PTirc plans to shut down port 6667 (plaintext) in the early 2019.
This decision is to reduce spam bots and increase the security of their users.
Therefore, all your users must use port 6697 to connect to the network.
The commands to connect to the network through some of the best known clients are:
/server irc.ptirc.org +6697
If you use other types of services, such as ZNC, to connect to the network, you must notify their providers to explicitly use port 6697 or they will be unable to connect.