Three-way handshake, waving the four basic network management command

Three-way handshake

　CLOSED state is not initiated before the client request, the server request is not received in the CLOSED state also

　　First, the client initiates a connection request to the server packet header time request packet SYN = 1, while selecting an initial sequence number SEQ = 1, then the client process passes FYN-SENT state, TCP SYN predetermined segment can not carry data, but it needs to consume a serial number

　　After the second time, the server receives the request message, if you agree connections, is a confirmation message. Acknowledgment packet should ACK = 1, SYN = 1, the acknowledgment number is ack = x + 1, and also initializes a sequence number seq = y for themselves, then the server into the SYN-RCVD state, it can not carry data packets , but the same consumption of a sequence number.

　　Third, the client receive a confirmation message, but also want to confirm the server side, there is an acknowledgment packet ACK = 1, ack = y + 1, its own serial number is seq = x + 1, this time, TCP connection established, the client enters eSTABLISHED, TCP provisions, ACK segment can carry data, if the data does not carry the serial number is not consumed

　　　　When the server receives confirmation client also enters ESTABLISHED state, after which the two sides can communicate

Four waving

　　The first client sends the last byte sequence number +1 disconnection request packet, when the packet header is FIN = 1, the sequence number of the last received data, then the client enters FIN-WAIT- 1 state, TCP packets a predetermined, FIN packets carry data without a need to consume sequence number

　　The second server receives the disconnection request message, a confirmation message is disconnected, ACK = 1, ack = u + 1, and bring its own sequence number seq = u, this time into the server CLOSE -WAIT state, TCP upper layers of the application process, the client has no data to send, but the server wants the client to send data, the client can also accept

　　　　When the client receives the server confirmation request, then the client enters the FIN-WAIT-2 state, waiting for the server to send packets to disconnect

　　After the third server transmits the final data is completed, the client sends a disconnection request FIN = 1 ack = u + 1, in which the server into the LSTE-ACK (acknowledgment last) state, the client waits for acknowledgment

　　第四次客户端收到服务器发来的断开报文后，必须确认，ACK=1,ack=w+1,而自己的序列号是seq=u+1，此时客户端进入了TIME-WAIT状态。注意此时TCP连接还没有断开，还要等待一段时间，当客户端撤销相应的TCP后，才进入CLOSED状态

　　　　服务器只要收到了客户端的确认报文后，立即进入CLOSED状态

子网划分

　　网络地址分为A,B,C,E类

　　　　A类　　0-127

　　　　B类　　128-191

　　　　C类　　192-223

　　　　D类　　224-239

　　　　E类　　240-254

　　主机数=2^主机ID -2

　　网络数=2^可变的网络ID位

ifconfig