Dhcp and ftp servers listen for and send network traffic on:
well known ports
registered ports
dynamic ports
ip ports

Well Known Ports

Explanation:Dynamic Host Configuration Protocol(DHCP) and File transfer protocol(FTP) are the protocols that have ports for their working , so if there is the request of the traffic over the network then there are well known ports which can receive it.

DHCP has the port 67 as the default port which is well known for receiving of the any default data and FTP has the port 21 as well defined port for receiving the default values.

i believe the two important functions that skype uses are nat traversal and user location.good luck        

explanation:

answer ingenuity;