This matches the routing realm.  Routing realms are used in complex routing
setups involving dynamic routing protocols like BGP.
.TP
.BI "--realm " "[!] " "value[/mask]"
Matches a given realm number (and optionally mask). If not a number, value
can be a named realm from /etc/iproute2/rt_realms (mask can not be used in
that case).
