1. A method of admitting a user session to a virtual network, the method comprising: receiving, at a mobility management function in a core network domain, an attach request from a User Equipment associated with the virtual network; transmitting an instruction, from the mobility management function towards the User Equipment, to begin an authentication challenge with an Authentication Function, the Authentication function both associated with and resident within the virtual network which is both associated with the User Equipment and distinct from the core network; and transmitting, by the mobility management function, an instruction to admit the user equipment to a user session in the virtual network in response to receipt of an indication of successful completion of the authentication challenge.