eth_open(port, srfd, get_userdata, put_userdata, put_pkt) finds an ethernet file descriptor that is free and associates the file descriptor with an ethernet port whose index within eth_port_table[] is port, eth_open()'s first parameter.
eth_open() is called by the ip code, the arp code and is called if an ethernet device file (e.g., /dev/eth) is opened directly.
Here are the relationships between various file descriptors and ports:
