Apparatus for connecting three or more computer networks (101, 102, 103), the apparatus comprising: a. three or more network interface machines (201, 202, 203) each arranged to be connected to a respective computer network with a bidirectional communications link (105, 106, 107) enabling the network interface machine to receive data from and transmit data to the computer network; b. at least two content checkers (210,211) connected together with the three or more network interface machines and configured to enable data to be transmitted from one network interface machine to another; wherein c. each network interface machine is connected to an associated first content checker by way of a first respective unidirectional communications link configured to enable data to be transmitted from the network interface machine to the associated first content checker, but not received by the network interface machine from the associated first content checker; d. each network interface machine is connected to an associated second content checker by way of a second respective unidirectional communications link configured to enable data to be transmitted from the associated second content checker to the network interface machine, but not received by the associated second content checker from the network interface machine; e. the apparatus is configured such that data transmitted from each network interface machine to any other network interface machine must be transmitted from the transmitting network interface machine via its first respective unidirectional communications link to its associated first content checker and data transmitted to each network interface machine from any other network interface machine must be transmitted to the receiving network interface machine from its associated second content checker via its second respective unidirectional communications link; and f. each network interface machine is arranged to transmit flow control data to another network interface machine thereby to regulate transmission of data between the network interface machines. a. three or more network interface machines (201, 202, 203) each arranged to be connected to a respective computer network with a bidirectional communications link (105, 106, 107) enabling the network interface machine to receive data from and transmit data to the computer network; b. at least two content checkers (210,211) connected together with the three or more network interface machines and configured to enable data to be transmitted from one network interface machine to another; wherein c. each network interface machine is connected to an associated first content checker by way of a first respective unidirectional communications link configured to enable data to be transmitted from the network interface machine to the associated first content checker, but not received by the network interface machine from the associated first content checker; d. each network interface machine is connected to an associated second content checker by way of a second respective unidirectional communications link configured to enable data to be transmitted from the associated second content checker to the network interface machine, but not received by the associated second content checker from the network interface machine; e. the apparatus is configured such that data transmitted from each network interface machine to any other network interface machine must be transmitted from the transmitting network interface machine via its first respective unidirectional communications link to its associated first content checker and data transmitted to each network interface machine from any other network interface machine must be transmitted to the receiving network interface machine from its associated second content checker via its second respective unidirectional communications link; and f. each network interface machine is arranged to transmit flow control data to another network interface machine thereby to regulate transmission of data between the network interface machines.