SSL/TLS

SSL stands for Secure Socket Layer, later this has been renamed to TLS (Transport Layer Security).

Why use SSL
When you use SSL for a connection all the traffic between the two endpoints is encrypted. Nobody can see/sniff/snoop the data (theoretically, anyway). This is important as traffic often includes things like passwords and other sensitive information.

You probably see https:// sites on the internet all the time. HTTPS is simply HTTP with SSL and it's used for banking, e-commerce sites and nowadays a lot of regular sites as well. The same technology (SSL) can be used for IRC.

How to use SSL
To use SSL on IRC you need two things:
 * An SSL port needs to be opened up on the server. The example configuration file opens up port 6697 for this (in the Listen block with listen::options::ssl)
 * You need an SSL-capable client

SSL-capable clients are widespread nowadays. mIRC, XChat, irssi all support SSL.

The UnrealIRCd team recommends to use SSL/TLS as much as possible. At the very least, use it for server to server traffic and for IRCOp client connections.

For real security you should validate certificates when you connect to servers and not blindly accept any SSL ceritificate. If you don't check them then you are still vulnerable to MitM attacks. That is, however, too off-topic to discuss here. See Wikipedia: Man-in-the-middle-attack for more background information. Clients like mIRC and XChat will show a popup prompt when a new (unknown) SSL certificate is detected, so that's good.