I believe you are referring to port numbers. TCP and UDP (layer-4 protocols) use addresses, commonly called port numbers.
Both TCP and UDP use port 53 for DNS. If you want to know which port numbers are registered for which service, you can look in the IANA Service Name and Transport Protocol Port Number Registry.
Type in any port number, and it will take you to the entries for that port.
According to the port registry, port 66 is registered for
If, on the other hand, you mean DHCP Option 66, that is for mass provisioning of VoIP phones. See DHCP Options in Plain English:
Carries the FQDN or IP address (or cluster identifier) that the device
should use to download the file specified in option 67. Note that
often the data put into option 66 does not actually appear in the DHCP
packet as option 66, but may have been moved into the “sname” field of
the DHCP packet. Additionally, the FQDN may have been resolved to an
IP address and also placed in the “siaddr” field of the DHCP packet.