So, the first two bits in the fourth octet are on and the remaining six are off. The number in the fourth octet is 192, and we can get to 192 by adding up 128 and 64. The first row shows base-2 exponents and the second row shows if an individual bit is on (1) or off (0). To find out how many bits are turned on in the fourth octet you can use the following table: 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 Here, part of the fourth octet is part of the network. The last octet is made up of all zeros and can be used for nodes. So, the first three octets are all ones and part of the network. To demonstrate, let’s convert the network mask 255.255.255.0. Once the subnet mask is a base-2 number you can instantly see which part of an IP address is the network and which part is used for nodes the network component is made up of all ones while the host part is made up of zeros. To make sense of subnet masks you need to understand base-2 numbers. The mask is used to mask out what part of an IP address is the network. Just like an IP address, a subnet mask is a 32-bit number divided into four-octets. Instead, the network address is defined by a subnet mask. SubnetsĪs said, with subnets the network component doesn’t always have to be exactly 1, 2 or 3 octets. When IPv6 is finally widely used the world should never again face an IP address shortage. The main other development that has happened since is the introduction of IPv6 addresses. CIDR was adopted in 1993 and is still widely used today. This makes it possible to divide networks into smaller parts, known as subnets. With CIDR the network part doesn’t always have to be an entire octet. The next attempt to solve the problem was a new architecture called Classless Inter-Domain Routing, or CIDR. As the internet kept growing rapidly it became clear that there weren’t enough IP addresses. At one end of the spectrum you had 254 class A addresses with a whopping 16,777,214 nodes each, while at the other end you had millions of class C addresses with just 254 nodes. Class A addresses still used only the first octet for the network class B used the first two and class C the first three. The primary classes were A, B and C, and each class defined a range of IP addresses and which octets were the network part. This scheme defined five classes of IP addresses. In the early 1980s a new architecture called classful network was adopted. Little did they know that billions of people would one day spend their lives on Facebook and connect doorbells and toasters to the internet. That limited the number of networks to just 254, but that seemed plenty at the time. In the early days of the internet (the ARPANET era) the first octet of an address was used for the network and the remaining three octets were used for nodes. IP addresses have two parts a network and a host part. You will be familiar with that if you develop websites locally on your computer – if you have a web server running on your machine then you can view websites you are working on via or Network classes These IP addresses are local IPs used for communication on your local network (LAN). For instance, chances are that your local machine has an IPv4 address starting with 192.168. Reserved and loopback IPsīoth IPv4 and IPV6 have so-called reserved addresses. The main difference is that you need to understand hexadecimals for IPv6 subnets. However, the basics are the same for both IPv4 and IPv6. That is mainly because subnets are more relevant for IPv4 addresses. In this article I focus on IPv4 addresses. The number of available IPv6 addresses is insanely large – so large that you might one day get a static IP address from your ISP. An IPv6 address is made up of eight 16-bit hextets (separated by colons). As you probably know, the world has pretty much run out of IPv4 addresses and IPv4 is slowly being replaced with IPv6. The most common type of IP address is still IPv4, which is a 32-bit address made up of four 8-bit octets separated by dots. IP addressesĪn IP address identifies a system on a network. And if you want to know how to calculate the number of IPs in a subnet then I got an article about that as well. Base-2 numbers play a large role in the story, so you may want to read my article about binary numbers first. It includes a brief history lesson about network classes that explains why subnets were invented (and why they are very useful). This article is a gentle introduction to IPv4 subnets and CIDR.
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