Client Port / Connections from other clients, Client to Client Source Exchange

You can change this port in Preferences -> Connection -> Client Port
This port has to be forwarded in a router. Changing this port in Preferences you must also change the forwarding in the router

Client Port / Connections to other clients, Client to Client Source Exchange

4662 is the default port, but other clients may have different settings. Change the remote port to any when configuring a firewall

extended eMule protocol, Queue Rating, File Reask Ping

This port has to be forwarded in a router. Changing this port in Preferences you must also change the forwarding in the router.
If you are not able to forward this port check the disable box in Preferences -> Connection -> UDP Port

extended eMule protocol, Queue Rating, File Reask Ping

4672 is the default port, but other clients may have different settings. Change the remote port to any when configuring a firewall

Connection to server

4661 is the default port of a server. Many server use different ports. Configuring a firewall the remote port again changes to any.

Servers using the default port 4661 TCP (see #5) automatically set their port for source asking to 4665 UDP. If a server uses a different port in #5 the corresponding UDP port is set to [Connection Port + 4]. For firewalls the remote port here is any.

UDP is a connectionless protocol, i.e. unlike TCP no connection is kept alive. If a server answers e.g. a source request this answer is treated as a new connection. All commonly used routers and firewalls in the home computer sector perform an UDP connection tracking therefore it is not necessary to specify an incoming port for UDP in the router's or firewall's configuration. More complex networks, especially if proxy servers are used, may require to define this local port to configure the router / firewall / proxy accordingly or to disable it at all if it is not possible to control them. It is possible to set this port in Preferences -> Preferences.ini but there is usually no need to change it.