1
00:00:00,240 --> 00:00:06,330
So in this example, are these devices in the same subnet or in different subnets?

2
00:00:06,510 --> 00:00:15,000
The PC on the left has an IP address of 10.1 1.1 with a subnet mask of 255.255.0.0.

3
00:00:15,180 --> 00:00:23,490
The device on the right has an IP address of 10.1 or 2.1 with a mosque of 255.255.020.

4
00:00:23,700 --> 00:00:29,910
So the device on the left does the following when it wants to communicate with the device on the right.

5
00:00:29,940 --> 00:00:33,500
It does a logical and on the network portion of the address.

6
00:00:33,510 --> 00:00:38,910
So at first determines which portion of its address is the network portion.

7
00:00:38,910 --> 00:00:44,910
And then it compares that to the network portion of the device that it's wanting to communicate with.

8
00:00:45,060 --> 00:00:48,990
So in this example, the network portion is 10.1.

9
00:00:49,290 --> 00:00:55,380
So in other words, the first two octets of the address on network, the device checks the first two

10
00:00:55,380 --> 00:01:01,130
octets of the other device to see if it's the same as its local network portion.

11
00:01:01,140 --> 00:01:03,390
And in this example, they are the same.

12
00:01:03,390 --> 00:01:10,110
So the device on the left will send traffic to the device on the right directly, and it will not try

13
00:01:10,110 --> 00:01:12,870
and send the traffic to its default gateway.

14
00:01:13,080 --> 00:01:20,910
What it will do is send an OPP message onto the local segment requesting the MAC address associated

15
00:01:20,910 --> 00:01:23,950
with IP address 10.1 to 2.1.

16
00:01:23,970 --> 00:01:32,310
It will try and communicate with 10.1 or 2.1 on the local segment directly and not send the traffic

17
00:01:32,310 --> 00:01:33,810
to a default gateway.

18
00:01:33,810 --> 00:01:39,420
And the reason for that is that the network portion of the addresses are the same.

19
00:01:39,420 --> 00:01:46,090
So the local device knows that the other device is on the same local segment as itself.

20
00:01:46,110 --> 00:01:50,070
In this example, the network mosque has changed.

21
00:01:50,070 --> 00:01:54,600
It's 255.255.255.0.

22
00:01:54,780 --> 00:02:01,650
So the device on the left will do a logical and and check whether the network portion of the device

23
00:02:01,650 --> 00:02:06,810
that it's wanting to communicate with is the same as its network portion.

24
00:02:07,050 --> 00:02:14,460
So based on the subnet mask, the local devices network portion is 10.11.

25
00:02:14,460 --> 00:02:21,210
The device it's wanting to communicate with has a network portion of ten .1.2.

26
00:02:21,210 --> 00:02:24,300
So in this case, the network portion is different.

27
00:02:24,390 --> 00:02:31,500
So the local host knows that the device that it's wanting to communicate with is on a different subnet

28
00:02:31,500 --> 00:02:32,580
to itself.

29
00:02:32,730 --> 00:02:39,060
So because these two devices are on different subnets, the local device will send its traffic to its

30
00:02:39,060 --> 00:02:40,380
configured default.

31
00:02:40,380 --> 00:02:47,310
Gateway PCs will send traffic to their default gateways when a default gateway is configured.

32
00:02:47,340 --> 00:02:53,550
In this example, let's assume that a default gateway is configured, and that's normally what happens

33
00:02:53,550 --> 00:02:55,560
in a real world implementation.

34
00:02:55,560 --> 00:03:01,890
So the PC is trying to talk to a device in a different subnet so it'll send its traffic to its default

35
00:03:01,890 --> 00:03:02,610
gateway.

36
00:03:02,820 --> 00:03:09,810
So in summary, the subnet mask allows the local device to determine whether the device that it's trying

37
00:03:09,810 --> 00:03:16,590
to communicate with is on the same subnet as itself or if it's on a different subnet.

38
00:03:16,920 --> 00:03:23,970
Now, Cisco and most network vendors do not support just contiguous subnet masks.

39
00:03:24,030 --> 00:03:29,250
A discontinuous subnet mask would look something like the following notice.

40
00:03:29,250 --> 00:03:38,040
In the binary we have binary ones, then binary zeros, then binary ones, binary zeros, binary ones,

41
00:03:38,040 --> 00:03:40,500
binary zeros and so forth and so on.

42
00:03:40,590 --> 00:03:48,770
This type of discontinuous subnet mask is not supported, only contiguous subnet masks are supported.

43
00:03:48,780 --> 00:03:57,480
In this example, we have contiguous or continuous ones in the binary and then contiguous or continuous

44
00:03:57,480 --> 00:03:59,400
zeros in the binary.

45
00:03:59,640 --> 00:04:06,030
Converting that to decimal gives us a value of 255 to 40.00.

46
00:04:06,060 --> 00:04:14,130
In this example, notice we have contiguous ones in binary and then contiguous zeros in binary, giving

47
00:04:14,130 --> 00:04:20,820
us a result in decimal of 255.255.192.0.

48
00:04:21,240 --> 00:04:28,050
So in a subnet mask we must start with binary ones and they must be contiguous.

49
00:04:28,320 --> 00:04:36,000
So you cannot have binary ones, then binary zeros, then binary ones and so forth and so on.

50
00:04:36,000 --> 00:04:40,740
They must be contiguous ones and then contiguous zeros.

51
00:04:41,010 --> 00:04:47,910
So you can't, for argument's sake, have a subnet mask, something like 0.0.0. 240.

52
00:04:47,910 --> 00:04:49,770
That's not supported.

53
00:04:49,950 --> 00:04:55,650
Subnet masks have to be contiguous ones, followed by contiguous zeros.

54
00:04:55,800 --> 00:04:59,280
This contiguous subnet masks are not supported.

55
00:04:59,280 --> 00:04:59,820
And thank.

56
00:04:59,890 --> 00:05:06,460
Goodness for that because it makes our lives as a network engineers a lot easier to have.

57
00:05:06,460 --> 00:05:08,680
Contiguous subnet masks.