IP 的地址自动补全
短 IP：
$ ping 127.1
PING 127.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.043 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.080 ms
ping x -> ping 0.0.0.x
ping x.y -> ping x.0.0.y
ping x.y.z -> ping x.y.0.z
短网址
http://1.1 -> http://1.0.0.1
http://192.168.1 -> http://192.168.0.1
