How Arp works in Linux


To understand how ARP translates IP addresses to MAC addresses, consider the following example. Assume host A has an IP address of and a MAC address of fc:99:47:49:d4:a0, and wants to send a packet to host B with an IP address of Note that the network number is the same for both hosts, so host A is able to send frames directly to host B.

The first time host A attempts to communicate with host B, the destination MAC address is not known. Host A makes an ARP request to the local network. The request is a broadcast with a message like this:

To: everybody (ff:ff:ff:ff:ff:ff). I am looking for the computer who has IP address Signed: MAC address fc:99:47:49:d4:a0.

Host B responds with a response like this:

To: fc:99:47:49:d4:a0. I have IP address Signed: MAC address 54:78:1a:86:00:a5.

Host A then sends Ethernet frames to host B.

You can initiate an ARP request manually using the arping command. For example, to send an ARP request to IP address

$ arping -I eth0
ARPING from eth0
Unicast reply from [54:78:1A:86:1C:0B]  0.670ms
Unicast reply from [54:78:1A:86:1C:0B]  0.722ms
Unicast reply from [54:78:1A:86:1C:0B]  0.723ms
Sent 3 probes (1 broadcast(s))
Received 3 response(s)

To reduce the number of ARP requests, operating systems maintain an ARP cache that contains the mappings of IP addresses to MAC address. On a Linux machine, you can view the contents of the ARP cache by using the arp command:

$ arp -n
Address                  HWtype  HWaddress           Flags Mask            Iface                ether   52:54:00:12:35:03   C                     eth0                ether   52:54:00:12:35:02   C                     eth0

The meanings of SR、LRM、LR、ER and ZR

SR、LRM、LR、ER、ZR are terms used in fiber optic communications, stand for the transmission distance of the 10G modules.

SR stands for Short Range, these transceivers support link length of 300m over multi-mode fiber and use 850nm lasers.

LRM means Long Reach Multimode, these transceivers support distance up to 220m over multi-mode fiber and use 1310nm lasers.

LR means Long Reach, these transceivers support distance up to 10km over single mode fiber and use 1310nm lasers.

ER means Extended Reach, the data rate of these transceivers support distance up to 40km over single mode fiber and use 1550nm lasers.

ZR also stands for Extended Reach which can transmit 10G data rate and 80km distance over single mode fiber and use 1550nm lasers.

Problem with SSD corruption on power failure .. flactuation

only enterprise-class SSDs can be relied upon (at all) for safe behavior on power fail.

The enterprise-class SSDs have super-capacitors that store enough power to write all data stored in the RAM within the SSD on power fail, and vendors charge three times as much as they do for consumer class SSDs.

Some vendors do certify their SSDs, but you should check them under real power-fail conditions, like pulling the plug.

It is better to use an Arduino with a relay for experimenting with cutting off power.

mysql python 3 mac

  548  pip install –egg‌​nnector-python-2.1.4‌​.zip

  549  pwd

  550  python 

  551  pip list

  552  pip install mysql-connector-python-rf

  553  python 

  554  pip install MySQL-python

  555  pip install mysql-python

  556  brew install mysql-connector-c 

  557  pip install mysql-python

  558  pip install MySQL-python

  559  pip install MySQL-python

  560  export PATH=$PATH:/usr/local/mysql/bin/

  561  pip install MySQL-python

  562  pip install MySQL-python

  563  pip install pymysql

  564  python 

  565  python 

  566  pip install MySQL-python

  567  pip3 install mysql-connect-python

  568  pip3 install

  569  pip install mysqlclient