1
00:00:00,280 --> 00:00:07,600
So a network masks helps you know which portion of the address identifies a network and which portion

2
00:00:07,600 --> 00:00:09,580
of the address identifies the node.

3
00:00:11,050 --> 00:00:18,970
An IPV four subnet mask consists of 32 bits, a sequence of one followed by a block of zero.

4
00:00:19,960 --> 00:00:25,390
The trailing block of zeros designates that part as being the host identifier.

5
00:00:26,690 --> 00:00:33,440
Class A, B and C networks have default masks, also known as natural masks.

6
00:00:33,440 --> 00:00:35,180
And these masks are.

7
00:00:36,110 --> 00:00:39,290
255.0.0.0 for class A.

8
00:00:39,890 --> 00:00:44,900
255.255.0.0 for class B and.

9
00:00:45,080 --> 00:00:50,240
255.255.255.0 for class C networks.

10
00:00:51,530 --> 00:00:55,670
So an IP address for class B network that is not been subnet.

11
00:00:56,240 --> 00:01:08,750
Would have an address mask pair similar to 131.107.1.2 being the address and 255.255.0.0 would be the

12
00:01:08,750 --> 00:01:10,100
subnet mask.

13
00:01:10,380 --> 00:01:10,930
Right.

14
00:01:11,720 --> 00:01:18,440
Now in order to see how the mask helps you identify the network and node parts of the address, convert

15
00:01:18,440 --> 00:01:23,150
the address and mask into binary numbers as seen on the slide.

16
00:01:25,470 --> 00:01:29,640
So once you have the address and the mask represented in binary.

17
00:01:30,310 --> 00:01:35,320
Then identification of the network and the host ID is a lot easier.

18
00:01:36,250 --> 00:01:42,760
Any address bits which have corresponding mask bits set to one represent the network ID.

19
00:01:43,680 --> 00:01:44,490
Any address.

20
00:01:44,490 --> 00:01:49,560
Bits that have corresponding mask bits set to zero represent the host ID.

21
00:01:50,800 --> 00:01:59,650
So in this example, the first part 131.107 is the network part of the IP address where the rest 1.2

22
00:01:59,650 --> 00:02:01,390
represents the host.

23
00:02:03,480 --> 00:02:09,570
All devices on this segment share the same network ID and the same subnet mask.

24
00:02:10,770 --> 00:02:16,770
Now you can customize the subnet mask to divide a network into many smaller portions.

25
00:02:17,900 --> 00:02:20,510
So here's a little brain tingler for you.

26
00:02:20,870 --> 00:02:29,840
The network ID may be expressed in classless Inter-domain routing or Cidr notation, written as the

27
00:02:29,840 --> 00:02:37,970
first address of a network followed by a slash character and ending with a bit length of the prefix.

28
00:02:38,850 --> 00:02:49,320
So in our example, 131.107.0.016 is the prefix of the IPV four network starting at the given address,

29
00:02:49,470 --> 00:02:56,880
having 16 bits allocated for the network prefix and the remaining 16 bits reserved for host addressing.