Shulou(Shulou.com)06/01 Report--

1. Network layer responsibilities:

Responsible for forwarding packets between different networks (based on IP address), not responsible for retransmission, not responsible for sequence.

Note: during the communication between the two hosts, each packet chooses a routing path separately.

2. The router can see the IP address, so it works at layer 3. Below the application layer (hubs, switches, routers) will not be poisoned, but will be affected by viruses, etc.

3. Related to the network layer:

Ip address: 10.0.0.0

Subnet mask: 255.0.0.0 (based on the subnet mask other than 255, it can be determined that the network segment is 0.0.0, and the ip of the gateway is the router is 10.0.0.1)

Gateway: 10.0.0.1

4. Computer communication process:

Starting with the analysis of the sender (pc4 in the figure above), the operation of the receiver (pc3) corresponds to the reverse:

1) Application layer: prepare the content to be sent and transmitted

2) Transport layer: segment and number the content of the application layer

3) Network layer: add the destination ip, and the source ip; uses its own subnet mask to determine which network segment its own address and the destination address are in. If it is the same network segment, use ARP protocol to obtain the other party's MAC address; otherwise, use ARP protocol to obtain the MAC address of the route (gateway). It is then passed to the link layer for transmission.

5. TCP/IP protocol stack:

Among them, the underlined part is the network layer protocol. It can be seen that the network layer is divided into three layers: ARP protocol, IP protocol and ICMP/IGMP protocol.

6. ARP/RARP (reverse ARP):

ARP:IP address-> MAC address (check whether the IP address is cached in the ARP cache. If so, return the corresponding MAC address directly. Otherwise, the local area network broadcasts the request packet whose destination MAC address is FF-FF-FF-FF-FF-FF and the ip address is the target ip, and the target machine that matches the ip address replies the response packet containing its own MAC address, thus obtaining the MAC address of the target host and storing the mapping in the ARP advanced cache for next use)

Note: ARP solves the relationship between ip address and mac address mapping of hosts and routers on the same local area network. For communication between different Lans, first find the gateway mac, and then the rest of the work will be done by the next network.

RARP:MAC address-> IP address (the host reads the MAC address from the network card, and then sends a broadcast packet of the RARP request on the network, requesting the RARP server to reply to the IP address of the PC; the RARP server receives the RARP request packet, assigns the IP address to it, and sends the RARP response to the host; after the host receives the RARP response, it uses the resulting IP address for communication. ).

Get ARP advanced cache: arp-a

Edit ARP Advanced Cache: arp-s ip address mac address

Clear ARP Advanced Cache: local connection-> repair

ARP spoofing: using ARP broadcast mode, using fake MAC address to intercept and forward data packets.

Tools: network marshals, P2P Terminators (return incorrect gateway MAC), ARP firewall

7. ICMP protocol: Internet control message protocol

IGMP protocol: Internet Multicast Management Protocol (for Multicast and Multicast, such as sending messages on TV at the same time, can not delay changing schedule, etc.) ), which can be used for video, etc., saving a lot of bandwidth resources.

8. Ping command: send ICMP packets to troubleshoot the network.

Time: delay

Ttl: prevents packets from being looped over the network, with up to 64 routes. For each route, ttl minus 1 (the default value of ttl varies with different operating systems, linux is 64, windows is 128, Unix is 255).

Example:

Ping /?

Ping 192.168.1.2-t (non-stop)

9. PathPing: track the packet routing path and calculate the packet loss

Tracert: tracks the packet routing path.

10. IP packet structure:

The protocol field indicates the protocol used for the content of the packet, such as ICMP:1,IGMP:2,TCP:6,UDP:17. The meaning and rules of other parts can be baidu.

Note: the maximum size of a single packet at the network layer is 65535 bytes, and the range of link layer packets is 46-1500 bytes. Therefore, the packet needs to be sliced, and if not sliced, the content is up to 1480 bytes.

11. IP protocol:

RIP: a distributed routing protocol based on distance vector.

Three main points: only exchange information with neighboring routers; the information exchanged is all the information currently known to this router, that is, its own routing table; and routing information is exchanged at regular intervals.

OSPF

12. Static routing, dynamic routing:

Static routing: the administrator sets the routing table (you need to set the next hop for all networks without Zhaopin, which is only suitable for small-scale networks)

View the routing table: show ip route

Edit routing table (operate router): config t

Ip route 192.158.1.0 255.255.0.0 172.16.2.0

Dynamic routing: based on the IP protocol, it can identify and update itself (router Zhou Qi broadcasts the routing table and updates the routing information every 30 seconds, with a maximum hop count of 16).

13. Network unblocked conditions:

Packets go back and forth. If there is a problem, check the routing table and gateway.

Note: this series of articles is to view the notes of Mr. Han Ligang's computer network basic course.

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