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Tech Stuff - 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 ( 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, static IPv6 addresses may need access to various conversion/tunnelling services to interwork with the huge base of IPv4 users so you need to carefully discuss their use with your ISP or Service Provider.

The answer as to whether you need Static or Dynamic IP Address(es) depends on at least a vague understanding of the role of DNS, local and global IP adresses and roughly what DSL or cable modems do. If you are comfortable with this stuff you can skip to the short answer. Otherwise you may be advised to read this next part and perhaps also want to have headache medication on hand.

IP Address and URLs (Names) When you access a service (web, ftp, mail and so on) the client application (such as a web browser or a mail client) typically uses a name (more correctly a Uniform Resource Locator or URL for short). As an example, you access this page by typing (or clicking a link) which refers to the URL The Internet, like all other networks, knows nothing about about names - it only knows about addresses. In the case of the Internet these are IPv4 or IPv6 addresses. So the start of the process to access, say this web site, is to translate from the URL to an IP address. In this case the part of the URL is translated (or resolved in the DNS jargon) by a Domain Name System (DNS) resolver, typically located at your ISP or Service Provider into its IPv4 (or IPv6) address. DNS data is normally static or changes very slowly (timescale may be measured in years) mostly for reasons of performance and service accessibility. If the IP address of your bank or google kept changing you might miss it due to latencies and caching (saved copies) - not terribly useful. See the section below on Dynamic DNS Providers under Alternative Approaches for exceptions to this general rule.

IP Address and Port Numbers: When a client (a web browser or email client) accesses a service (web, ftp, mail etc.) it sends a message which, among lots of other data, contains the client PC IP address and an arbitrary or random port number in the range 1024 to 65535 (this is the source address information - so that the receiving system, say the web server, knows where to return the answer or response) 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. When the local PC (or server) is connected to the Internet using a Cable or DSL modem a lot of other stuff happens next.

Local and Global (Remote) IP Addresses It gets a bit messier now. If you are connected to the Internet using a DSL or Cable modem then your local PC(s) has/have been allocated a local IP address (typically 192.168.x.x) by the modem using a protocol called Dynamic Host Control Protocol (DHCP). Local addresses of the form 192.168.x.x (there are specific groups of IP addresses allocated exclusively as local IP addresses) cannot be routed across the Internet - they are only usable on your local (domestic) network. To route traffic (messages) from your PC to the Internet the local address must be converted (or translated) to a global IP address (which can be routed across the Internet). This is done by another piece of software in your DSL or Cable modem called a Network Address Translator (NAT). The translation may be to a static or a dynamic IP address - it depends on what service you are paying your ISP or Service Provider for. In the vast majority of cases you will be using a dynamic IP address and, as described below, it will change from time to time based on the policy of your ISP or Service Provider.

Static and Dynamic IP Address Properties

As the name implies Static IP addresses are the same every time you connect or send a message. Static IP Addresses are used by all major web sites, email services, FTP services and so on. There are no exceptions to this. You will pay handsomely for static IP addresses. Dynamic IP addresses may change each time you connect to the Internet or even, in extreme cases, every time you send a message. 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. You will see no operational effect in normal user browsing or access when the IP address changes - but neither will you be able to stop the IP address change 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 or Service Provider. For example, a change of network policy or installation of new equipment or because it's Tuesday. You do not control the IP address change policy, your ISP/Service Provider does.

Typical Flows

The following diagram shows a simplified data flow across a network. This diagram may, or may not, help to understand the traffic flows and who translates what, where.

network flow diagram

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 off site mail server or service (the normal method and typically using your ISP's mail server) or a typically web based mail service such as gmail, hotmail, yahoo mail and many others);

  3. If you download or upload files (using ftp or http);

  4. If you use Instant message services or chat services;

  5. If you run any services, such as web or FTP servers, that ONLY require access from your local area network (single site) or perhaps within a multi-site corporate network, 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:

  1. You run one or more Web server's directly on your site that require external access (from the Internet);

  2. You run one or more E-mail server's directly on your site that require external access (from the Internet);

  3. You run one or more FTP server's directly on your site that require external access (from the Internet);

  4. You run one or more DNS server's directly on your site that require external access (from the Internet);

  5. You run any other service or 'Application' that requires external access (from the Internet). Some Financial Terminal Client and other licensed applications require that you have a static IP address for licensing purposes;

  6. 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 may not need a static IP address. Verify the details with the client/application supplier.

Note: If you are using DSL/Cable to host local services, be aware that these services normally provide 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) at, relatively, slow speed and get back a lot of data from a web site or FTP server - a file download or a web page - at a,relatively, fast speed. If you are providing a service using DSL or Cable the opposite is true. Users send you (incoming) a small amount of data (a single URL) at a, relatively, fast speed and your local service sends (outgoing) a lot of data at a, relatively, slow speed. 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.

  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. Each service uses a different well known port number, for instance web/http uses port 80, and most DSL and cable modems embed a port mapper (a.k.a NAT-PAT);

  2. If you run one copy of any application which uses Fixed (or well known) 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.

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, and so on - but do lots of homework before you choose. Increasingly, external hosting is being provided using virtualized services which typically allows very high levels of user control over the hosted service since, in effect, you may be the sole user of the virtual host (even though the physical host is shared by 10s or even 100s of virtual hosts).

Dynamic DNS Services

All externally visible (global) 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 into their web browser it is translated via a DNS service to a specific IP address using a forward map. In most cases the IP address in the DNS is a static IP address. However, there are a number of organizations (Dynamic DNS providers) that will map names, such as or to a dynamic IP address by constantly monitoring your Dynamic IP address and changing the addresses in the DNS whenever your IP address changes. While this can be very effective in terms of cost savings there will always be a time lag (will vary with the Dynamic DNS provider) between the Dynamic IP address change and when it is recognized by the Dynamic DNS Service which can interrupt external user access. This may not be problematic for many uses but in time sensitive services it may be catastrophic. In such 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.

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Page modified: March 01 2014.


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