>>62
RFC 791のこの部分じゃなくて？
> Addresses are fixed length of four octets (32 bits). An address
> begins with a network number, followed by local address (called the
> "rest" field). There are three formats or classes of internet
> addresses: in class a, the high order bit is zero, the next 7 bits
> are the network, and the last 24 bits are the local address; in
> class b, the high order two bits are one-zero, the next 14 bits are
> the network and the last 16 bits are the local address; in class c,
> the high order three bits are one-one-zero, the next 21 bits are the
> network and the last 8 bits are the local address.