-- Channel And User Modes --

Channels can have additional constraints, which can be set by the MODE command. To understand this, recall that the first person that joined a channel effectively creates it and is, at least initially, in charge of the channel. He or she becomes a Channel Operator or chanop or 'op'. He can set constraints to the channel and make other people chanops as his wishes. The command that allows channel operators to change channel modes, or any user to change their personal mode is :

/MODE {channel|nickname} [{+|-}{modechars} [{parameters}]]
A + or - sign determines whether the mode should be added or deleted.

Channels can be moderated, secret, private, with a limited number of users, anonymous, invite-only, topic-limited, with a list of banned users...

/mode {channel} +b {nick|address} ban somebody by nickname or address mask (nick!account@host)
/mode {channel} +i channel is invite-only
/mode {channel} +l {number} channel is limited, with {number} users allowed maximal
/mode {channel} +m channel is moderated, only chanops and others with 'voice' can talk
/mode {channel} +n external /MSGs to channel are not allowed
/mode {channel} +p channel is private
/mode {channel} +s channel is secret
/mode {channel} +t topic limited, only chanops may change it

/mode {channel} +o {nick} makes {nick} a channel operator
/mode {channel} +v {nick} gives {nick} a voice

The MODE command also allows you to modify your personal parameters, your "user mode". You can check your usermode with the command "/MODE {yournick}". Note that user mode +i may be the default on some servers, in order to protect privacy of users. This should not be seen as a problem, since any user can change his/her personal mode from whatever defaults a server may set.

/mode {yournick} +r sets your access to restricted. You cant do a -r mode change !!
/mode {yournick} +i makes yourself invisible to anybody that does not know your nickname
/mode {yournick} +o gives IRC-Operator status, can only be set by IRC-ops with OPER
/mode {yournick} +s lets you receive server notices
/mode {yournick} +w makes you receive wallops; messages to IRC-Ops (abused and deprecated)


-- What do these Channel and User Modes mean? --

A channel is PUBLIC by default. Anyone can notice a public channel, see its users and join the conversation. In a list of channels you can see a public channel's topic. When someone is on a public channel, he can be easily found by all other users as long as his personal user mode is not set to invisible (see below).

An INVITE-ONLY channel can only be joined if you are invited by one of its channel operators.

PRIVATE channels turn up normally in the channels list. People can see you are on a private channel somewhere, but they can never find out on -which- private channel you are unless they search all (private) channels by brute force. With the names command your nickname will not show up, but it will with the /who {channel_name} command unless you hide by setting your personal user mode to 'invisible'.

SECRET channels do not show up in a channels list and you cannot find out its topic unless you join it. If you are on a secret channel, someone who is not on the same channel can't see that you are there, regardless what your personal user mode is set to. Your name does not show up in a names list of people on IRC if you are on secret channels only.

Your user mode can be set to INVISIBLE meaning that other people cannot find you by searching on IRC unless they know your exact nickname. No (wildcarded) search on you by the /who command on your IP Address or real name will deliver your current nickname or other likewise info to others.

When your user mode is restricted you do not have the power normal channel operator have on channels (you cant use /mode, /kick, ..etc.) and you can not change nickname (/nick). But you can chat normally.