The TFTP server uses a subnet-directed broadcast address as the destination address.
It also uses a well-known port as the port of datagrams sent to clients that have requested the subnet broadcast option. The clients listen for and receive datagrams on the well-known port. The keyword for the well-known port is subntbcst_tftp, and its decimal value is 247.
The TFTP server sends subnet-directed broadcast datagrams to clients that request the subnet broadcast option. The source ports from which the TFTP server sends these datagrams do not have to be unique. They can be arbitrarily allocated.
Some routers filter or block subnet-directed broadcast datagrams. In support of router filters, you can define restricted ports for the QTFTP profile. If you define restricted ports for the QTFTP profile, the TFTP server uses only the defined restricted ports as the source ports for the subnet-directed broadcast datagrams. Network administrators define router filtering rules to allow subnet-directed broadcast datagrams to pass through router filters based on the source port of subnet-directed datagrams being one of the restricted ports defined for the QTFTP profile.