IP Subnets
If you've looked into colocation before, you may have found that, when it comes to getting IP addresses for your equipment, the colocation service provider has said that they will assign you one or more subnets for your use. What does that mean?
An IP 'subnet' (short for 'sub network'), is just a group of contiguous IP addresses of a pre-determined size that has been carved out of a larger group of IPs, in a way that allows it to be treated separately from those that come before and after it. There is a more concise technical explanation to be sure, but it's really not necessary to go into that. The important point is that, rather than giving you a random scattering of IPs from widely varying ranges, a subnet is a uniform chunk of addresses, all from the same range, assigned solely to you. The size of the subnet can be varied, from as little as a single IP address, on up to thousands or millions... but in all likelihood, your provider will assign you a subnet of perhaps eight, 16, or 32 IP addresses, depending upon how many you ask for.
When you get an IP subnet, you'll find that not all the IPs in the range can be used as hosts. A few of the addresses are used for internal networking tasks that are important to making subnetting possible. These will be called out as the network address (also called the base address, the first address in the range), broadcast address (last address in the range), and likely a gateway address (the second address in the range). What's left are IPs that are usable by you. So, if you are assigned a subet of 16 IP addresses, keep in mind that only 13 of the addresses will be usable for your hosting needs.
The size of a subnet - meaning the number of IPs included in the subnet - is determined by applying a network mask (commonly called an netmask). The netmask is essentially a number string that follows the dotted notation used by IP addresses. A longer network mask will 'black out' more of a range, resulting in a smaller subnet (fewer usable IPs). A shorter netmask will result in a larger subnet (more IPs). A unique subnet is denoted by its network address, followed by a '/', followed by the length of it's subnet mask, i.e. 192.168.1.0/24.
If you are interested in learning more technical details of subnets, there is a good article here: http://articles.techrepublic.com.com/5100-1035-6089187.html. A nice little cheat sheet for figuring out the particulars of any subnet can be found here: http://krow.net/dict/subnet.html.
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