IP packet structure and OSI layer 3 Network layer in Phase 5

one。 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