# Ethernet header contains a type fileld of 0x0800, which identifies the frame as an IPv4 frame.
# 若支持dual-stack的server既有IPv4又有IPv6。则IP层让server透明地既可处理IPv4又可处理IPv6.该server需要绑定到wildcard且未设置IPV6_V6ONLY socket option.
# UNP Page359 Figure 12.5 Summary of interoperability between IPv4 and IPv6 clients and servers.
# 尽可能用IPv6, since an IPv6 client can still communicate with IPv4 servers, and vice versa.