The cheaper 'Home' or 'USB' ADSL services provide a 'dynamic' IP address, which may alter each time that a connection is made to the ADSL service. In April 2001, BT altered the way that they configured DHCP, so that a user would probably be given the same IP address as before, if the interval before reconnecting was not more than about 20 minutes and if there were not many requests for IP addresses at the same time.

Several companies provide ways to map dynamic 'numeric' IP addresses to a fixed domain name. This enables people on Internet to access computers that do not have a fixed numeric IP address by using their domain name, which 'follows' the numeric IP address, as it changes.
Dynu Systems Inc in Minnesota, USA provide a free service with software called 'Dynu Basic Client' Version 2.0. Their commercial software is called ' Dynu Web Client' and provides the 'Dynu Premium Dynamic DNS Service'.

As soon as the client computer connects to Internet, the software sends the name of the computer and its new dynamic IP address to Dynu DNS servers, which update the DNS mapping tables with the new IP address, so that other computers can continue to connect to it using a 'fixed' name.
In this way, you can host your own Web, FTP, Mail Server, Game Server or other TCP/IP Service. The software will provide a fixed address for a Cable modem, xDSL, ISDN or Dialup services. Dynu can also provide a domain name. Similar services are provided by 'XDISK' in the UK, 'DNS2GO' in Michigan, USA (who also sell Wingate software), 'DHS International' in Hong Kong and 'Selfhost' in France.

The above services map your computer to a domain name under the control of the company that provides the service. If you want to map your computer on an ADSL connection to your own domain name, this can be done in a complex manner: A BT Openworld FAQ notes that hammernode.com provide a service to update automatically your IP address to your own domain name.

1. First create a 'vanity account' login with hammernode.com.
2. Second, set up a 'vanity domain' (yourname.hn.co.uk) which will be used by the redirection process.
3. Third, create a 'Virtual Domain Mapping login', using the box on the right of the hammernode.com home page.
4. Then you have to alter the settings at the site where you registered your domain name, to use the name servers at hammernode.com. For example, if you have a domain name that is registered at ukreg.com, logon to it and alter the name server entries for your domain name from the ukreg.com name servers to the hammernode.com name servers. Wait for about 24 hours for the name server root to be updated then log onto hammernode.com and point your new root-verified domain at your recently created vanity domain.
5. Finally, install a progam on your PC, such as 'DNS2GO', which will update the IP data when it is dynamically changed by BTOpenworld. If you use yiPost for this function, set one option there to 'Check local IP address'.

It is possible, but not necessarily acceptable to the service provider, to set up an email (STMP, POP3 and Finger) server on a dynamic IP ADSL line using, for example, the free software from 'argosoft' called 'ArGoSoft Mail Server' and 'Mercury'.

A disadvantage of dynamic IP addresses is that you may acquire the IP address of someone else who had set up a service on that IP address, which will then 'map' to your computer when you acquire their IP address. When someone runs a game server, that IP address is registered in some central location and becomes visible to anyone in the world wishing to play that type of game. Typically, anyone looking for a game will ping (actually more than a ping, but that is what the gamers call it) every available server, sometimes repeatedly and will then pick one game and play. So every single player in the world looking for that game will make requests to every registered server. Several hundred an hour is not an unreasonable number for some games.

Most often, these attempts to access your computer will be logged by ZoneAlarm as 'attacks', even though they are not a cause for alarm. UDP is a broadcast protocol used for games servers, video and sound broadcast, etc. The problem will continue until their machines realised that the service is discontinued, so it is better to get another IP address.

Another cause of multiple attempts to access a computer is when you start using Napster or other peer-to peer software. These attempted connections are other users eager to download MP3 files, etc.