What's the meanings of the Routing tables' data in the Mac OS?

I use netstat to check the routing tables: $ netstat -rn Routing tables Internet: Destination Gateway Flags Refs Use Netif Expire default 10.10.10.1 UGSc 20 16 en0 10.10.10/24 link#4 UCS 1 0 en0 10.10.10.1/32 link#4 UCS 1 0 en0 10.10.10.1 bc:67:1c:40:3:f6 UHLWIir 21 14 en0 1193 10.10.10.104/32 link#4 UCS 0 0 en0 10.10.10.107 a8:6b:ad:6b:c4:a5 UHLWI 0 0 en0 820 127 127.0.0.1 UCS 0 0 lo0 127.0.0.1 127.0.0.1 UH 54 2734893 lo0 169.254 link#4 UCS 0 0 en0 172.16.111/24 link#11 UC 1 0 vmnet1 192.168.220 link#12 UC 1 0 vmnet8 224.0.0/4 link#4 UmCS 2 0 en0 224.0.0.251 1:0:5e:0:0:fb UHmLWI 0 0 en0 239.255.255.250 1:0:5e:7f:ff:fa UHmLWI 0 784 en0 255.255.255.255/32 link#4 UCS 0 0 en0 But I have several questions about this: 1. I do not understand the Flags, Refs, Use and Expire's meaning. 2. You see the 10.10.10.1 bc:67:1c:40:3:f6, so does this mean the IPv4 can divert to IPv6 address directly? 3. Whats the link#4 and link#12 meaning in the *Gateway* field? 4. What's the Expire field meaning?