A network system that, by synchronizing and updating the NAPT tables of a center-side router and a base-side router, reduces NAPT table management load and can prevent setting errors and reduce workload. According to the present invention, on the basis of equipment information for communication equipment that is connected to a base-side router, a center-side router sets a virtual port number that will be used in communications between the center-side router and the base-side router, uses the virtual port number to renew its own NAPT table, and transmits the virtual port number to the base-side router. The base-side router uses the virtual port number received thereby to update its own NAPT table.
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