The PortRange setting defines the ports the TFTP server will use for sending and receiving data.

PortRange=51000-51100          ; Default: AUTO

The default is AUTO and specifies that the port numbers are allocated automatically on demand as they are made available by windows sockets (winsock). This can be a problem with firewalls, because the port numbers can not be explicitly opened in the firewall if they are unknown at configuration time. Therefore it is possible to specify a range of ports like PortRange=51000-51100. This tells the TFTP function to select port numbers out of the given range for sending and receiving data. Please make sure that the specified port numbers are not confilicting with other network services running on the server.