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Just to add a little to Mark's post, the default gateway should only be used when the destination host's IP is considered not on the same subnet as the source host's.
Since subnets are local to the host, and multiple subnets can be on the same "wire", you can have some "odd" situations, where one host uses the default gateway and the other doesn't, or hosts only use the gateway once and the gateway informs the source it doesn't need the gateway, but normally, as Mark noted, the source should just ARP for the destination's MAC.
PS: BTW, there can be other "odd" situations, that would preclude two hosts on the same subnet from directly communicating, like Cisco's private VLAN feature.
