Imagine I want to build a network that will support up to 30 devices in different segments. We can use those borrowed bits to create additional networks, resulting in smaller-sized networks. Subnetting allows you to create smaller network (sub networks subnets) inside a large network by borrowing bits from the Host ID portion of the address. That issue of IP address wastage brings us to the topic at hand – Subnetting. This wastage of IP addresses is even worse for Class B (65,534 usable IP addresses per network) and Class A (16,777,214 usable IP addresses per network)! What if you only need 10 IP addresses? You still get a Class C network. So in those days, anyone who needed a network that supports up to 254 hosts can use a Class C network. A simple formula to calculate the number of hosts supported by a network is: However, two of these addresses cannot be assigned to hosts because the first (all 0s) represents the network address while the last (all 1s) represents the broadcast address. For example, a Class C network will ideally support up to 256 host IDs i.e. Looking at the Host ID portion of the classes, we can determine how many hosts (or number of individual IP addresses) a network in each class will support. For example, the 192.168.1.250 IP address clearly falls into the Class C range. With these classes, a computer/device can look at the first three bits of any IP address and determine what class it belongs to. Note: The range of Class A is actually 1-126 because 0.x.x.x and 127.x.x.x are reserved. The three leftmost bits are set to “110”. Class C: Uses the first 24 bits for the Network portion leaving 8 bits for host IDs.Class B: Uses the first 16 bits for the Network portion leaving 16 bits for host IDs.
Class A: Uses the first 8 bits for the Network portion leaving 24 bits for host IDs.Multicast addresses are assigned from the Class D range while Class E is reserved for experimental use, leaving us with Class A to C: So what does this mean? Well, IPv4 addresses were originally designed based on classes: Class A to Class E. Therefore, to a computer, the IPv4 address 192.168.1.250 is actually “11000000 10101000 00000001 11111010” (I only put the spaces to make it readable the computer doesn’t see spaces):Ī unicast IPv4 address such as 192.168.1.250 can be divided into two parts: Network portion and Host ID.
To make them more readable for humans, IPv4 addresses are represented in dotted decimal notation where the 32 bits are divided into 4 blocks of 8 bits (also known as an octet), and each block is converted to a decimal number.įor example, 01110100 in binary is 116 in decimal: Therefore, even though we see an IP address represented like 192.168.1.250, it is actually just a string of bits – 32 bits in total for IPv4 addresses. To help your understanding of IP addresses and subnetting, you need to resolve the following fact in your head: Computers think in binary, that is, 0s and 1s. Also, features like Network Address Translation (NAT) allow one IP address to be shared by multiple devices. Traffic sent to Multicast (one-to-many) and Broadcast (one-to-all) IP addresses can be delivered to multiple devices. Note: This is an oversimplification of things just for understanding sake and refers to Unicast (one-to-one) IPv4 addresses. Just like a house number uniquely identifies a house on a street, an IP address uniquely identifies a device on a network.įor example, any traffic with a destination IP address of 192.168.1.101 will be delivered to PC1, while traffic addressed to 192.168.1.250 will be delivered to SERVER. Subnetting deals with IP addresses and so, it is natural to start any discussion on subnetting with IP addresses. Moreover, subnetting in IPv6 is more of a want rather than a necessity because of the large address space. The same concepts explained here can be applied to IPv6. Note: While subnetting applies to both IPv4 and IPv6, this article will only focus on IPv4.
To make this article as practical as possible, we will go through many examples.
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In this article, we will discuss what Subnetting is, why it came about, its usefulness, and how to do subnetting the proper way. Another reason is that many people have not had enough practice with subnetting. One of the reasons this happens is that one has to perform (mental) calculations in decimal and also binary. If there’s one topic that trips people up (both new and experienced) in the networking industry, it is that of Subnetting.
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