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5.2.4. About Port Offsets for Socket Binding Groups
Port offsets are a numeric offset added to the port values given by the socket binding group for that server. This allows a single server to inherit the socket bindings of the server group that is belongs, with an offset to ensure that it does not clash with the other servers in the group. For instance, if the HTTP port of the socket binding group is 8080, and your server uses a port offset of 100, its HTTP port is 8180.