This matches the routing realm.  Routing realms are used in complex routing
setups involving dynamic routing protocols like BGP.
.TP
[\fB!\fP] \fB\-\-realm\fP \fIvalue\fP[\fB/\fP\fImask\fP]
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).
Both value and mask are four byte unsigned integers and may be specified in
decimal, hex (by prefixing with "0x") or octal (if a leading zero is given).