SIP ALG is a Session Initiation Protocol Application Layer Gateway. This feature is commonly found in routers and firewalls and is designed to assist in the setup and management of VoIP calls. SIP is a protocol used to establish and control VoIP calls, and it relies on specific port numbers and packet formats. SIP ALG is intended to solve problems by inspecting and modifying SIP packets as they pass through the router or firewall (to help ensure proper handling of the call setup and signaling). It can also translate private IP addresses to public ones and open/close firewall ports as needed for the call.