一.数据链路层功能
The error frame is your reply data frame check
two .MAC address 
your MAC address 48-bit binary number
three Ethernet frame format, the MAC address header data 
preamble: port accepts and revenue, the frame start delimiter: this start and end by identifying the binary language, computer
destination address: MAC address, source address you send someone else host: host your own MAC address, data: data layer of your application. Frame Check Sequence: You check the data frame is complete.
Overview of the four switches. 
Two-story: the data link layer, three layers: the network layer, look at these series beginning with the letter, the bigger the better. 
Second type: There are 24 ports, dual media: an optical fiber, electrical signals can, Gigabit Ethernet speed.
V. principle switch 
first to our Host A and Host B through a switch to transfer data
to a new switch do not know anything, A host needs to broadcast their own data frames transmitted to each port, which includes your source MAC address and MAC address of the host B, this time will switch you to learn the source address of the host a and 1-port recorded in the entry in. Host B a look at this is my MAC address immediately to respond, the MAC address of the own source address and host A hair out, this time switch and start learning, the source MAC address and port records Host B in the entry in this time the two hosts on the data transmitted through the switch.
Why do we use a switch: A host can only pick a line and a host transferring data exchange, with the switch multiple hosts can exchange data transmission.