A network device comprising: a mounting rail of the network device providing a single device mounting surface or opposed device mounting surfaces for receiving and supporting modules (30, 32), wherein the single device mounting surface or each of the opposed device mounting surfaces having at least one mounting location, each of the at least one mounting locations defined by one or more device reference locators that define a mounting direction relative to that mounting location, and each mounting location including a device connector attached to a device printed circuit board, PCB, and positioned to have a connecting direction parallel to the mounting direction; at least one module (30, 32) for mounting to one of the mounting locations, wherein the module (30, 32) includes corresponding one or more module reference locators each adapted to engage one of the one or more device reference locators when the module (30, 32) is positioned to be mounted to the mounting location, and wherein the module (30, 32) includes a module connector attached to a module PCB and positioned to have a connecting direction parallel to the mounting direction; wherein the module connector and the device connector are positioned such that when the one or more module reference locators are each engaged with the corresponding device reference locators, and the module (30, 32) is advanced in the mounting direction towards the mounting location, the module connector and the device connector connect as the module (30, 32) mounts the mounting location to connect the module (30, 32) to the network device, wherein the module (30, 32) is constrained to move in a direction perpendicular to the device mounting surface of the mounting rail, parallel to a mounting axis of the reference locators. a mounting rail of the network device providing a single device mounting surface or opposed device mounting surfaces for receiving and supporting modules (30, 32), wherein the single device mounting surface or each of the opposed device mounting surfaces having at least one mounting location, each of the at least one mounting locations defined by one or more device reference locators that define a mounting direction relative to that mounting location, and each mounting location including a device connector attached to a device printed circuit board, PCB, and positioned to have a connecting direction parallel to the mounting direction; at least one module (30, 32) for mounting to one of the mounting locations, wherein the module (30, 32) includes corresponding one or more module reference locators each adapted to engage one of the one or more device reference locators when the module (30, 32) is positioned to be mounted to the mounting location, and wherein the module (30, 32) includes a module connector attached to a module PCB and positioned to have a connecting direction parallel to the mounting direction; wherein the module connector and the device connector are positioned such that when the one or more module reference locators are each engaged with the corresponding device reference locators, and the module (30, 32) is advanced in the mounting direction towards the mounting location, the module connector and the device connector connect as the module (30, 32) mounts the mounting location to connect the module (30, 32) to the network device, wherein the module (30, 32) is constrained to move in a direction perpendicular to the device mounting surface of the mounting rail, parallel to a mounting axis of the reference locators.