Accessing a network host computer from outside the network with improved security

An authorised user of a host network 11 such as an office network has a pager in the form of a PC card slotted into a user terminal T 1 . To access office network 11 from outside, for example through a public switched network 12, the authorised user is given a one-time password by host computer 13 which is stored in the PC card pager. The PC card pager is then removed from the user terminal T 1 and inserted into a mobile terminal MT 1 (for example a notebook computer). Using the mobile terminal the user requests access to the network 11 from outside by sending a username and encrypted authentication information comprising pager ID and one-time password to the host. The user is authenticated and allowed to log in to the network if this information corresponds to information registered in the host computer. Encryption is performed by the PC card pager using a hash function, the authentication information and a random number generator. The mobile terminal may have a wireless or wired connection to the network. A desktop computer may also be used. The one-time password may be transferred to the PC card pager through the paging system.