Static vs. Dynamic IP addresses...

This FAQ discusses when you need may Static (or fixed) IP addresses (and how many) and when you can use dynamic IP addresses. IPv4 addresses are in increasingly short supply and most organizations charge for the use of Static IPv4 addresses. IPv6 addresses are becoming more common and are readily available - often in very large numbers.

Definitions

As the name implies Static IP addresses are the same every time you connect. Dynamic IP addresses may change each time you connect to the network (typically Internet). Some organizations change dynamic IP addresses every 24 hours others chnage less frequently (some even as long as one month). Check your local ISP's policy.

When you do NOT need static IP addresses

If you ONLY do things from the following list you do not need static IP addresses.

1. If you browse the Internet.
2. If you send and receive e-mail via an offsite server (the normal method).
3. If you download or upload files.
4. If you use Instant message services or chat services.

When you need Static IP Addresses

You need one or more Static IP addresses if any of the following are true:

1. You run one or more Web server's directly on your site that require external access (from the Internet, or Extranet but not typically an Intranet) . 
2. You run one or more E-mail server's directly on your site that require external access (from the Internet, or Extranet but not typically an Intranet) . 
3. You run one or more FTP server's directly on your site that require external access (from the Internet, or Extranet but not typically an Intranet) .
4. You run one or more DNS server's directly on your site that require external access (from the Internet, or Extranet but not typically an Intranet).
5. You run any other service or 'Application' that requires external access (from the Internet or an Extranet). Some Financial Terminal Client applications require that you have a static IP address for licensing purposes. 
6. You allow 'incoming' Video or Audio services you may need to use static IP addresses.

How many IP Addresses do you need

The following 'rules of thumb' apply to calculate the number of required IP addresses.

1. If you run one of any type of server (FTP, Web, E-mail etc.) on site then you only require a single Static IP address since most DSL and other modems embed a port mapper.
2. If you run one copy of any application which uses Fixed Port Numbers you only need a single Static IP address.
3. In all other cases you need as many Static IP addresses as there are Applications or Servers that use the same Port Number.

If you have any doubts discuss them with your ISP's technical specialists.

All externally visible IP addresses are forward-mapped (from name to IP) and in some cases reverse-mapped (from IP to name) via a DNS service. There are a number of organizations that will map site application services, such as web or email) to a dynamic IP address by constantly monitoring and changing the addresses in the DNS. While this can be very effective in terms of cost savings there will always be a time lag between address changes which can interrupt external user service. In time sensitive cases it may be better to host the service externally.