Network basic concept

最近在設定 iptables ， 要有一些基本知識，就翻了些網頁，順便做點筆記，以後忘掉時才有東西可查

Ref：鳥哥私房菜 - 網路基礎概念、Network Sorcery - IP、RFC791、酷學園 - TCP 與 UDP、Network Sorcery - TCP、RFC793

A Class :
  0xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx  ==> NetI_D 的開頭是 0
  |-Net-ID|-----------Host-ID-------|
B Class :
  10xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx  ==> NetI_D 的開頭是 10
  |-----Net-ID-----|----Host-ID-----|
C Class :
  110xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx  ==> NetI_D 的開頭是 110
  |----------Net-ID---------|-Host-ID|

三種分級在十進位的表示：
A Class :   0.xx.xx.xx ~ 126.xx.xx.xx
B Class : 128.xx.xx.xx ~ 191.xx.xx.xx
C Class : 192.xx.xx.xx ~ 223.xx.xx.xx

Network : Host-ID all '0'  , first IP in that that network
Broadcast : Host-ID all '1' , last IP in that that network
Netmask : Net-ID all '1' , Host-ID all '0'
Network / Netmask :  Network / bits , bits = How many Net-ID bits

example.
a.
192.168.0.0 / 24
available ip -- 192.168.0.0 ~ 192.168.0.255
b.
192.168.0.128 / 25  is the same with 192.168.0.128/255.255.255.128
25 bits fo Net-ID , available ip -- 192.168.0.128 ~ 192.168.0.255

private IP
A Class：10.0.0.0    - 10.255.255.255
B Class：172.16.0.0  - 172.31.255.255
C Class：192.168.0.0 - 192.168.255.255

three-way handshake：
1.
Server is listening , Client sent TCP packet (SYN , seq = 1200)
# 1200 is the initial sequence number ,
# server expect the first data byte seq = 1200+1
# server also replies a sequence number to client
2.
Server replies packet ( SYN ,ACK, seq = 2340 ack = 1201  )
# Client will also expect server to sent data  for seq = 2340+1
3.
Client replies packet (ACK , ack = 2341, seq = 1201 )