If you're registered a nickname on an Internet Relay Chat server equipped with Nickserv and someone else is using it, you can regain that nickname by sending a Regain message to Nickserv. You can also tell Nickserv not to let anyone else use your nickname by sending an Enforce message in advance. Register your nickname in advance with a Register command.
Registering Your Nickname
Nickserv registers and protects your nickname -- the name that you use in the IRC server's chat channels. If you don't register your nickname and log off, you can return to the server to find someone else using it, either maliciously trying to impersonate you or simply coincidentally. Also, to reduce spam and harassment, some IRC channels may not let you join or may not let you send messages unless you use a properly registered nickname.
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To communicate with Nickserv, send messages just as if you were sending private messages to another user on the server, using the "/msg" command.
To register with Nickserv, log in with your desired nickname and message Nickserv by typing /msg Nickserv register
Some servers also require that you send your email address to Nickserv.
If you receive a message back from Nickserv saying that it also requires your email address, send another Register command including your email address by typing /msg Nickserv register
When you receive an email from the operators of your IRC network, follow the instructions in the email to send a Verify Register command to Nickserv with a verification code from the email and complete registration. Do this by typing /msg Nickserv verify register
After your nickname is registered, confirm you're its registered user by identifying yourself to Nickserv when you connect to your IRC server to log in with your chosen password.
Send an Identify command to Nickserv with your password by typing /msg Nickserv identify
Take Back Your Nickname
If you've registered your nickname and connect to your IRC server only to find somebody else is using it, you can take it back by messaging a "Regain" command to Nickserv.
Type /msg Nickserv regain
Enforce Your Registration
To prevent people from using your registered nickname even when you're not logged on to the server, you can tell Nickserv to enforce your registration, requiring anyone who tries to use your nickname to identify themselves within a short amount of time or be required to use another name.
To activate enforcement, type /msg Nickserv set enforce on.