Thank you for your answer.
(I have a little break using mirc and now come back to meet this problem again)
I changed the network1 with the name of the server in format xxx.xxxxxx.xxx and <password> with my password but this command still does not work
I got the message : -NickServ- This nickname is registered. Please choose a different nickname, or identify via /msg NickServ identify <password>.
What might be wrong?