In a mainframe class data processing system having multiple logical partitions and a port to a network, a host-network interface is established for reducing network overhead at the multiple partitions. The host-network interface includes, for example, a host channel connection coupling the multiple partitions of the host system to a communications adapter having a network device driver for each network coupled to the adapter. The adapter also includes an address resolution protocol (ARP) cache designed to hold predetermined media headers for the clients coupled to the network(s) for use in forwarding an internet protocol (IP) datagram across the network to one of the clients from a partition of the host system. If not predetermined, the adapter is provided with a capability to dynamically determine the needed media header for transmission of the IP datagram across the network to the client. In Ethernet format, this dynamically determining includes forwarding an Ethernet 802.3 ARP request and an Ethernet DIX ARP request from the IP datagram. Provision is also made for partition-to-partition communication of IP datagrams by storing IP addresses of the logical partitions as HOME addresses in the ARP cache of the adapter.