It’s inevitable… someday, if you haven’t already experienced this, you will be working through a solution for your network and you’ll to determine how many IP addresses you will need. When you start working on the solution, how do you know how many usable IP addresses are in a /X? How about if there are any other requirements that need to be met? Can you meet them, or do you need to invest in a service to help you?
You can use Google to comb for information, but for the most part you’ll get a list of sites that, if you weren’t well versed in IP Engineering, would look like an ancient scroll. You can call around your company, consult with colleagues and hope someone can provide you the right answer…but that’s not the fastest way to a solution.
Let’s start with getting to the answer of how many IP addresses are in a /18, /20, /8 etc. I could provide you a couple of shortcut ways to calculate this yourself, but that would be like providing you a Russian to Greek Translation guide and telling you to look it up. For those readers who know Russian or Greek, one of the formulas goes something like this:
A subnet has 32 bits (that never changes). There is a relationship between hosts and networks so when you want to know how many IP’s in a /25 for instance follow this:
(32-X)=Y (where X= the number after the /) Then calculate 2^y to get the number of IP Addresses
- 32 – 25 = 7 bits
- Starting with 1, double the last number seven times for the 7 bits
- Answer = 128 IP’s in a /25 and 126 useable
- Useable IP’s will always be total amount of IP’s – 2.
The easiest way to get to an answer, however, is to reference the table below:
|IP Addresses||Usable IP Addresses|
Now that you know what your requirements are from a space perspective, the next question should be: “Are there any requirements that need to be met?” Depending on the overall size of the block required, the answer could be yes. Given the scarcity of IPv4 addresses, ARIN (American Registry for Internet Numbers) requires justification for assigning IP blocks to end users. This was implemented to prevent hoarding of these scarce resources. If you have requested a large quantity of IP Addresses you may be familiar with this. For smaller blocks, getting this directly from your provider is the best route. Every provider has slightly different bars related to the need for justification forms based on space they have available but if you’re requiring a /23 or greater from a network provider, you can expect to be going through the Justification process.
Whether you go directly to ARIN or request larger quantities of space from your provider, the justification forms are pretty similar and will go through the roughly the same decision making process. The Justification form is looking for you to identify immediate need as well as future need amongst other requirements. If you are interested in the details, you can find the ARIN policy manual here: ARIN Number Resource Policy Manual OR you could ask your network or hosting vendor to provide some guidance with the justification form. If they’re like EarthLink and really interested in being a true partner, they will jump through hoops to assist (truly…contact us if you need help navigating an issue like this!).