Syntax: /mode nickname +/- mode(s)
+d Deaf in all channels.
You won't "hear" a thing that's said in the channels. Private messages get through though. Primary reason is making the job easier for service bots that don't want to process channel messages.
+i Makes you invisible.
Invisible people won't show up when searched for with /who, /who #chan and not with /whois with wildcards, unless you can see them already by sharing a channel. To put it briefly, the invisible mode prevents people from finding you unless they know your exact nickname or are on the same channel as you. You can hide by changing nick, people can't mass-message everyone on your ISP and you are not visible from outside a channel. Today more than half of the nicks on Undernet are +i. When not set to +i, a fellow user can locate you through the /who or through the /names #channelname command. IRC Operators also have the ability to see someone regardless of whether they are invisible or not.
+s Receives server notices.
You receive various information sent between servers, mostly kills and G-lines, but also nickname collisions, invalid usernames, splits, etc. Setting yourself +s is a request to the server to send you all the generic server messages and information about oper kills. Undernet has stopped allowing regular users to use +s.
+w Receives wallops.
The IRCops can broadcast messages with this. Wallops are network wide messages sent by IRC Operators to each other. Any user who is set to +w will also be able to observe the conversation. Wallops have become quite extinct on undernet and are rarely sent. However, undernet did 'broadcast' the 10th Anniversary celebration on wallops, and may be used again for broadcasting large events.
+x is a usermode that allows logged in users to hide their real host, replacing it with @username.users.undernet.org. Log into X (/msg X@channels.undernet.org login Username Password) then type //mode $me +x or /mode nick +x. Hiding your hostname will shield you against several types of attacks often carried out on IRC, but be aware that taking part in a DCC chat or file transfer will reveal your IP address. The +x usermode remains as long as you’re connected to the IRC server, so that channel service can recognize you even if your server splits off and rejoins the net. No need to login to X all over again.
+o o nick
To op someone. A channel operator is designated by the @ next to their nick. Those with a @ can give and remove the +o mode.
+v v nick
Voice someone. When a channel is moderated (+m), only the channel ops and the channel voices can send messages to the channel. Only channel ops can add or remove voice.
+b To ban someone and display the reason.
+b stands for ban and allows a channel operator to deny a person (or a group of people) access to the channel. If someone is banned while in the channel, they will not be able to talk.
Syntax: /mode #channelname +/-b [hosts]
Example: /mode #mIRCHelp +b-bb *!*@annoying.pest.com
If you don't specify any hosts, you will be shown a list of channel bans, even if you're outside the channel.
+i Channel is invite-only.
Setting a channel to invite only will only allow those who have been invited to the channel (with the /invite command) to join the channel. On undernet, you must have ops to invite someone into a channel. To do this, use /INVITE nickname #channel.
+k Channel is keyed or password protected.
The key of the channel is a word that you must specify in your /join command when attempting to enter the channel. If the key was incorrect or if no key was specified then you will be denied access to this channel.
Syntax: /mode #channelname +/-k key
Example: /mode #mIRCHelp +k khaled
Anybody joining the channel will need to use the /join #channelname key command. In this case, it would be /join #mIRCHelp khaled.
+l Channel is limited to <limit> participants at the same time.
The channel limit allows you to specify the maximum number of users who can be in the channel at a time. Once this number has been reached, anyone who attempts to join the channel will receive an error message of the channel being full.
Example: /mode #channelname +/-l number (/mode #mIRCHelp +l 25)
+m Channel is moderated.
Only ops and voiced people can talk. A moderated channel only allows the ops (@) and voices (+) to send messages to the channel. All other messages will be blocked. Anyone who is present in a moderated channel and who is not an op or voice there will also be prevented from changing their nickname.
+n No messages or notices are allowed to be sent to the channel from anyone not in the channel.
The +n stands for no external messages. If +n is not set then it's possible for someone to send messages to the channel even if they are not present inside the channel. On undernet this mode is now almost universally used.
+p Channel is private.
A private channel is not shown on the /whois output unless the person requesting the /whois is also on the channel. A /LIST command will not return anything for a +p channel. A display of /NAMES will return Prv: *" in place of the channel name, but the users will be listed. In other words, a private channel prevents people on the outside from finding out its name, not who is in it.
+r Channel is restricted.
+r is a channel mode where only users who are logged into X (i.e. have a registered username) can join the channel. The command to set it is: /mode #channel +r .. to unset it: /mode #channelname -r. The +r channelmode can be overridden with an invite to a +r channel.
+s Channel is secret.
The only difference between a secret (+s) channel and a private (+p) channel is that you can do "/topic #channel" for +p channels from outside.
+t Topic change by ops only.
No one but the ops can change the channel topic.