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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.
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.
If you ONLY do things from the following list you do not need static IP addresses.
You need one or more Static IP addresses if any of the following are true:
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.
Use the following 'rules of thumb' to calculate the number of required IP addresses.
If you have any doubts discuss them with your ISP's technical specialists.
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.
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.
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