IP packet header analysis and static routing

IP Baotou analysis

 IP Number of header bytes （20-60）  It's usually 20

edition ： Declare this ip Baotou is ipv4 still ipv6

The length of the first ： identification IP baotou The length of   How many bytes

Priority and service type ： Not commonly used , When using differentiated services , This field only works

Total length ： Whole IP package （ message ） The length of

  identifier ： The data from the upper layer arrives IP layer （ The network layer ） Will be sliced , Each segment is the same byte , And it will produce a ID It is used to identify that these pieces belong to the same ip package , Used for classification and segmentation , After segmentation, all data grams can be reinstalled correctly

The following figure   1 2

  sign ： There are three   The first bit is not enabled For ever 0  Second 1 It means not divided 0 It's for fragmentation The last one said Is this segment the last segment   If 1 Represents not the last fragment    Such as 001  It stands for segmentation And there are subsequent slices

  In the picture above 0  1480   2960   Segment bias

TTL： Time to live , Every time you go through a router, it's going to decrease 1,TTL Reduced to 0, Packets will be discarded ,TTL The function of value is to prevent packets from permanently circulating on the network , commonly TTL value 100 Here is Linux System ,100 The above is windows System

Agreement No ： Identify the upper layer protocol , The encapsulated data used to identify the upper layer is UDP still TCP

Head check sum ： Verify the three layers IP baotou

Teardrop attack is to target i The computer sending is damaged ip The package is then identified by the computer , sign , Slice offset , The computer crashed due to the failure of the reorganization
Firewall on ip The packet is forbidden to pass in pieces, and only 010 adopt , Then the network layer at the sending end will not be responsible for fragmentation , Instead, the software at the application layer splits the data and sends it , In order to achieve the purpose of no partition

  route

route ： Router is IP The process of Baotou selecting the path （ Select by routing table ）

Routing table

• A collection of routing entries maintained in a router
• The router chooses the route according to the routing table

The formation of routing table

• Direct connection network segment
  To configure IP Address , port UP state , Form a direct route
• Non direct network segment

  The following figure shows the routing tables of the two routers

c Representative direct connection   s It's static

Optimize routing tables   Use default routing s*0.0.0.0/0 When the router cannot find the routing entry of the target network in the routing table , The router forwards the request to the default routing interface

For example, the routing table on the left of the above figure You can replace the last five lines with s*0.0.0.0/0

Multiple lines can also be connected between routers

 

Static routing

conf t

         ip route       Target segment           Subnet mask              Next jump IP

  Such as     ip route          70.1.1.0        255.255.255.0      20.1.1.2

Default route

conf t

ip route 0.0.0.0 0.0.0.0 Next jump IP

Such as ：

ip route 0.0.0.0 0.0.0.0 20.1.1.2

Floating route ：

Add a space after the static or default route + Numbers （ Positive integer ）

ip route 0.0.0.0 0.0.0.0 20.1.1.2

ip route 0.0.0.0 0.0.0.0 30.1.1.2 2