I have an isy 99 and a NAS and want to check them periodically when I am gone but the ip address changes sometimes and I can't get in. Someone suggested DynDNS. Could someone explain to me how this works? Thanks.
So you want to be able to access your NAS and ISY99 (home automation system?) remotely when your not at home?
Well there's two stages to this. The first is to be able to communicate to your internet access point at home. And the second is to be able to get from their to the relevant device.
The first stage is probably most easily achieved by getting a static IP for your internet connection at home. This way you have a set IP to use to "go home." You usually arrange this through your Internet Service Provider. There are other alternatives with services such as DynDNS which basically redirect traffic for a particular domain to an ever changing IP address. I've heard about these services, but not sure how reliable or quick they are.
So once you can connect back home, you get to the second stage. This is the bit that Soulphire is talking about. Basically you need to configure your router so that when it gets communication on particular ports, it redirects it to the relevant device on your home network (eg your ISY99 or NAS). Most modern routers can do this, and this feature is often called Port Forwarding or Mapping.
Note that you will need to setup static IP's for your devices on your home network as well as know what Ports your NAS and ISY99 use to be able to configure this properly.
Since you are able to connect to these services when you know the correct external IP, it sounds like you've got the port forwarding part figured out and hopefully you've assigned static internal IPs to the servers so I'm going to leave that subject for now.
Yeah, just sign up for a dynDNS account and add the details to the relevant section on the router configuration. If your router does not have the ability to do that, you can set up a machine on your network to do it (I used my IPcop firewall).
You should note that this will not give you a static external IP, only your ISP can do that but it will provide you with a name which effectively emulates this.