in code/include/swoc/IPAddr.h [942:948]
inline IP6Addr::IP6Addr(IP4Addr addr) {
_addr._store[MSW] = 0;
_addr._quad[QUAD_IDX[4]] = 0;
_addr._quad[QUAD_IDX[5]] = 0xffff;
_addr._quad[QUAD_IDX[6]] = addr.host_order() >> QUAD_WIDTH;
_addr._quad[QUAD_IDX[7]] = addr.host_order();
}