Purpose of netmask in dynmic DHCP statement [duplicate]

        What is the purpose of the netmask in a dynamic DHCP statement:

ip nat pool poolname 209.165.200.226 209.165.200.240 netmask 255.255.255.224

I know, that I should choose the netmask to be larger then the range of the IP adresses given in the statement. So can I just use netmask of 255.255.255.0 as it is restricted to the range anyways?