Question : Port forwarding in combination with DHCP

I am using a USR broadband router. Port forwarding is possible on this router (it is called virtual server)however the ip address of the pc to which a particular port is forwarded to should be fixed. Now I do use the DHCP server from the router with mac address mapping so the ip addresses ar more or less fixed.
Unfortunately this does not seem to work, I cannot access my webserver form the internet.

Now I have two questions:

Is mac address mapping technicaly the same as a fixed ip or do I realy need to configure the webserver PC with a fixed ip?

Can I still use DHCP with one PC in the network with a fixed ip and the others dynamic or will this sooner or later cause an ip conflict?

Answer : Port forwarding in combination with DHCP

It's best to set the IP address statically on the web server...

You will need to exclude the static (fixed) IP address on the DHCP server so it doesn't assign the same address to another workstation.  If it is not excluded from the DHCP range, it will be used and you will have a conflict on the network and both hosts will be down.  You should be able to exclude IP addresses from the DHCP range on your router.