Certain channel modes can block people from joining channels. In alphabetical order, they are:
1. +b - Channel ban.
Upon trying to join, you will get a message saying that you are banned from the channel. To get unbanned, you can try asking a channel operator to unban you (find one with the /names #channel command). If you are already banned despite having never joined the channel before, the channel operators may have set ISP-wide bans.
Do NOT evade the ban via vhosts and other methods, for that is a network offense and will lead to a network ban.
Do NOT ask IRC operators to intercede. We do not mediate channel issues, and channel bans, including ISP-wide bans, are considered a channel issue.
2. +i - Invite only.
You will get a message saying that the channel is invite only. A channel operator has to invite you. Again, you can try finding a chanop that is willing to invite you by using the /names command.
3. +k - Channel key.
You will get a message saying that you have a bad channel key. This is equivalent to having a password to enter the channel.
4. +l - Channel user limit.
You will get a message saying that the channel is at its user limit and therefore cannot allow you to connect immediately. If the channel is not +s, you can /list #channelname to see if anyone has left the channel and freed up a spot for you.
5. Jupitered channels.
You will get a message saying that the channel is temporarily unavailable. These channels have been permanently suspended by network services staff due to various abuse by the channel.
6. +O - Opers only
You will get a message saying that you can't join the channel because the +O mode is set. Only operators can join these channels.
7. +R - Registered only
You will get a message saying that you can't join the channel because it's for registered users only. Read this topic if you don't know how to register your nickname
8. +S - SSL only
You will get a message saying that you can't join the channel because you're not connected via SSL.
Original post by mink.