1 00:00:00,000 --> 00:00:05,000 The 1st class is once again Class A when looking at the 1st octet 2 00:00:05,000 --> 00:00:11,000 of an IPv4 Address, in other words if we've got x.x.x.x 3 00:00:11,000 --> 00:00:15,000 we're looking at the 1st octet or 1st 8 bits 4 00:00:15,000 --> 00:00:19,000 the 1st octet always starts with the binary 0 5 00:00:19,000 --> 00:00:24,000 so in the 1st octet or the 1st 8 bits 6 00:00:24,000 --> 00:00:27,000 the 1st bit is set to 0, now if we go through all 7 00:00:27,000 --> 00:00:31,000 the combinations of binary in the 1st octet that means we'll end 8 00:00:31,000 --> 00:00:37,000 up with combinations from 0 in decimal up to 127 in decimal 9 00:00:37,000 --> 00:00:42,000 So the range for Class A Addresses is from 0.0.0.0 10 00:00:42,000 --> 00:00:47,000 up to 127.255.255.255 11 00:00:47,000 --> 00:00:50,000 So for completeness let's briefly look at that 12 00:00:50,000 --> 00:00:54,000 Class A in the 1st octet has binary values 13 00:00:54,000 --> 00:00:56,000 where the 1st bit is set to 0 14 00:00:56,000 --> 00:01:02,000 so the 1st 8 entry would be 00000000 which equates to 0 in decimal 15 00:01:02,000 --> 00:01:08,000 the next binary entry is 00000001 which equates to decimal 1 16 00:01:08,000 --> 00:01:11,000 going through all the combinations 2, 3, 4 and so forth 17 00:01:11,000 --> 00:01:15,000 and so on will end up at 127 18 00:01:15,000 --> 00:01:20,000 So in the 1st octet a Class A Address always starts with 0 19 00:01:20,000 --> 00:01:26,000 for the 1st bit and the combinations are from 0 to 127 in decimal 20 00:01:26,000 --> 00:01:30,000 Now as always in life there are some exceptions 21 00:01:30,000 --> 00:01:34,000 So in Class A there are exception where 127 is reserved for 22 00:01:34,000 --> 00:01:37,000 the loopback, I'll explain the loopback Address 23 00:01:37,000 --> 00:01:42,000 in more detail in a moment, you cannot as an example configure 24 00:01:42,000 --> 00:01:48,000 an IP Address of 127.0.0.1 on a PC 25 00:01:48,000 --> 00:01:51,000 that is not an address that's available for you to configure 26 00:01:51,000 --> 00:01:54,000 manually as a Static Address on a PC 27 00:01:54,000 --> 00:01:59,000 0 is reserved for the default Network so that can't be used 28 00:01:59,000 --> 00:02:02,000 either to configure an IP Address on a PC 29 00:02:02,000 --> 00:02:07,000 So you couldn't give your PC an IP Address of 0.1.1.1 30 00:02:07,000 --> 00:02:09,000 So the actual range for Class A Addresses 31 00:02:09,000 --> 00:02:13,000 is the range 1.0.0.0 going through all the combinations 32 00:02:13,000 --> 00:02:20,000 up to 126.255.255.255 33 00:02:20,000 --> 00:02:25,000 So in the first octet, the values are from 1 to 126 34 00:02:25,000 --> 00:02:27,000 for Class A Addresses 35 00:02:27,000 --> 00:02:31,000 Notice that on my PC as an example 36 00:02:31,000 --> 00:02:35,000 if I go to the local area connection properties and select 37 00:02:35,000 --> 00:02:39,000 IPv4 and then I'll try and configure IP Address of 127 38 00:02:39,000 --> 00:02:44,000 The PC tells me that that's not possible, IP Addresses starting 39 00:02:44,000 --> 00:02:48,000 with 127 are not valid because they're reserved for Loopback 40 00:02:48,000 --> 00:02:52,000 Addresses, we need to specify an IP Address in the range 41 00:02:52,000 --> 00:02:57,000 1 to 223, in the same way if we try and configure an IP Address 42 00:02:57,000 --> 00:03:02,000 of 0, Windows in this example tells us that that IP Address 43 00:03:02,000 --> 00:03:07,000 is not valid and we need to specify a value between 1 and 223 44 00:03:07,000 --> 00:03:14,000 So Class A Addresses 1 to 126 in the 1st octet 45 00:03:14,000 --> 00:03:17,000 So in summary, in a Class A Address 46 00:03:17,000 --> 00:03:23,000 the 1st 8 bits denote Network and the last 24 bits denote host 47 00:03:23,000 --> 00:03:25,000 this is determined by the IANA 48 00:03:25,000 --> 00:03:31,000 So an address like 10.1.1.1 implies that this is a Class A 49 00:03:31,000 --> 00:03:34,000 Address, the 1st 8 bits is Network 50 00:03:34,000 --> 00:03:37,000 and the remaining 24 bits is host 51 00:03:37,000 --> 00:03:39,000 The next Class of Address is Class B 52 00:03:39,000 --> 00:03:43,000 Class B Addresses start with Binary 10 53 00:03:43,000 --> 00:03:49,000 Please note that's not 10 in decimal it's binary 10 54 00:03:49,000 --> 00:03:53,000 Class A Addresses at the 1st bit in the 1st octet 55 00:03:53,000 --> 00:03:58,000 set to 0, on Class B Addresses the 2nd bit position 56 00:03:58,000 --> 00:04:01,000 in the 1st octet is set to 0 57 00:04:01,000 --> 00:04:04,000 If you go through all the combinations, you'll end up with 58 00:04:04,000 --> 00:04:08,000 numbers in the range 128 to 191 59 00:04:08,000 --> 00:04:11,000 So Class B Addresses are therefore in the range 60 00:04:11,000 --> 00:04:15,000 128 to 191 in the 1st octet 61 00:04:15,000 --> 00:04:21,000 So in Class B Addresses the 1st 16 bits denote Network 62 00:04:21,000 --> 00:04:25,000 the 2nd 16 bits denote the host portion 63 00:04:25,000 --> 00:04:30,000 So 1st 2 octets are Network, the last 2 octets are the host 64 00:04:30,000 --> 00:04:32,000 portion of the address 65 00:04:32,000 --> 00:04:37,000 So for example 172.16.1.1 66 00:04:37,000 --> 00:04:42,000 the 172 in the 1st octet tells us that this is a Class B Address 67 00:04:42,000 --> 00:04:46,000 because it's in the range 128 to 191 68 00:04:46,000 --> 00:04:50,000 So 172.16 is the Network portion 69 00:04:50,000 --> 00:04:54,000 1.1 is the host portion of this Address 70 00:04:54,000 --> 00:04:59,000 and we know that because this is the Class B Address