1
00:00:00,560 --> 00:00:07,730
‫So a network masks helps you know which portion of the address identifies a network and which portion

2
00:00:07,730 --> 00:00:09,470
‫of the address identifies the node.

3
00:00:11,240 --> 00:00:18,860
‫An IPv4 subnet mask consists of 32 bits, a sequence of one followed by a block of zero.

4
00:00:20,230 --> 00:00:25,270
‫The trailing block of zeros designates that part as being the host identifier.

5
00:00:26,900 --> 00:00:35,060
‫Class A, B and C networks have default masks, also known as natural masks, and these masks are.

6
00:00:36,410 --> 00:00:39,200
‫Two five five zero zero zero for class.

7
00:00:40,190 --> 00:00:50,090
‫Two five five two five five zero zero for Class B and 255.255.255.0 for Class C networks.

8
00:00:51,800 --> 00:00:58,790
‫So an IP address on a class-A network that has not been subnet would have an address mask pair similar

9
00:00:58,790 --> 00:01:09,350
‫to one three one one zero 7.1.2 being the address and two five 5.0.2 0.0 would be this subnet mask,

10
00:01:09,890 --> 00:01:10,190
‫right?

11
00:01:11,410 --> 00:01:17,890
‫Now, in order to see how the mask helps you identify the network and node parts of the address, convert

12
00:01:17,890 --> 00:01:22,450
‫the address and mask into binary numbers as seen in the slide.

13
00:01:25,170 --> 00:01:28,920
‫So once you have the address in a mask represented in binary.

14
00:01:30,000 --> 00:01:34,590
‫Then identification of the network and the host ID is a lot easier.

15
00:01:35,840 --> 00:01:42,110
‫Any address bits which have corresponding mask bits set to one represent the network ID.

16
00:01:43,270 --> 00:01:48,880
‫And he addressed bits that have corresponding mask bits set to zero represent the host ID.

17
00:01:50,490 --> 00:01:56,610
‫So in this example, the first part, one three one one zero seven is the network part of the IP address,

18
00:01:57,180 --> 00:02:00,660
‫where the rest one that two represents the host.

19
00:02:03,140 --> 00:02:08,870
‫All devices on this segment share the same network ID and the same subnet mask.

20
00:02:10,360 --> 00:02:16,120
‫Now you can customize the subnet mask to divide a network into many smaller portions.

21
00:02:17,610 --> 00:02:19,740
‫So here's a little brain Tingler for you.

22
00:02:20,610 --> 00:02:29,730
‫The Network ID may be expressed in classless inter domain routing or sidr notation written as the first

23
00:02:29,730 --> 00:02:37,170
‫address of a network followed by a slash character and ending with a bit length of the prefix.

24
00:02:38,500 --> 00:02:46,270
‫So in our example, one three one one zero seven eight zero zero one six is the prefix of the IPV four

25
00:02:46,270 --> 00:02:53,500
‫network, starting at the given address, having 16 bits allocated for the network prefix and the remaining

26
00:02:53,500 --> 00:02:56,170
‫16 bits reserved for host addressing.