's the actual frame. 24 00:03:13,030 --> 00:03:22,270 A quick recap of terminology to be precise and to be correct for the CCNA exam at layer 1 on the 25 00:03:22,270 --> 00:03:23,460 OSI model. 26 00:03:23,560 --> 00:03:31,630 We talk about bits at layer 2 in the OSI model we talk about frames at layer 3 we talk 27 00:03:31,630 --> 00:03:39,400 about packets and at layer 4 we talk about segments and then we typically talk about data at 28 00:03:39,400 --> 00:03:41,050 higher layers. 29 00:03:41,050 --> 00:03:48,520 I'm often using terms interchangeably here but if you wanna be very precise about terminology at 30 00:03:48,520 --> 00:03:56,110 layer 1 it's bits, layer 2 its frames, layer 3 it's packets at layer 4 it's segments. 31 00:03:56,110 --> 00:04:00,990 So notice at layer 2 the frame has a destination address of a broadcast. 32 00:04:03,350 --> 00:04:09,980 That's causing problems with the frame that was sent by PC 1. 33 00:04:10,040 --> 00:04:17,660 We've got a collision taking place here so there's a problem with the frames because of the collisions, 34 00:04:18,769 --> 00:04:24,530 only one device can access the network at any time. So here 35 00:04:24,540 --> 00:04:31,260 PC 1 is sending the ICMP message and a reply is sent back to PC 1 36 00:04:34,180 --> 00:04:39,100 so run the simulation again, before I do that 37 00:04:39,100 --> 00:04:49,720 I'm gonna make sure that PC 2 can ping PC 4 make sure that it's ARP cache is populated so 38 00:04:49,720 --> 00:04:50,670 both 39 00:04:50,800 --> 00:05:02,770 PC 2 and PC1 have PC 4s MAC address in the ARP cache and then what I'll do in simulation mode 40 00:05:03,550 --> 00:05:15,130 is get PC 1 to ping PC 4 and get PC 2 to ping PC 4 so they both can send an ICMP packet. 41 00:05:16,750 --> 00:05:20,470 When that hits the hub we have a collision. 42 00:05:20,860 --> 00:05:24,490 You have a single collision domain when you have a hub. 43 00:05:24,910 --> 00:05:31,830 So a hub is a single broadcast domain as well as a single collision domain. 44 00:05:31,870 --> 00:05:38,620 We're gonna have problems with lots of collisions taking place as you add more and more devices to 45 00:05:38,620 --> 00:05:39,790 a hub. 46 00:05:39,790 --> 00:05:42,120 So be careful with hubs. 47 00:05:42,250 --> 00:05:50,320 They are single collision domains and single broadcast domains so we can save for question 11, network 48 00:05:50,320 --> 00:05:56,770 1 equals a single collision domain. 49 00:05:56,770 --> 00:06:06,160 Be careful using hubs today we don't use hubs in wide infrastructures we use switches which we'll see 50 00:06:06,160 --> 00:06:07,030 in a moment. 51 00:06:07,090 --> 00:06:09,430 Have multiple collision domains.