Additional Prefixes
You can specific additional prefixes in addition to that of the addr-prefix parameter you configure for a realm. For example, you can configure a global realm with additional address prefixes to specify the IP addresses (or ranges of addresses) of the media gateways that are allowed to send and receive early media. This overrides the SIP interface realm’s early media blocking settings.
You can also enter additional prefixes in non-global realms. These additional prefixes function the same as would multiple values in the addr-prefix parameter (which only takes one value), except addresses in additional-prefixes are not used for SIP NATs.