1 00:00:00,710 --> 00:00:03,230 Now what about subnet masks. 2 00:00:03,230 --> 00:00:06,490 I've mentioned subnet masks a few times. 3 00:00:06,650 --> 00:00:08,200 What is the subnet mask. 4 00:00:08,390 --> 00:00:09,590 What does it do. 5 00:00:09,920 --> 00:00:18,140 Well a subnet mask is used to determine which part of an IP address is the network portion and which 6 00:00:18,140 --> 00:00:21,160 part of the address is the host portion. 7 00:00:21,170 --> 00:00:29,240 This allows a PC for example to determine whether a device that it wants to communicate with is a remote 8 00:00:29,420 --> 00:00:32,330 device or a local device. 9 00:00:32,330 --> 00:00:39,590 So here's an example we have a PC with IP address 10 dot one that wanted that one and another PC with 10 00:00:39,590 --> 00:00:46,430 an IP address of 10 to one to two to one when the PC on the left wants to forward traffic to the PC 11 00:00:46,430 --> 00:00:50,170 on the right with the IP address tendered one to two that one. 12 00:00:50,210 --> 00:00:57,570 Does the PC for the traffic onto the local segment or does it forward it to its default gateway. 13 00:00:57,710 --> 00:01:05,060 If these two devices are in the same subnet they can communicate directly without the use of a default 14 00:01:05,060 --> 00:01:06,270 gateway. 15 00:01:06,380 --> 00:01:13,370 But if they are on different subnets the PCs will forward their traffic to their default gateways which 16 00:01:13,370 --> 00:01:15,240 will do the interview LAN routing. 17 00:01:15,290 --> 00:01:21,470 If they're on the local land or Local Area Network as an example all rocked the traffic if the traffic 18 00:01:21,470 --> 00:01:24,350 is forwarded to a traditional router. 19 00:01:24,380 --> 00:01:31,850 So at least three switch may do the routing between two villans way router my router the traffic between 20 00:01:31,850 --> 00:01:32,960 these two devices. 21 00:01:33,050 --> 00:01:39,920 If they are on different subnets but how would you determine if these two devices are on the same subnet 22 00:01:40,250 --> 00:01:43,300 or if they're on different subnets. 23 00:01:43,310 --> 00:01:47,690 Now I'm going to explain this in more detail in the next few minutes. 24 00:01:47,960 --> 00:01:57,100 But as an example 10.0 wondered 1.1 and Tendo wonder 2.1 are in the same subnet if they are using a 25 00:01:57,120 --> 00:01:59,270 slash 16 mosque. 26 00:01:59,300 --> 00:02:07,430 However if they are using a 24 mosque that means that the devices are on different subnets. 27 00:02:07,430 --> 00:02:14,690 So let me explain that in more detail in Dittrich mosque allows a device to determine once again which 28 00:02:14,690 --> 00:02:21,560 portion of the address is the host portion and which portion of the address is the network portion. 29 00:02:21,560 --> 00:02:29,150 This allows a local PC as an example to determine whether the device it wants to communicate with is 30 00:02:29,150 --> 00:02:35,110 on a remote network and is thus reachable via the default gateway. 31 00:02:35,360 --> 00:02:43,100 Or if the device is on the local subnet and therefore does not require the use of a default gateway 32 00:02:43,700 --> 00:02:50,130 say PCa and PC be on the same subnet no default gateway is required. 33 00:02:50,210 --> 00:02:57,410 But if they are on different subnets then a default gateway would typically be required to do the routing 34 00:02:57,410 --> 00:02:59,630 between the two PCs. 35 00:02:59,690 --> 00:03:03,220 So that's essentially what a network mask does. 36 00:03:03,290 --> 00:03:12,230 Now as I've explained Plus a B and C networks have default mosques which are also known as natural mosques 37 00:03:12,860 --> 00:03:21,500 in a class A address the first octet is the network in a class B address the first two octets are network 38 00:03:21,920 --> 00:03:27,830 and with a Class C address the first three octets of the network portion. 39 00:03:27,850 --> 00:03:32,450 I'll explain more complicated subnet masks in the submitting videos. 40 00:03:32,450 --> 00:03:37,050 But let's first start with some simple examples. 41 00:03:37,310 --> 00:03:46,760 In this example we have a class a network that hasn't been submitted across a network the default mosque 42 00:03:46,910 --> 00:03:49,990 is to far 5.0 0.0. 43 00:03:50,030 --> 00:03:57,890 So if we look at a address such as 10 dot one to 1 one and convert that into binary it's going to look 44 00:03:58,100 --> 00:03:59,060 as follows. 45 00:03:59,880 --> 00:04:09,540 Now look at the following mosque 2 4 5 in binary equates to 8 binary ones 0 in decimal equates to 8 46 00:04:09,630 --> 00:04:11,280 binary zeros. 47 00:04:11,370 --> 00:04:20,070 So converting the mosque into binary shows us that the network portion consists of contiguous ones or 48 00:04:20,070 --> 00:04:23,910 continuous ones starting from the left hand side. 49 00:04:24,300 --> 00:04:33,680 A 1 in binary in the network mosque indicates network a zero in binary in the network mosque indicates 50 00:04:33,690 --> 00:04:34,690 host. 51 00:04:34,710 --> 00:04:41,340 So in this example this portion of the address is a network and this portion of the address is host. 52 00:04:41,410 --> 00:04:49,420 Hence this device with IP address tendered one but one that one is on network 10.00 at 0. 53 00:04:49,440 --> 00:04:53,180 This is the network portion and this is the host portion. 54 00:04:53,370 --> 00:05:03,150 This device with IP address 10.0 1.1 one is on network 10 so when working out the network and host portions 55 00:05:03,360 --> 00:05:11,960 of an address follow these two simple rules any address bits that have a corresponding mosque that set 56 00:05:12,030 --> 00:05:20,550 to one in binary represents the network in the address bits that have a corresponding mosque but set 57 00:05:20,670 --> 00:05:23,680 to zero represents the host. 58 00:05:23,820 --> 00:05:29,520 So one in binary means network 0 in binary means host. 59 00:05:29,550 --> 00:05:35,870 So in this example 10 is the network because they are ones in the mosque in binary. 60 00:05:36,030 --> 00:05:38,960 So the network ID is set to 10. 61 00:05:38,970 --> 00:05:43,980 Notice these octets are populated by binary zeros that means host. 62 00:05:44,070 --> 00:05:48,280 So the host ID is equal to 1 dot 1.1. 63 00:05:48,420 --> 00:05:50,900 So in summary the network is 10. 64 00:05:50,970 --> 00:05:55,130 The host portion of the address is 1.1 1. 65 00:05:55,140 --> 00:05:56,490 Here's another example. 66 00:05:56,490 --> 00:06:04,870 Remember any address bits that have a corresponding mosque but set to 1 in binary represents makework 67 00:06:05,310 --> 00:06:13,380 any address bits that have a corresponding mosque but set to 0 in binary represent node ID. 68 00:06:13,380 --> 00:06:15,640 So here we've got a class A address. 69 00:06:15,740 --> 00:06:17,680 One got one that wanted one. 70 00:06:17,880 --> 00:06:19,920 But note the difference. 71 00:06:19,920 --> 00:06:30,810 The network mosque in this case is true 5 5 5.5 5.0 etc. so converting 1.1 at 1.1 to binary gives us 72 00:06:30,810 --> 00:06:32,220 the following. 73 00:06:32,220 --> 00:06:38,080 Taking the network mosque and converting that to binary gives us the following. 74 00:06:38,080 --> 00:06:45,660 Notice 205 equates to 8 binary ones which therefore means that those Portion of The address is a network 75 00:06:45,990 --> 00:06:53,070 so the network ID is one dot one and looking at the remaining part of the address which is populated 76 00:06:53,070 --> 00:07:00,930 with binary zeros in the network mosque means that one door one is the host portion of the address. 77 00:07:00,930 --> 00:07:07,410 In other words the network is one dot 1.0 to zero with a host portion of one. 78 00:07:07,410 --> 00:07:10,830 Got one on that network. 79 00:07:10,830 --> 00:07:14,750 The mosque is 2 5 5 5 0 0. 80 00:07:15,060 --> 00:07:22,140 In this example its easy to see the network portion of the address because we have 2 4 5 2 5 5 in the 81 00:07:22,140 --> 00:07:23,500 network mosque. 82 00:07:23,760 --> 00:07:29,730 Just be aware that things can get a lot more complicated than what we are seeing in these examples. 83 00:07:29,730 --> 00:07:35,910 You'll see that when we get to the submitting videos these two examples are simple because it's easy 84 00:07:35,910 --> 00:07:40,120 to recognize which portion is a network and which portion is host. 85 00:07:40,260 --> 00:07:46,440 In the submitting videos I'm going to show you much more complicated examples and in those examples 86 00:07:46,620 --> 00:07:52,740 it's more difficult to determine which portion is network and which portion is host. 87 00:07:52,740 --> 00:07:59,420 So in summary how does a device know whether another device is local remote to itself. 88 00:08:00,250 --> 00:08:06,430 So the first thing it will do is check the network portion of its local address and then compare that 89 00:08:06,520 --> 00:08:08,710 to the address of the other host. 90 00:08:08,920 --> 00:08:15,550 If the network portion of the address is the same the local device knows that the other device is local 91 00:08:15,550 --> 00:08:16,560 to itself. 92 00:08:16,870 --> 00:08:22,940 If the network portion is not the same the local device knows that the other device is remote.