Virtual Internet
In fact, the computer network is intricate
The physical equipment uses the IP protocol, shielding the differences between physical networks
When the hosts in the network use the IP protocol to connect, you do not need to pay attention to the network details
IP protocol
MAC address (48 bits) is usually in hexadecimal
IP addresses are usually expressed in dotted decimal
For a computer, the IP address is variable and the MAC address is unique.
The ip address is represented by four dotted decimals, and the range of each dotted decimal is 0 to 255
4-bit header length: the unit is 4 bytes, the maximum is 15x4 = 60 bytes
16-bit total length: header length + data length
8-bit protocol: indicate what protocol is specific to the IP protocol (tcp, udp, etc.)
Header checksum: check whether the header is wrong
Routing table
IP protocol forwarding process
So how do you know E's MAC address based on E's IP address?
This uses the ARP cache table in the ARP protocol, which caches the mapping of IP addresses to MAC addresses.
The records in the ARP cache table are not permanently valid and have a certain period.
F to C is the same as E to F
At each hop of the data frame, the MAC address changes (source and destination)
The IP address of the IP datagram does not change at each hop (source and destination)