1. The IP header is encapsulated at least 50 bytes
*
Version: Version number ipv4
*
Header length:
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Priority and service type: Confirm priority
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Total length: 65535 bytes of the entire ip packet length
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Identifier: Identification function
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Mark segment offset Migration: related to IP fragmentation
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TTL: life cycle, every time a route passes through TTL-1, the packet is lost when it is 0 (to prevent routing loops and network storms)
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Protocol Number: What is the upper layer protocol
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Checksum: Detection Whether the data is damaged
2. ICMP: Internet control message protocol
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ping uses the ICMP protocol, the firewall sometimes blocks the ICMP protocol
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ICMP is a network layer protocol (controversial)
3. ARP
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IP→MAC address protocol, Pc1 communicates with Pc2, ARP broadcasts to get the corresponding MAC address
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ARP cache table: IP and MAC corresponding side on the host
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Pc1 initializes ARP request → broadcast FFFFFFFFFFFF → pc2 cache table records pc1 IP and MAC Address correspondence → pc2 reply pc1 → pc1 cache table record
Correspondence between pc2IP address and MAC address → Unicast
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ARP attack: PC in the same broadcast domain sends a false response to the host, the host updates the ARP cache table, and then the communication fails
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ARP spoofing: Forward the traffic, and then control and view the traffic Private information
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defense method: manually write the target IP and MAC address into the ARP cache table
4. Principle and configuration of static routing
* 路由器功能：路径选择（最佳路径而非最短路径）
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There are routing entries in the routing table for path selection (the corresponding relationship between network disconnection and interface/next hop routing is recorded in the routing table)
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Two network cards in the router are in different network segments
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①Direct connection: only need to configure the network card information and configure the IP can use
②Indirect connection: add manually
5. Features of static routing
* 手动配置，单向
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Lack of flexibility
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can avoid routing loops
6. Dynamic/default routing
* When the
router routing table cannot find a network routing entry, the router forwards the request to the default routing interface (stub network)
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When the destination address is a huge collection of addresses, the default route can be set, and then the next one Routing processing.
*The
default route can only be set on the router connected to the PC
7. The distinction between three types of tables
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ARP cache table IP address ⇆ MAC address terminal device
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MAC address table MAC address ⇆ interface switch
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routing table target network disconnect ⇆ interface / next hop router / PC
8. Static routing and default routing configuration
9. Static/default routing configuration case
R1, R3 (stub network) use the default route, R2 uses the static route to
configure static, configure the IP/gateway and MAC address of the host and routing network card before the default.
- R1
Router>en
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#host R1
R1(config)#int f0/0
R1(config-if)#ip add 192.168.10.1 255.255.255.0
R1(config-if)#no sh
R1(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
R1(config-if)#int f0/1
R1(config-if)#ip add 10.0.0.1 255.0.0.0
R1(config-if)#no sh
R1(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/1, changed state to up
R1(config-if)#exit
R1(config)#ip route 0.0.0.0 0.0.0.0 10.0.0.254
R1(config)#
2.R2
Router>en
Router#host R2
^
% Invalid input detected at ‘^’ marker.
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#host R2
R2(config)#int f0/0
R2(config-if)#ip add 10.0.0.254 255.0.0.0
R2(config-if)#no sh
R2(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
R2(config-if)#int f0/1
R2(config-if)#ip add 20.0.0.1 255.0.0.0
R2(config-if)#no sh
R2(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/1, changed state to up
R2(config-if)#exit
R2(config)#ip route 192.168.10.0 255.255.255.0 10.0.0.1
R2(config)#ip route 192.168.20.0 255.255.255.0 20.0.0.254
R2(config)#
3.R3
Router>en
Router#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#host R3
R3(config)#int f0/0
R3(config-if)#ip add 20.0.0.254 255.0.0.0
R3(config-if)#no sh
R3(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
R3(config-if)#int f0/1
R3(config-if)#ip add 192.168.20.1 255.255.255.0
R3(config-if)#no sh
R3(config-if)#
%LINK-5-CHANGED: Interface FastEthernet0/1, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up
R3(config-if)#ip route 0.0.0.0 0.0.0.0 20.0.0.1
R3(config)#