In this video, you will learn to describe the four octet format used by IPv4 IP addresses, describe the difference between class A, B, C, D, and E networks. Let's begin by talking about Internet protocol version 4. IPv4 uses a 32-bit addressing schema that is divided into four octets of eight bits each. Now you should remember from the previous video that eight digits of base two, ones and zeros can have values ranging from 0-255 in decimal notation, which is two raised to the eighth power. Expressed in decimal format, this gives IPv4 addresses arranged from 0.0.0.0 with all bits off to 255.255.255.255 with all bits on. Since the value of any of the four octets could range between 0 and 255, this gives IPv4 a very large number of possible addresses, 4,294,967,296 to be exact. This seems like a very big number, but we're already getting short of IPv4 addresses. If we convert this IP address to binary, the way the computer sees it, this will be its representation. Using the same method we used in the last video, we can convert each octet from decimal to binary. For the decimal number 10, we check the 16 place holder. Sixteen is greater than 10, so we put a zero there. So the next place holder is eight, and we can subtract 8 from 10. So that place holder gets a one and we have two left over. You cannot subtract 4 from 2, so the four place holder gets a zero. You can subtract 2 from 2, so the two place holder gets a one and we have zero left over. With zero left over, we're done. So any remaining place holders get a zero. You can see each octet contains eight bits, which is why it is called an octet. An IP address is divided into a network portion and a host portion, which is something that you can configure on your own computer. But most of the time computers are set up now to allow DHCP or Dynamic Host Configuration Protocol to dynamically configure IP addresses for you. So let's take a look at this in action. By logging into our server, server 100, let's take a look at the interface IP address. My IP address is 192.168.52.3, four octets, and we have this slash 24. This whole number is called the CIDR range. The slash 24 defines how many bits of the IP address are dedicated to the network portion of the address. So each IPv4 address has a network portion and a host portion. The size of the host portion defines how many hosts or endpoints this network segment can hold. In this example, the network portion uses 24 bits of the 32-bit address, which leaves eight bits for the host portion. Two raised to the eighth power is 256. So that's the largest number of hosts that this network segment can support, 0-255. In the early days of IPv4, networks used the classful addressing schema, which allowed for only five different address ranges. Class A goes from 0.0.0.0 to 127.255.255.255. This is for special use and unicast. The default subnet mask is 255.0.0.0, which we will explore more later on. This is the class B, the class C, the class D, and class E, use these address ranges. Class D is reserved for multicast groups. So you will see protocols like that bios using addresses and this range to communicate. Finally class E, which is reserved for research development and future uses. So this is classful addressing. In class A networks, the first octet is used for the network portion and the last three octets are used for the host portion. In class B networks, the first two octets we dedicated to the network and the last two to the host. Class C networks have the first three octets dedicated to the network, and only the last octet is dedicated to the host.