How was the magic network ID value chosen?

Question

Messages on the Bitcoin network are identified by the magic value 0xD9B4BEF9, and on the testnet by 0xDAB5BFFA. Why were these values chosen?

Answer 1

main.cpp carries this comment:

// The message start string is designed to be unlikely to occur in normal data.
// The characters are rarely used upper ascii, not valid as UTF-8, and produce
// a large 4-byte int at any alignment.
unsigned char pchMessageStart[4] = { 0xf9, 0xbe, 0xb4, 0xd9 };