Static and 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. IP addresses may be either the familiar IPv4 style (192.168.2.1) or the newer IPv6 style (2001:db8:0:1::3f). IPv4 addresses are in increasingly short supply and most organizations charge for the use of Static IPv4 addresses - sometimes quite a lot. IPv6 addresses are becoming more common and are readily available - often in very large numbers and usually at significantly lower prices than IPv4 addresses. However, local IPv6 addresses may need access to various conversion/tunelling services to interwork with the huge base of IPv4 users so you need to carefully discuss their use with your ISP or Service Provider.

IP Address and Port Numbers: When a client accesses a service (web, ftp, mail etc.) it sends its IP address and a port number (the source address information) and the IP address and port number of the desired service (the destination address information). Services such as web, ftp, mail and so on use Well Known Port Numbers (defined in a list maintained by IANA). In the case of a web service this Well Known Port Number is 80, FTP uses port 21 (and port 20), mail uses port 25 (SMTP) for outgoing mail and either port 110 (POP3) or port 143 (IMAP) for incoming mail. Since each port is unique they can all be supported on a single IP address either by running all the services on a single server or by using a port mapping service such as NAT-PAT (provided by most DSL and cable modems) to translate incoming port numbers to a unique (internal IP address). When a client accesses any service, such as a web service, it sends the request from its IP address (which may be Static or Dynamic) and a dynamically allocated port number typically in the range 1024 to 65535.

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 Internet. Dynamic IP addresses are the normal customer access method used by most ISPs or Service Providers. When using dynamic IP addresses, even if you are permanently connected (always-on) some ISPs/Service Providers change dynamic IP addresses every 24 hours, others change less frequently (monthly or even longer in certain cases). Check your local ISP's policy on IP address change frequency. The change of IP address is typically carried out between your ISP/Service Providers network and your local (on-site) DSL or other modem using the Dynamic Host Control Protocol (DHCP). You will see no operational effect when the IP address changes - but neither will you be able to stop the process.

Note: Even if, by observation, a dynamic IP address does not change frequently it still can at any time solely at the discretion of your ISP. For example, a change of network policy or installation of new equipment. You do not control the IP address change policy, your ISP/Service Provider does.

When you do NOT need static IP addresses

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

If you browse the Internet.
If you send and receive e-mail via an offsite mail server or service (the normal method and typically using your ISP's mail server(s) or a mail service such as gmail, hotmail, yahoo mail etc.).
If you download or upload files.
If you use Instant message services or chat services.
If you run any services, such as web or FTP servers, that ONLY require access from your local area network (single site) and that DO NOT require or ALLOW access from the Internet.

When you need Static IP Addresses

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

You run one or more Web server's directly on your site that require external access (from the Internet or an Extranet).
You run one or more E-mail server's directly on your site that require external access (from the Internet or an Extranet).
You run one or more FTP server's directly on your site that require external access (from the Internet or an Extranet)
You run one or more DNS server's directly on your site that require external access (from the Internet or an Extranet).
You run any other service or 'Application' that requires external access (from the Internet or an Extranet). Some Financial Terminal Client and other licensed applications require that you have a static IP address for licensing purposes.
You require or allow 'incoming' Video or Audio services. In this case you may need to use static IP addresses. As an example, if you run an on-site IP-PBX you will typically need one or more static IP address(es). However, in some cases audio and/or video client applications (including some IP-PBXs) will require you to connect, or register, with an external server which will forward incoming traffic. When using these types of clients/applications you do not need a static IP address. Verify the details with the client/application supplier.

Note: If you are using DSL to host local services, be aware that DSL normally provides asymmetric speeds and that incoming speed (from the Internet) is normally faster than outgoing (to the Internet) speed. In some cases the difference in speed is significant. In the case of browsing and most client services this difference works in your favor. You send a small amount of data (a single URL) and get back a lot of data from a web site. If you are providing a service the opposite is true. Users send you (incoming) a small amount of data (a single URL) and your local service sends (outgoing) a lot of data. Check with your ISP/Telecom supplier for the details.

How many IP Addresses do you need

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

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 (aka NAT-PAT).
If you run one copy of any application which uses Fixed Port Numbers you only need a single Static IP address.
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.

Alternative Approaches

External Hosting

Hosting services range in price from less than $10 per month to many $100's of dollars. You get what you pay for in terms of reliability, software supported, permitted access, volume of data allowed, bandwidth, etc. - but do lots of homework before you choose. Increasingly external hosting is being provided using virtualized services which typically enables very high levels of user control over the hosted service.

Dynamic DNS Services

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. Thus, if someone types www.example.com into their web browser it is translated via a DNS service to a specific IP address using a forward map. 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 rather than try and map it to a dynamic IP address.

Problems, comments, suggestions, corrections (including broken links) or something to add? Please take the time from a busy life to 'mail us' (at top of screen), the webmaster (below) or info-support at zytrax. You will have a warm inner glow for the rest of the day.