Hostmask
Users are distinguished by
<nickname>!<ident>@<hostname/IP>
(ident is always "Mibbit" if they use a Mibbit client on a webirc enabled server, the hostname or IP usually is cloaked), e.g.,
theNick!Mibbit@9C75DB05.72B5B948.80A677AE.IP
or
some1!xyz@mib-CAC25864.mbb.telenor.rs
Those tripplets are called "hostmasks".