Transfer byte-buffer with ENC28J60
Assuming you are using the EtherCard library you can send a UDP packet with:

ether.sendUdp(packet_data, length, srcPort, hisip, dstPort );   


Assuming you have already set up your source port and have the destination IP address and port in hisip and dstPort.

The prototype for the function is:

void sendUdp (char *data,uint8_t len,uint16_t sport, uint8_t *dip, uint16_t dport);


The example udpClientSendOnly.ino with the library shows you UDP sending in action.