1. A method utilising the known Online Certificate Status Protocol (OCSP) to enable a network-accessible computing device utilising OCSP responder functionality to perform network access control and determine whether attempted communication should be allowed between other computing devices appropriately configured with known Internet Protocol Security (IPsec) with digital certificates and OCSP client software, useful to help protect computing devices from communicating with other computing devices that are considered to be untrustworthy, malicious, compromised or otherwise potentially harmful, with the determination as to whether such communication should be allowed based on a set of rules considering the role or roles of the computing devices attempting to communicate and whether the computing devices attempting to communicate have previously exhibited suspicious or undesirable behaviour, with the method consisting essentially of the following new steps performed by the computing device utilising OCSP responder functionality when it receives an OCSP request from a computing device implementing OCSP client functionality: determining both the role or roles of the OCSP client computing device issuing the OCSP request, and the role or roles of the computing device possessing the digital certificate questioned in the OCSP request; determining whether communication should be allowed between the OCSP client computing device that issued the OCSP request and the computing device possessing the digital certificate questioned in the OCSP request, based on whether the role or roles of these two computing devices mean that they have a legitimate reason to communicate, and based on whether either of these two computing devices have previously exhibited suspicious or undesirable behaviour; and, indicating to the OCSP client computing device that issued the OCSP request that communication between these two computing devices should not be allowed, by not sending any response or responding with an error message or an answer of “revoked”, or alternatively indicating to the OCSP client computing device that issued the OCSP request that communication between these two computing devices should be allowed by sending a response with an answer of “good”.