Notes:
1. ICMP IP IGMP 均为网络层
2. 以太网区分上层协议利用类型域(16 bits)，IP 区分上层协议依靠protocol字段， TCP/UDP区分上层协议利用port(2 bytes)
3. 135.252.*.* 属于B类地址
4. As a general rule, TCP servers are concurrent, and UDP servers are iterative, but there are a few exceptions. TCP服务器多为并发型，UDP多为迭代型
5. An example of the difference between an Internet-wide service and a Unix-specific service is the difference between Telnet and Rlogin.
6. Most TCP/IP implementations allocate ephemeral port numbers between 1024 and 5000. 客户端的临时端口号
7. well-known ports in /etc/services
8. when the same service is provided using both TCP and UDP, both port numbers are normally chosen to be the same.
9. The lowercase internet means multiple networks connected together, using a common protocol suite. The
uppercase Internet refers to the collection of hosts (over one million) around the world that can communicate
with each other using TCP/IP. While the Internet is an internet, the reverse is not true.