cmSysExpIpSubNet is a read-write IP address that the system interprets as a class C subnet (A.B.C.0) used internally to auto-assign IP addresses to expansion SCMs so they can load their software. An administrator configures this before adding expansion shelves, making sure the chosen subnet doesn't overlap with any addresses used elsewhere on the network, since the description explicitly warns operators to prevent that conflict and filter external access to it. For example, setting cmSysExpIpSubNet to 10.0.99.0 would let the system hand out addresses like 10.0.99.1 and 10.0.99.2 to newly installed expansion SCMs during their boot-load process.