1 00:00:00,330 --> 00:00:06,750 Now that we've discussed address clauses in IP version four, we're going to continue the discussion 2 00:00:06,750 --> 00:00:15,180 looking at special addresses such as the local broadcast address, loopback addresses and other special 3 00:00:15,180 --> 00:00:18,570 addresses that you'll encounter in IP version four. 4 00:00:18,750 --> 00:00:25,920 We'll also look at network mosques and CIDR or classless into domain routing, and we'll see how that 5 00:00:25,920 --> 00:00:31,200 affects the network and host portion of addresses in IP version four. 6 00:00:32,560 --> 00:00:39,460 So now let's look at some of the special addresses that you'll encounter in your networking career. 7 00:00:39,850 --> 00:00:43,420 The first one is directed broadcast address. 8 00:00:43,690 --> 00:00:52,600 A directed broadcast address is used by hosts to send data to all devices on the specific subnet or 9 00:00:52,600 --> 00:00:56,880 specific network in direct broadcast addresses. 10 00:00:56,890 --> 00:01:02,440 The entire host portion of the address is populated with binary ones. 11 00:01:02,440 --> 00:01:17,140 So as an example, if we have a network of 172.310.0, the directed broadcast address is 172. 30 1255.255. 12 00:01:17,500 --> 00:01:20,530 Notice, because this is a class B address. 13 00:01:20,530 --> 00:01:28,150 The first two octets denotes network and the last two octets denotes host portion of the address. 14 00:01:28,390 --> 00:01:32,470 So the host portion is filled with binary ones. 15 00:01:32,560 --> 00:01:41,560 255 in decimal equates to eight binary ones, so the host portion is therefore populated with binary 16 00:01:41,560 --> 00:01:44,530 ones in both the third and fourth octet. 17 00:01:44,740 --> 00:01:50,470 So the address now becomes 172. 31 two 55.255. 18 00:01:50,860 --> 00:01:59,080 Routers can be configured to route directed broadcasts, but by default, directed broadcasts are not 19 00:01:59,080 --> 00:02:06,070 routed from one physical interface to another physical interface or from one VLAN to another VLAN. 20 00:02:06,730 --> 00:02:15,490 There are hacking utilities that you can download and use to launch denial of service attacks or DDoS 21 00:02:15,490 --> 00:02:23,170 attacks by using directed broadcasts and thus for security reasons, it's recommended that the forwarding 22 00:02:23,170 --> 00:02:25,900 of directed broadcasts be disabled. 23 00:02:25,900 --> 00:02:33,400 This is the default on modern versions of the Cisco iOS, so routers and switches will not forward directed 24 00:02:33,400 --> 00:02:38,800 broadcasts from one VLAN to another or route them from one interface to another interface. 25 00:02:39,160 --> 00:02:41,050 So here's a sample network. 26 00:02:41,050 --> 00:02:53,830 Notice this device 172. 30 120.1 is on network one 72.30 1020172 is a class B network. 27 00:02:53,830 --> 00:03:02,260 So the network portion of the address is one 72.31 and the host portion of the address is 0.0. 28 00:03:02,900 --> 00:03:13,310 This device is sending a directed broadcast to 172. 31 to 50 5 to 55 using a hacking tool such as Smurf 29 00:03:13,310 --> 00:03:14,450 as an example. 30 00:03:14,630 --> 00:03:19,670 In other words, it's sending a broadcast to this subnet. 31 00:03:19,670 --> 00:03:22,490 172. 31 .0.0. 32 00:03:23,170 --> 00:03:30,730 Now a router or switch configured to forward directed broadcasts will forward that directed broadcast 33 00:03:30,790 --> 00:03:35,500 to network one 72.31 0.0. 34 00:03:35,860 --> 00:03:45,490 And all devices on that subnet, including this device 172 31 0.1 will receive that broadcast. 35 00:03:45,580 --> 00:03:52,330 So all hosts on that segment will receive the directed broadcast will accept it. 36 00:03:52,330 --> 00:03:57,760 So in other words, the network interface cards will accept the broadcast and forward it to higher lay 37 00:03:57,760 --> 00:03:59,260 protocols for processing. 38 00:03:59,290 --> 00:04:06,220 The CPUs of every device will be interrupted to process the directed broadcast. 39 00:04:06,610 --> 00:04:13,840 Now, normally, attackers would send the directed broadcast from the device that they want to attack. 40 00:04:13,840 --> 00:04:17,740 In other words, they may be using a different IP address. 41 00:04:17,740 --> 00:04:21,279 For example, one 72.16 .0. ten. 42 00:04:21,279 --> 00:04:29,110 But if they wanted to attack this device 172 16 zero one, they would send directed broadcasts to the 43 00:04:29,110 --> 00:04:33,040 subnet 172. 30 1.0.0. 44 00:04:33,040 --> 00:04:41,470 In other words, they would launch lots of traffic with a source IP address of 172 16 0.12. 45 00:04:41,470 --> 00:04:46,030 Destination 172 30 1255 255. 46 00:04:46,030 --> 00:04:55,150 All devices on this subnet would then reply back to the source address one 7216 zero one causing a denial 47 00:04:55,150 --> 00:04:57,940 of service attack on that device. 48 00:04:58,150 --> 00:05:05,350 So a hacker is getting legitimate hosts on the network to cause a denial of service attack on another 49 00:05:05,350 --> 00:05:06,970 host on the network. 50 00:05:07,150 --> 00:05:13,780 Now, once again, directed broadcasts are not permitted by Cisco Devices these days to prevent these 51 00:05:13,780 --> 00:05:17,020 kind of attacks using applications such as Smurf. 52 00:05:17,020 --> 00:05:23,980 Smurf is an example of an application that allows you to launch denial of service attacks using directed 53 00:05:23,980 --> 00:05:25,030 broadcasts. 54 00:05:25,540 --> 00:05:33,310 That's not as common today because routers and switches drop directed broadcast traffic by default.