What do the octets mean in an ip address

The act of creating large numbers from groups of binary units or bits is called binary arithmetic. Learning binary arithmetic helps you understand how your computer sees IPs or any numbers greater than one. In binary arithmetic, each bit within a group represents a power of two.

Specifically, the first bit in a group represents 2 0 [Editor's note for non-math majors: mathematicians stipulate that any number raised to the power of zero equals 1], the second bit represents 2 1 , the third bit represents 2 2 , and so on. It's easy to understand binary because each successive bit in a group is exactly twice the value of the previous bit. The following table represents the value for each bit in a byte remember, a byte is 8 bits. In binary math, the values for the bits ascend from right to left, just as in the decimal system you're accustomed to:.

Now that we know how to calculate the value for each bit in a byte, creating large numbers in binary is simply a matter of turning on certain bits and then adding together the values of those bits. So what does an 8-bit binary number like represent? The following table dissects this number. Remember, a computer uses 1 to signify "on" and 0 to signify "off":.

In the table above, you can see that the bits with the values 64, 32, 8, 4 and 2 are all turned on. As mentioned before, calculating the value of a binary number means totaling all the values for the "on" bits. Binary arithmetic is pretty easy once you know what's going on. So now that you understand a bit about binary pun intended , you can understand the technical definition of an IP address.

To your computer, an IP address is a bit number subdivided into four bytes. Remember the example of an IP above, Using binary arithmetic, we can convert that IP address to its binary equivalent. This is how your computer sees that IP:. Understanding binary also provides you with some of the rules pertaining to IPs.

We wondered why the four segments of an IP were called octets. Well, now that you know that each octet is actually a byte, or eight bits, it makes a lot more sense to call it an octet. And remember how the values for each octet in an IP were within the range of 0 to , but we didn't know why? Using binary arithmetic, it's easy to calculate the highest number that a byte can represent.

Table summarizes the possible network numbers, the total number of each type, and the number of hosts in each Class A, B, and C network. There are several reserved cases. For example, networks 0. Networks Memorizing the contents of Table should be one of the first things you do in preparation for the CCNA exam s. Engineers should be able to categorize a network as Class A, B, or C with ease.

Also memorize the number of octets in the network part of Class A, B, and C addresses, as shown in Table You need to know how it works and how to "do the math" to figure out issues when subnetting is in use, both in real life and on the exam. Chapter 12 covers the details of subnetting concepts, motivation, and math, but you should have a basic understanding of the concepts before covering the topics between here and Chapter So, this section describes the basics.

IP subnetting creates vastly larger numbers of smaller groups of IP addresses, compared with simply using Class A, B, and C conventions. Subnetting treats a subdivision of a single Class A, B, or C network as if it were a network itself.

By doing so, a single Class A, B, or C network can be subdivided into many nonoverlapping subnets. Comparing a single network topology using subnetting with the same topology without subnetting drives home the basic concept. Figure shows such a network, without subnetting. The design in Figure requires six groups, each of which is a Class B network in this example.

Additionally, the two serial interfaces composing the point-to-point serial link between routers C and D use the same network because these two interfaces are not separated by a router. Finally, the three router interfaces composing the Frame Relay network with routers A, B, and C are not separated by an IP router and would compose the sixth network.

In fact, this design would not be allowed if it were connected to the Internet. You more likely would get a couple of Class C networks, and the NIC would expect you to use subnetting. Figure illustrates a more realistic example that uses basic subnetting. Figure Using Subnets. As in Figure , the design in Figure requires six groups. Unlike Figure , this figure uses six subnets, each of which is a subnet of a single Class B network.

This design subnets Class B network To perform subnetting,the third octet in this example is used to identify unique subnets of network Notice that each subnet number in the figure shows a different value in the third octet, representing each different subnet number.

In other words, this design numbers or identifies each different subnet using the third octet. This field is created by "stealing" or "borrowing" bits from the host part of the address.

The host part of the address shrinks to make room for the subnet part of the address. Figure shows the format of addresses when subnetting. Now, instead of routing based on the network part of an address, routers can route based on the combined network and subnet parts. Finally, IP addressing with subnetting uses a concept called a subnet mask. A subnet mask helps define the structure of an IP address, as shown in Figure Chapter 12 explains the details. I would like to receive exclusive offers and hear about products from Cisco Press and its family of brands.

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The bytes of the IP address are further classified into two parts: the network part and the host part. Figure shows the component parts of a typical IP address, This part specifies the unique number assigned to your network. It also identifies the class of network assigned. In Figure , the network part takes up two bytes of the IP address. This is the part of the IP address that you assign to each host. It uniquely identifies this machine on your network.