Server protocol:Server ID
Server id's (SID's) are unique id's that identify a server. They start with a digit and are followed by 2 alphanumerical characters.
When a server is introduced and SIDs are enabled, the syntax is as follows:
:prefix SID servername hopcount sid :serverinfo
Example leaf introduction:
:00A SID leaf.domain.tld 2 00B :Mynetwork Leaf
Other examples of valid server IDs: 001, 0AA, 9ZZ, ..
You can (theoretically) use a SID anywhere a server name is used, like:
:server.name NOTICE #chan :Hello
:001 NOTICE #chan :Hello
Older UnrealIRCd versions used server numerics which was a very similar feature but required base64 encoding/decoding. With SID's this is not necessary.