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IP packet header analysis and static routing
2022-07-05 13:54:00 【Cwxh0125】
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
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