Methods, modems, and systems for blocking data transfers unless including predefined communications to provide access to a network

The transfer of data through a modem can be blocked in the modem during a safe mode unless the data includes predefined communications such as a request for a network address to maintain access to the network for the host system or a response to the request that includes the network address. Accordingly, the safe mode can protect a host system from unauthorized access from the network, while allowing the network service to be maintained for the host system. In particular, requests for renewals of leases, such as Dynamic Host Configuration Protocol (DHCP) requests and responses thereto, on Internet Protocol (IP) addresses used by the host system may not be blocked by the modem during safe mode. Furthermore, requests and response for addresses of systems on the network to which the DHCP requests are transmitted, such as Address Resolution Protocol (ARP) requests and responses thereto, may also not be blocked during safe mode. Moreover, in some embodiments according to the present invention, the blocking is provided at the modem so that multiple host systems can be protected by the modem. Related methods, modems, and systems are disclosed.