Is the WAN IP of an ADSL modem likely to remain the same if it is not rebooted?
To specifically answer your first question:
The modem doesn't need to be rebooted by the user in order for the IP to change when the connection is assigned a dynamic IP. The ISP can initiate the change and you may notice a slight "blip" in your connection, but not necessarily a complete drop out and loss of connectivity.
Is an ISP likely to change the WAN IP while the modem is in session?
To specifically answer your second question:
Assuming you have a dynamic IP, this would depend if the ISP has set intervals for changing the IP or if they wait for inactivity over a period of time.
To elaborate on the topic in general:
Whether your connection has a dynamic IP or static IP is something that is controlled by the ISP. Most modems (if not all modern modems) will display basic information about the WAN connection, including IP address, DNS servers, ADSL connection details, and whether the IP is dynamic or static. To find out whether you have a static or dynamic IP, I see that you can do one of three things:
- Assuming the modem has a web interface, check if the WAN details are displayed in the configuration pages,
- If you know who the ISP is, have a look at their website to see if their Service Terms mention if they offer static or dynamic IPs (some ISPs offer one or the other, some offer both),
- If you're the connection owner, just call the support desk of the ISP.
In the event you have a dynamic IP, the frequency that it will change is determined by the ISP. I have been with an ISP that changed dynamic IPs once a month, and another ISP that changed much more frequently. I have also been with an ISP where the IP would change when I rebooted the modem.
If you need to RDP to a computer that is on a connection that has a dynamic IP, I would recommend using the services of http://www.dyndns.com. If you are not familiar with DynDNS, it is a free service that "allows you to point a hostname to a dynamic or static IP address or URL".
For example, instead of keeping record of what the dynamic IP is, you can use DynDNS to map a hostname (such as cjohnstone.dyndns.com) to the dynamic IP and simply RDP to the hostname from that point on. And should the dynamic IP change at some point, a small piece of software is used to update DynDNS at regular intervals so the correct IP is always mapped to your chosen hostname.