A method, system and computer program product providing port scrambling for securing communications in internal computer networks are disclosed. A transformation function is applied on an identifier of a first port at which an outgoing communication is designated to be received, whereby an identifier of a second port the outgoing communication is directed to be received at is obtained. The transformation function depends on at least one parameter shared among a plurality of devices in a computer network, whereby a device receiving the communication at the second port is enabled to apply an inverse transformation function on the identifier of the second port to obtain the identifier of the first port and redirect the communication thereto. The transformation function is applied in condition that transmittal of the outgoing communication was requested by an application program listed in a list of authorized application programs for the plurality of devices.