OSI Seven-layer reference Model-Network layer

Network layer

Carry on the logical address addressing, realize the communication between different networks, define the IP address, choose the best path for data transmission, and the router works in the network layer.

I. IP packet format

Priority and service type (8 bits): priority and service type

Identifier, flag, segment offset: these fields are used to identify the packet, so that when the data reaches the destination, it will not be out of order.

Header length: the minimum length of ip packet header is 20 bytes

Total length (16):

Protocol number: UDP is 17. TCP is 6.

First checksum:

TTL: lifecycle field. When the TTL value of a router is minus 1, when 0, the packet is dropped. To prevent a packet from looping indefinitely in the network.

II. Network layer protocols

1. ARP protocol

Resolve a known IP address to an MAC address

ARP-an in Windows system: view the ARP cache table

2. Agent ARP

IP address resolves to MAC address of gateway interface

3 、 ICMP

Internet Control message Protocol

1) the connection is established successfully:

Reply from 192.168.1.1:bytes=32 time