Internet protocol, or IP addresses, are numeric codes used by computers to route messages to one another. Groups of IP addresses, known as IP ranges, are allocated to specific companies and networks. There are different ways to represent an IP range depending on the purpose you have in mind and whether you're communicating with humans or computers.
Understanding a Network IP Address
Traditional IP addresses, known as IPv4 addresses, consist of four numbers separated by periods. Each of the four numbers is a decimal between 0 and 255, and it can also be represented as eight binary digits, or bits. The entire IP address can be represented as a 32-bit binary number, which is useful for some computations.
When you send a message to a computer online, whether you're transmitting an email or downloading something from a web page, you usually type in a domain name such as www.example.com. The internet's Domain Name System translates these human-readable and easily memorable domain names into IP addresses understood by computers.
Internet routers know in which direction to send messages for various IP addresses according to routing information circulated across the internet. Usually, they do so by matching rules that refer to the first digits of the IP address.
Understanding IP Ranges
To make routing more efficient, IP addresses are normally divided into contiguous ranges, such as 127.0.0.1 through 127.255.255.255 or 192.168.0.1 through 192.168.0.255. Those are allocated to a single network or group of networks so that routers can match the initial bits of the IP address and know where to send messages for that group of systems.
IP ranges can be written using dash notation, but they can also be written using what's called a netmask or subnet mask, which looks like an IP address that represents the leftmost digits of the IP address, which are always set to 1 for a particular network. There is usually an accompanying number indicating how many of the bits are to be considered part of the network's prefix. The rest vary among IP addresses in the network. 126.96.36.199/12 is an example.
Traditionally, IP addresses were divided into ranges based on classes, where networks had fixed numbers of IP addresses in them, but this method proved inefficient over time. The modern system, with an arbitrary number of bits in a network prefix, is called classless interdomain routing, or CIDR. The acronym is often pronounced "cider."
You can use an online netmask calculator or subnet calculator tool to translate between notations or figure out whether an IP address matches a subnet.
Reserved IP Addresses and Ranges
Certain IP addresses are reserved for specific purposes.
For example, the ranges 10.0.0.0 through 10.255.255.255, 172.16.0.0 through 172.31.255.255 and 192.168.0.0 through 192.168.255.255 are reserved for internal private use on networks. They can't be routed across the internet. That's useful for assigning addresses to computers only connected to the internet by an intermediate router without having to worry about conflicts with external devices or using up an organization's supply of routable IP addresses.
The range of addresses from 127.0.0.0 through 127.255.255.255 is reserved for computers to send messages to themselves. They're called loopback addresses.